JPWO2015190030A1 - Operational amplifier and charge amplifier using the same - Google Patents

Abstract

Provided are an operational amplifier that can be of an input type with high impedance without impairing speed performance, and that can further suppress generated noise, and a charge amplifier to which the operational amplifier is applied. First and second buffers (11) and (12) arranged in parallel, each having a bipolar transistor and a bias diode, which are connected in a complementary manner by connecting emitters to each other, and the connection between the emitters between the two buffers A resistor (Re) connected between the points, a first current-voltage conversion circuit (13) and a second current-voltage conversion circuit (14) for connecting the first and second buffers to the positive power supply line and the negative power supply line ), First and second junction field effect transistors (Q15), (Q25) for controlling the current flowing in the diode, and an output stage (15) connected to the outputs of the first and second current-voltage conversion circuits And the gates of the first and second junction field effect transistors are used as input terminals.

Description

  The present invention relates to an operational amplifier having high speed, low input bias current, and low noise characteristics, and a charge amplifier using the operational amplifier.

As sensors using the principle of suppressing minute capacitance changes, there are various physical quantity sensors that detect various physical quantities using a MEMS (Micro Electro Mechanical System) structure that have been developed and researched in recent years.
The capacitance variable type physical quantity sensor is very simple and has a configuration in which a weight and a minute spring supporting the weight are formed by a Si process or the like, and a fixed electrode is provided so as to face the weight. Since the weight is supported by the spring, it can move to some extent with respect to the external force, and when the external force is applied, the capacitance between the weight and the fixed electrode changes as a result. The principle of the capacitance variable physical quantity sensor using MEMS is to detect the displacement of the weight by reading this capacitance with a charge amplifier.

  When a charge amplifier is configured using an operational amplifier to observe a capacitance change in this way, noise is reduced at high frequencies. In order to ensure a high SN, it is desirable to measure the capacity change at 100 kHz or higher. As an approximate trend of increasing SN, a single digit higher frequency is required to improve the single digit SN. In other words, it is shown that a high SN can be achieved by adopting a circuit configuration capable of operating at a high frequency.

  Current feedback operational amplifiers are known as operational amplifiers that can operate at high frequencies. This current feedback operational amplifier has a configuration shown in FIG. 12 when simplified. That is, it has a buffer BU in which complementary npn-type bipolar transistor Q101 and pnp-type bipolar transistor Q102 having emitters connected to each other are connected in series.

In this buffer BU, the collector of the bipolar transistor Q101 is connected to the positive power supply line Lp via the diode D101 in the forward direction, and the collector of the bipolar transistor Q102 is connected to the negative power supply line Ln via the diode D102 in the forward direction. ing.
Two diodes D103 and D104 are connected in series in the forward direction between the bases of the bipolar transistors Q101 and Q102. Here, the anode of the diode D103 is connected to the constant current source CI101 connected to the positive power supply line Lp, and the cathode of the diode D104 is connected to the constant current source CI102 connected to the negative power supply line Ln.

A connection point between the cathode of the diode D103 and the anode of D104 is connected to the positive input terminal + tin. A connection point between the emitters of the bipolar transistors Q101 and Q102 is connected to the negative side input terminal -tin.
Further, the connection point between the collector of the bipolar transistor Q101 of the buffer BU and the cathode of the diode D101 is connected to the base of a pnp bipolar transistor Q103 constituting the output stage. Similarly, the connection point between the collector of the bipolar transistor Q102 of the buffer BU and the anode of the diode D102 is connected to the base of an npn-type bipolar transistor Q104 constituting the output stage.

The bipolar transistors Q103 and Q104 have collectors connected to each other, the emitter of the bipolar transistor Q103 is connected to the positive power supply line Lp, and the emitter of the bipolar transistor Q104 is connected to the negative power supply line Ln.
Further, the connection point between the collectors of the bipolar transistors Q103 and Q104 is connected to the output terminal tout via the output stage buffer BU2 having a high impedance. A parallel circuit of a resistor R101 and a capacitor C101 is connected between a connection point between the collectors of the bipolar transistors Q103 and Q104 and between the input side of the output stage buffer BU2 and the ground.

Furthermore, a load resistor R is connected between the negative input terminal -tin and the ground, and a connection point between the load resistance R and the negative input terminal -tin is connected to the output side of the buffer amplifier BU2 via the feedback resistor Rf. A current feedback type operational amplifier is configured by being connected.
In this current feedback operational amplifier, when the input voltage at the positive input terminal + tin increases, the increase in the current flowing into the load resistor R is folded as it is to increase the potential of the output stage buffer BU2 having a high impedance. When the potentials at the input terminals + tin and -tin become equal, the increase in current disappears, and the output is stabilized at that potential. In the current feedback operational amplifier, amplification is performed by switching the current, and the reason for the high speed is that the current of the bipolar transistor can be switched faster than the voltage.

  By the way, the current feedback operational amplifier having the above configuration cannot be applied to a charge amplifier that detects a minute capacitance change. There are two reasons for this. First, the first reason is that the impedance of the negative side input terminal -tin is very low (approximately 10 to 100Ω), and in principle, a gain cannot be obtained unless a current is supplied reliably. The second reason is that the impedance of the positive input terminal + tin is not high enough to be applied to the charge amplifier.

On the other hand, in order to improve the slew rate while reducing the power consumption during the steady operation of the operational amplifier, the level conversion circuit that converts the differential input signal level and outputs it differentially, There has been proposed an operational amplifier that includes a differential amplifier having inputs connected in series and a current-voltage conversion circuit connected to the positive side and the negative side of the differential amplifier (see Patent Documents 1 and 2). . In this case, the differential amplifier connects two p-channel MOS field effect transistors (hereinafter also referred to as “pMOS”) in parallel and two n-channel MOS field effect transistors (hereinafter also referred to as “nMOS”) in parallel. The source of each pMOS and nMOS connected in parallel is connected directly or via a resistor.
In this operational amplifier, since the two input terminals are connected to the gate of the pMOS, both input terminals can be set to high impedance. The first reason and the second reason in the operational amplifier of FIG. Can be resolved.

JP 2009-171479 A US Pat. No. 7,982,537

However, in the conventional examples described in Patent Documents 1 and 2, the differential amplifier, the level conversion circuit, and the current-voltage conversion circuit that constitute the operational amplifier are configured by NMOS and PMOS, and thus are generated by 1 / f noise. There is an unsolved problem that the noise is large and is not suitable for obtaining a high SN and cannot be applied to a charge amplifier that detects a minute capacitance change.
Therefore, the present invention has been made paying attention to the unsolved problems of the above-described conventional example, and an operational amplifier capable of suppressing the generated noise and the input type having a high impedance without impairing the speed performance. The purpose of this is to provide a charge amplifier using this.

In order to achieve the above object, one aspect of an operational amplifier according to the present invention includes a diode inserted in series so that a current flows in a forward direction between a positive power supply line and a negative power supply line, and a mutual emitter. First and second buffers having an npn bipolar transistor having a base connected to an anode of a diode on the side of the dead power line and a pnp bipolar transistor having a base connected to a cathode of the diode on the negative power line side A resistor interposed between connection points between the emitters of the npn bipolar transistor and the pnp bipolar transistor in the first and second buffers, the npn bipolar transistor in the first and second buffers, and the High potential among pnp bipolar transistor collectors A first current-voltage conversion circuit connected to the positive power supply line connecting the first current-voltage conversion circuit, a second current-voltage conversion circuit connected to the negative power supply line connecting the lower potential side of the collector, the first and first N-channel first and second junction field effect transistors having a source connected to a connection point of the anode of the diode and a base of the npn bipolar transistor in the second buffer, and a drain connected to the positive power supply line side; An output stage electrically connected to the outputs of the first and second current-voltage conversion circuits and controlled by the first and second current-voltage conversion circuits. The gates of the first and second junction field effect transistors are used as input terminals, and the output of the output stage is electrically connected to the output terminal to form a voltage feedback differential amplifier configuration.
In one embodiment of the charge amplifier according to the present invention, at least the operational amplifier is applied as an operational amplifier constituting the integrating circuit.

According to the present invention, the first and second junction field effect transistors are connected to the connection points between the bases of the npn bipolar transistors or the bases of the pnp bipolar transistors and the diodes constituting the first and second buffers. Since the gates of the first and second junction field effect transistors are used as input terminals, it is possible to provide an operational amplifier that can increase input impedance and reliably suppress noise without impairing high-speed performance. it can.
In addition, since the charge amplifier is configured by applying the operational amplifier having the above effect as the operational amplifier constituting the integration circuit, a charge amplifier capable of reliably suppressing the influence of external noise while ensuring the operation speed is provided. can do.

1 is a circuit diagram showing a first embodiment of an operational amplifier according to the present invention. It is a characteristic diagram which shows the relationship between a bipolar transistor current and a voltage noise density when the electric current of a junction field effect transistor is 1 mA. It is a characteristic diagram which shows the relationship between a bipolar transistor current and a voltage noise density when the electric current of a junction field effect transistor is 10 mA. It is a circuit diagram which shows the modification of 1st Embodiment. It is a circuit diagram showing a charge amplifier using the operational amplifier of the first embodiment. It is a circuit diagram which shows the conventional charge amplifier. 4A and 4B are diagrams illustrating frequency characteristics of a charge amplifier according to the present embodiment and a conventional example, where FIG. 5A is a characteristic diagram illustrating a relationship between frequency and gain, and FIG. 5B is a characteristic diagram illustrating a relationship between frequency and phase. It is. It is a characteristic diagram which shows the output characteristic of the charge amplifier of this embodiment and a prior art example. It is a characteristic diagram which shows the relationship between the frequency of a charge amplifier, and noise. 1 is a circuit diagram showing a differential charge amplifier using an operational amplifier according to a first embodiment. FIG. It is a characteristic diagram which shows the power supply voltage dependence characteristic with and without a cascode transistor. It is a circuit diagram which shows the conventional operational amplifier.

Next, an embodiment of the present invention will be described with reference to the drawings. In the following description of the drawings, the same or similar parts are denoted by the same or similar reference numerals.
Further, the following embodiments exemplify devices for embodying the technical idea of the present invention, and the technical idea of the present invention is the material, shape, structure, arrangement, etc. of the component parts. Is not specified as follows. The technical idea of the present invention can be variously modified within the technical scope defined by the claims described in the claims.

First, a first embodiment of an operational amplifier that represents one aspect of the present invention will be described.
In the operational amplifier 1, as shown in FIG. 1, a first buffer 11 and a second buffer 12 arranged in parallel are connected to a positive power supply line Lp via a first current-voltage conversion circuit 13. At the same time, it is connected to the negative power supply line Ln via the second current-voltage conversion circuit 14.
Further, a connection point between the first buffer 11 and the first current-voltage conversion circuit 13 and a connection point between the first buffer 11 and the second current-voltage conversion circuit 14 are individually provided in the output stage 15. It is connected to the.

The first buffer 11 includes a complementary connection npn-type bipolar transistor Q11 and pnp-type bipolar transistor Q12 whose emitters are connected to each other. The collector of the npn-type bipolar transistor Q11 is connected to the first current-voltage conversion circuit 13, and the collector of the pnp-type bipolar transistor Q12 is connected to the second current-voltage conversion circuit 14.
Further, the base of the npn type bipolar transistor Q11 and the base of the pnp type bipolar transistor Q12 are connected to the bias circuit 21 to constitute a class B push-pull circuit.
The bias circuit 21 includes an npn bipolar transistor Q13 and a pnp bipolar transistor Q14 that operate as diodes.

The collector of the npn-type bipolar transistor Q13 is connected to the positive power supply line Lp via the npn-type bipolar transistor Q16 that is cascode-connected to the first n-channel junction field effect transistor Q15, and the npn-type bipolar transistor Q11 Connected to the base.
The collector of the pnp bipolar transistor Q14 is connected to the negative power supply line Ln via the current mirror circuit 22, and is also connected to the base of the pnp bipolar transistor Q12.

The base of npn bipolar transistor Q13 is connected to the connection point between the collector of npn bipolar transistor Q13 and the base of npn bipolar transistor Q11. Similarly, the base of the pnp bipolar transistor Q14 is connected to the connection point between the collector of the pnp bipolar transistor Q14 and the base of the pnp bipolar transistor Q12. Here, the collector and base of the npn-type bipolar transistor Q13 operating as a diode are the anode, and the emitter is the cathode. Further, the emitter of the pnp bipolar transistor Q14 operating as a diode is the anode, and the collector and base are the cathode.
The positive input terminal + tin is connected to the gate of the first n-channel junction field effect transistor Q15. The power supply circuit 23 is connected to the base of the cascode-connected npn bipolar transistor Q16, and the power supply circuit 23 sets the base potential of the npn bipolar transistor Q16 to an intermediate potential Vcas1 between the ground potential and the power supply potential. Yes.

The second buffer 12 has the same configuration as that of the first buffer 11 described above, and has complementary npn-type bipolar transistor Q21 and pnp-type bipolar transistor Q22 whose emitters are connected to each other. The collector of the npn-type bipolar transistor Q21 is connected to the positive power supply line Lp via the current-voltage conversion circuit 13. The collector of the pnp bipolar transistor Q22 is connected to the negative power supply line Ln via the second current-voltage conversion circuit 14.
Further, the base of the npn-type bipolar transistor Q21 and the base of the pnp-type bipolar transistor Q22 are connected to the bias circuit 24 to constitute a class B push-pull circuit.

The bias circuit 24 includes an npn bipolar transistor Q23 and a pnp bipolar transistor Q24 that operate as diodes.
The collector of the npn-type bipolar transistor Q23 is connected to the positive power supply line Lp via the npn-type bipolar transistor Q26 that is cascode-connected to the second n-channel junction field-effect transistor Q25, and the npn-type bipolar transistor Q21 Connected to the base.

  The collector of the pnp bipolar transistor Q24 is connected to the negative power supply line Ln via the current mirror circuit 22, and is also connected to the base of the pnp bipolar transistor Q22. The base of npn bipolar transistor Q23 is connected to the connection point between the collector of npn bipolar transistor Q23 and the base of npn bipolar transistor Q21. Similarly, the base of the pnp bipolar transistor Q24 is connected to the connection point between the collector of the pnp bipolar transistor Q24 and the base of the pnp bipolar transistor Q22. Here, the collector and base of the npn-type bipolar transistor Q23 operating as a diode are the anode, and the emitter is the cathode. The emitter of the pnp bipolar transistor Q24 operating as a diode is the anode, and the collector and base are the cathode.

A connection point between the emitters of the npn-type bipolar transistor Q11 and the pnp-type bipolar transistor Q12 of the first buffer 11 and a connection point between the emitters of the npn-type bipolar transistor Q21 and the pnp-type bipolar transistor Q22 of the second buffer 12 Are connected via an emitter connection resistor Re.
Further, the negative input terminal -tin is connected to the gate of the first junction field effect transistor Q25. Further, a power supply circuit 25 is connected to the base of the cascode-connected npn bipolar transistor 26, and the power supply circuit 25 fixes the base potential of the npn bipolar transistor 26 to an intermediate potential Vcas2 between the ground potential and the power supply potential. Yes.

Note that the fixed potentials Vcas1 and Vcas2 of the npn-type bipolar transistors Q16 and Q26 that are cascode-connected are set to the lowest voltage in consideration of the operation (threshold voltage: Vth) of the n-channel junction field effect transistors Q15 and Q25. However, it is set to Vth + 1V, and the upper limit voltage is set to ensure (power supply voltage-1) V.
The first current-voltage conversion circuit 13 includes a pnp bipolar transistor Q31 having an emitter connected to the positive power line Lp and a collector connected to the first buffer 11, and an emitter connected to the positive power line Lp. And a pnp bipolar transistor Q32 having a collector connected to the second buffer 12. The bases of these pnp bipolar transistors Q31 and Q32 are connected to each other, and the connection point of both bases is connected to the collector of the pnp bipolar transistor Q32 to form a current mirror circuit.

  The second current-voltage conversion circuit 14 has an npn bipolar transistor Q33 whose emitter is connected to the negative power supply line Ln and collector connected to the first buffer 11, and an emitter connected to the negative power supply line Ln. And an npn-type bipolar transistor Q34 having a collector connected to the second buffer 12. The bases of these npn-type bipolar transistors Q33 and Q34 are connected to each other, and the connection point of both bases is connected to the collector of the npn-type bipolar transistor Q34 to form a current mirror circuit.

The current mirror circuit 22 includes an npn bipolar transistor Q35 inserted between the constant current source 31 and the negative power supply line Ln, an npn bipolar transistor Q36 having a base connected to the base of the npn bipolar transistor, and Q37.
The npn bipolar transistor Q35 has a collector and a base connected to a constant current source 31 that supplies a constant current Iref, and an emitter connected to a negative power supply line Ln.

The npn bipolar transistor Q36 has a base connected to the base of the npn bipolar transistor Q35, and a collector connected to the base of the pnp bipolar transistor Q12 of the first buffer 11 and the collector and base of the pnp bipolar transistor Q14. The emitter is connected to the negative power line Ln.
The npn bipolar transistor Q37 has a base connected to the base of the npn bipolar transistor Q35, and a collector connected to the base of the pnp bipolar transistor Q22 of the second buffer 12 and the collector and base of the pnp bipolar transistor Q24. The emitter is connected to the negative power line Ln.

The output stage 15 includes a first voltage-current conversion circuit 41 that converts a voltage connected to the positive power supply line Lp into a current, and a second voltage current that converts a voltage connected to the negative power supply line Ln into a current. And a conversion circuit 42.
The first voltage-current conversion circuit 41 includes pnp bipolar transistors Q41 and Q42. The pnp bipolar transistor Q41 has an emitter connected to the positive power supply line Lp, a base connected to a connection point between the first buffer 11 and the first current / voltage conversion circuit 13, and a collector connected to the second voltage / current conversion. The npn-type bipolar transistor Q43 of the circuit 42 is connected to the collector.
The pnp bipolar transistor Q42 has an emitter connected to the positive power supply line Lp, a collector and a base connected to the base of the pnp bipolar transistor Q41 and a connection point between the first buffer 11 and the first current-voltage conversion circuit 13. Connected between.

The second voltage-current conversion circuit 42 includes npn-type bipolar transistors Q43 and Q44. The npn bipolar transistor Q43 has a collector connected to the collector of the pnp bipolar transistor Q41 of the first voltage-current conversion circuit 41 described above, an emitter connected to the negative power supply line Ln, and a base connected to the first buffer 11 and It is connected to a connection point between the second current-voltage conversion circuits 14.
The npn bipolar transistor Q44 has an emitter connected to the negative power supply line Ln, and a collector and a base connected to the base of the npn bipolar transistor Q43 and a connection point between the first buffer 11 and the second current-voltage conversion circuit 14. Connected between.

A connection point between the collector of the pnp bipolar transistor Q41 of the first voltage-current conversion circuit 41 and the collector of the npn-type bipolar transistor Q43 of the second voltage-current conversion circuit is connected to the output terminal tout, and a capacitor It is connected via C to the connection point between the emitters of the bipolar transistors Q23 and Q24 in the bias circuit 24 of the second buffer 12.
At least the first buffer 11, the second buffer 12, the first current-voltage conversion circuit 13, the second current-voltage conversion circuit 14, the output stage 15, the junction field effect transistors Q15 and Q25, and the current mirror circuit 22 A voltage feedback type differential amplifier is configured.

Next, the operation of the first embodiment will be described.
In the first embodiment, the collector currents of the first buffer 11 and the second buffer 12 using the complementary bipolar transistors Q11, Q12 and Q21, Q22 are converted into the first current-voltage conversion circuit 13 and the second current, respectively. The current-voltage conversion circuit 14 has a folded structure with a current mirror circuit. Therefore, when the voltage at the positive input terminal + tin rises, the increase in the current flowing into the emitter connection resistor Re is folded as it is, and the potential of the buffers (Q41 and Q43) of the output stage 15 having high impedance is raised.

When the potentials of the positive electrode side input terminal + tin and the negative electrode side input terminal -tin become equal, the increase in current disappears, and the output is stabilized at that potential. In this embodiment, amplification is performed by switching the current, and the current of the bipolar transistor can be switched faster than the voltage, so that high-speed operation is possible.
At this time, a voltage feedback operational amplifier is realized by providing the first buffer 11 and the second buffer 12 and using these as current paths. What are the four transistors Q11, Q12 and Q21, Q22 connected by the emitter connection resistor Re and the current mirror circuit constituting the first current-voltage conversion circuit 13 and the second current-voltage conversion circuit 14 arranged on the power supply side? Very fast due to current operation. Since the voltage is finally converted by the bipolar transistors Q41 and Q43 in the output stage 15, the speed is limited in this portion, but high speed amplification is possible by optimally setting the area and bias current of each bipolar transistor. Become.

By connecting the input terminals + tin and -tin to the gates of the n-channel junction field effect transistors Q15 and Q25, the input impedance can be increased, and the first buffer 11 and the second buffer can be formed using bipolar transistors. 12, the first current-voltage conversion circuit 13, the second current-voltage conversion circuit 14, and the output stage 15 constitute a voltage feedback operational amplifier with almost no loss of high-speed performance.
In order to realize a high SN, it is important to reduce the noise of the operational amplifier itself. For that purpose, it is important to appropriately set a bias current value to be applied to the transistor. In particular, when used in a charge amplifier, high-frequency voltage noise density is important.

In the case of non-feedback, the self-noise of the operational amplifier is given by the sum of noises generated in all devices, but is usually used with limited gain by applying feedback. In this case, noise generated from a device outside the negative feedback loop is corrected by the negative feedback and can be ignored in the frequency band where the negative feedback effect is present. Therefore, when considering noise, it is only necessary to focus on noise generated by devices in the feedback loop. Therefore, it is only necessary to consider noise generated from devices from the positive input terminal + tin to the negative input terminal −tin. Specifically, in FIG. 1, the current path is Q15-Q11-Re-Q21-Q25, and the other is the current path of Q15-Q13-Q14-Q12-Re-Q22-Q24-Q23-Q25.
The voltage noise Vfet of the junction field effect transistors Q15 and Q25 is a function of frequency and can be expressed by the following equation (1).

Here, k is the Boltzmann constant, g _mJFET is the mutual conductance of the junction field effect transistor, and T is the absolute temperature. K _JFET is a constant attributed to the shape of the junction field effect transistor, and changes depending on each process.
The voltage noise of the bipolar transistor can be expressed by the following formula (2).

_{Here, g mBJT} transconductance, _{r b} of the bipolar transistor is the base resistance (usually about 5 [Omega).
Further, the voltage noise of the resistor can be expressed by the following formula (3), where R is a resistance value.

にて表すことができるが、これはモデル上のもので、物理的な抵抗ではない。したがって、熱ノイズへの寄与は考えなくて良い。
以上より、ノイズを加算すればよいのだが、バイポーラトランジスタＱ１１，Ｑ１２（Ｑ２１、Ｑ２２）は並列なので半分のノイズになることに注意する。結果としてトータルノイズＶｎは、以下の式（５）で表すことができる。Resistance r _d of the diodes in the small signal, q the elementary charge, as current through the diode I _d

This is on the model, not a physical resistance. Therefore, it is not necessary to consider the contribution to thermal noise.
From the above, it is only necessary to add noise, but it should be noted that the bipolar transistors Q11 and Q12 (Q21, Q22) are in parallel because of the parallel noise. As a result, the total noise Vn can be expressed by the following equation (5).

Here, 2K _JFET / f is 1 / f noise, and if an appropriate junction field effect transistor is selected, noise that causes a problem at the measurement frequency can be prevented. The other terms are the sum of the resistance components, and the thermal noise due to the resistance is calculated.
A common junction field effect transistor has a conductance gm of about Is / 1V (Is is a source current), and even a high-performance type has Is / 0.2V, whereas a bipolar transistor has a well-known conductance gm. Thus, Ic / 26 mV. This indicates that the equivalent resistance of the junction field effect transistor is 10 to 40 times higher than that of the bipolar transistor when the same current flows.

For example, when a high-performance junction field effect transistor having a mutual conductance of about 5 mS (about 270 Ω in terms of resistance in the equation (5)) is used when the source current is 1 mA. Take the example.
FIG. 2 shows the result of calculating the voltage noise density of the operational amplifier of FIG. 1 while ignoring the 1 / f noise term in the equation (5) when the current I _JFET of the junction field effect transistor is 1 mA. As is apparent from FIG. 2, when the current I _BPT of the bipolar transistor is 0.1 mA or more, which is 1/10 of the current I _JFET of the junction field effect transistor, it takes a substantially constant value. Noise increases suddenly. FIG. 3 shows the case where the current I _JFET of the junction field effect transistor is set to 10 mA, but when the current I _{BPT of the} bipolar transistor is reduced to 1 mA which is 1/10 of 1/10 of the current I _JFET of the junction field effect transistor, Noise increases.

Noise design is performed as follows. First, the current of the junction field effect transistor is determined from the amount of noise of the required device. This corresponds to setting the noise of the flat portion in FIGS. Here not only calculate two cases, but if the current _{I JFET} junction field effect transistor of Fig 2 is 1mA, 2.2nV / √Hz, the current _{I JFET} junction field effect transistor of FIG. 3 10 mA of In this case, 1 nV / √Hz.

Next, the current value of the bipolar transistor is determined. As is clear from FIGS. 2 and 3, even if the current I _{BPT of the} bipolar transistor is set to be higher than the current I _JFET of the junction field effect transistor, noise does not decrease and current consumption such as temperature rise increases. The harmful effects caused by this increase. Normally, the amount of current is reduced to reduce noise, but the noise increases when the current amount of the bipolar transistor becomes 1/10 or less of the current _IJFET of the junction field effect transistor. The meaning of disappears. For this reason, the current I _{BPT of the} bipolar transistor is set to a value that is equal to or slightly smaller than the current I _JFET of the junction field effect transistor, thereby suppressing the adverse effect of temperature rise due to an increase in current consumption while suppressing the voltage noise density. can do.

  By the way, when a MOS field effect transistor is applied instead of the n-channel junction field effect transistors Q15 and Q25, the 1 / f noise of the MOS field effect transistor is large, so that the voltage noise density can be suppressed. It is impossible to construct an operational amplifier with low noise characteristics. In addition, when a MOS field effect transistor is applied in place of each bipolar transistor, the voltage noise density is further increased, and an operational amplifier having low noise characteristics cannot be constructed at all.

  In this embodiment, the connection point between the emitters of the complementary transistors Q11 and Q12 of the first buffer 11 and the connection point between the emitters of the bipolar transistors Q21 and Q22 of the second buffer 12 are connected. They are connected via an emitter connection resistor Re. As described above, by arranging the emitter connection resistor Re, even if the characteristics of the four bipolar transistors Q11, Q12 and Q21, Q22 that are connected in a complementary manner have a certain degree of variation, the emitter connection resistor Re absorbs the variation in characteristics. And a stable circuit.

  Incidentally, when the emitter connection resistor Re is omitted and the connection point between the emitters of the complementary transistors Q11 and Q12 and the connection point between the emitters of the complementary transistors Q21 and Q22 connected directly are bipolar. The current flowing through the connecting portion is not stable according to variations in the characteristics of the transistors Q11, Q12 and Q21, Q22, and normal operation is impaired.

Further, in this embodiment, npn-type bipolar transistors Q16 and Q26 are cascode-connected to the positive-side power supply line Lp side of the n-channel junction field effect transistors Q15 and Q25, and the base potentials of the bipolar transistors Q16 and Q26 are connected to the ground potential and the power supply. It is fixed at an intermediate potential.
Therefore, it is possible to prevent the bias voltages of the bias circuits 21 and 24 of the first buffer 11 and the second buffer 12 from being affected by the power supply voltage fluctuation from the positive power supply line Lp. 11 and the first current-voltage conversion circuit 13 and the second current-voltage conversion circuit 14 can be prevented from changing in output voltage.

  In the first embodiment, the case where npn-type bipolar transistors Q16 and Q26 having the same polarity are connected between the n-channel junction field effect transistors Q15 and Q25 and the positive power supply line Lp has been described. The present invention is not limited to this, and p-channel junction field effect transistors having different polarities from the n-channel junction field effect transistors Q15 and Q25 may be applied.

  In the first embodiment, the case where the npn bipolar transistors Q16 and Q26 are cascode-connected between the n-channel junction field effect transistors Q15 and Q25 and the positive power supply line Lp has been described. However, the present invention is not limited to the above configuration, and a constant current source may be applied instead of the npn-type bipolar transistors Q16 and Q26.

  In the first embodiment, the n-channel junction field effect transistors Q15 and Q25 and the npn bipolar transistors Q16 and Q26 are connected between the first buffer 11 and the second buffer 12 and the positive power supply line Lp. Inserted. However, the present invention is not limited to the above configuration, and as shown in FIG. 4, between the bias circuits 21 and 24 of the first buffer 11 and the second buffer 12 and the positive power supply line Lp. A current mirror circuit 22 is inserted, and p-channel junction field effect transistors Q51 and Q52 are inserted between the bias circuits 21 and 24 of the first buffer 11 and the second buffer 12 and the negative power supply line Ln. The pnp bipolar transistors Q53 and Q54 having the same polarity may be cascode-connected to the p-channel junction field effect transistors Q51 and Q52. Also in this case, the same effect as that of the first embodiment described above can be obtained. Of course, p-channel junction field effect transistors may be applied instead of the pnp bipolar transistors Q53 and Q54.

Next, a second embodiment of the present invention will be described with reference to FIG.
In the second embodiment, a charge amplifier is configured using the operational amplifier 1 in the first embodiment described above.
In the second embodiment, the negative side input terminal -tin of the operational amplifier 1, that is, the gate of the n-channel junction field effect transistor Q25 is connected to the input terminal tin via the variable capacitance C _SENS of a minute capacitance of, for example, 1 pF of the variable capacitance sensor. It is connected to the. Here, as the variable capacitance sensor, a sensor that detects a physical quantity such as acceleration or vibration using a MEMS (Micro Electro Mechanical System) structure is applied.

Further, a parallel circuit of a feedback resistor Rf and a capacitor Cf is provided between the connection point between the variable capacitor C _SENS and the gate of the n-channel junction field effect transistor Q25 and the connection point between the output terminal tout and the output stage 15. The charge amplifier 50 is configured by being connected. The output signal of the operational amplifier 1 is negatively fed back to the negative input terminal -tin by the resistor Rf and the capacitor Cf.

で表される。
したがって、入力電圧Ｖｉｎを一定値に制御すれば、可変容量Ｃ_ＳＥＮＳの変化量に比例した出力電圧Ｖｏｕｔを得ることができる。Thus, by configuring the charge amplifier 50 using the operational amplifier 1, the relationship between the output voltage Vout output from the output terminal tout in the operation region and the input voltage Vin input to the input terminal tin is

It is represented by
Therefore, if the input voltage Vin is controlled to a constant value, an output voltage Vout proportional to the amount of change of the variable capacitor C _SENS can be obtained.

According to the second embodiment, the charge amplifier 50 is configured by using the operational amplifier 1 having the low noise characteristics while having the high speed performance in the first embodiment described above. It becomes a charge amplifier that inherits performance and low noise performance.
In order to confirm the high-speed performance of the charge amplifier 50 in the second embodiment, a performance comparison with the conventional charge amplifier 110 shown in FIG. 6 was performed. Here, the configuration of the conventional charge amplifier 110 includes a differential amplifier circuit 111 in which n-channel junction field effect transistors Q111 and Q112 and a series circuit of resistors R111 and R112 are connected in parallel. In the differential amplifier circuit 111, the opposite sides of the resistors R111 and R112 to the n-channel junction field effect transistors Q111 and Q112 are connected to each other and connected to the positive power supply line Lp. The sources of the n-channel junction field effect transistors Q111 and Q112 are connected to each other and connected to the negative power line Ln via the constant current source 113.

The differential output of the differential amplifier circuit 111 is connected to the inverting input terminal and the non-inverting input terminal of the operational amplifier 114, and the output terminal tout is connected to the output side of the operational amplifier 114.
Further, the gate of the n-channel junction field effect transistor Q111 is grounded, and the gate of the n-channel junction field effect transistor Q112 is connected to the input terminal tin via the variable capacitor C _SENS of the variable capacitance sensor, and the variable capacitors C _SENS and A parallel circuit of a feedback resistor Rf and a capacitor Cf is connected between a connection point between the gate of the n-channel junction field effect transistor Q112 and the output side of the operational amplifier 114.

Then, the gain and phase of the output waveform are measured when the input voltage Vin of 8 V and 10 MHz is applied to the input terminal tin of the charge amplifier 50 of the present invention shown in FIG. 5 and the conventional charge amplifier 110 shown in FIG. did.
The measurement results of these gains and phases are shown in FIGS. 7 (a) and 7 (b). As shown in FIG. 7A, in the conventional charge amplifier 110, the gain begins to gradually decrease when the frequency exceeds 1 MHz as shown by the broken line, but in the charge amplifier 50 of the present invention, the gain is shown as the solid line. When the frequency exceeds 60 MHz, for example, it sharply decreases.

Further, as shown in FIG. 7B, the phase starts to decrease when the frequency exceeds 500 kHz in the conventional charge amplifier 50, but starts to gradually decrease when the frequency exceeds 3 MHz, for example. .
As a result, it was confirmed that the charge amplifier 50 of the present invention operates satisfactorily with a phase delay time of about 3 nsec and no gain reduction even when the frequency of the input voltage Vin is 10 MHz. However, the conventional charge amplifier 110 cannot perform a sufficient operation because the phase delay time is 16 nsec and the gain is reduced to 61%.

Further, as shown in FIG. 8, the output voltage Vout has an output waveform that changes within a range of ± 4 V as shown by the solid line in the charge amplifier 50 of the present invention. As shown by the broken line, the output waveform changes within a range of ± 2 V, which is half the output voltage of the charge amplifier 50 of the present invention.
Furthermore, the noise characteristics when the charge amplifier 50 of the present invention is applied to variable capacitance measurement are as shown in FIG. As for the noise characteristics of the conventional charge amplifier 110, if an n-channel junction field effect transistor having the same input is applied, a noise characteristic almost the same as that of the present invention can be obtained. In FIG. 9, the flat portion where the noise of 10 kHz or more does not change is the portion where the noise of the operational amplifier itself is dominant, and the noise indicated by the thin dotted line in the figure extended with the same gain is hidden in the large noise. Existing.

The noise increasing at 100 kHz or less is due to the feedback resistor Rf, and is unavoidable when configuring a charge amplifier. Therefore, when applied to a variable capacitance sensor, the lowest noise characteristic can be obtained if it is used in the frequency region where the flat portion shown in the figure appears.
In the conventional charge amplifier 110, since the gain and phase characteristics deteriorate at 10 MHz, it must be used at 1 MHz or less. Moreover, since feedback resistance noise increases at 10 kHz or less, it is desirable to use it at a center frequency of about 100 kHz. However, this condition is a case where 1000 MegΩ can be secured as the resistance value of the feedback resistor Rf. A high resistance of 1000 MegΩ can be produced industrially, but in actual use, there is a high possibility that the resistance will decrease due to aging and the parasitic resistance of the substrate or wafer. End up.

  In the present invention, as described above, the frequency can be increased by about one digit compared with the conventional type. This means that even if the measurement frequency is 1 MHz, the operation can be performed with a sufficient margin. As a result, the noise can be increased due to the feedback resistor Rf. In FIG. 9, a thick broken line indicates a noise characteristic when the feedback resistance Rf is set to 100 MegΩ. When set to 100 MegΩ, the frequency at which the noise rises is 100 kHz, but there is still room for the 1 MHz operating point. Since the resistance drop due to deterioration over time is allowed up to 30 MegΩ, a charge amplifier having no practical problem can be provided.

Next, a third embodiment of the present invention will be described with reference to FIG.
In the third embodiment, a differential charge amplifier is configured using the operational amplifier in the first embodiment described above.
That is, in the third embodiment, like the second embodiment described above, the feedback resistor Rf and the feedback resistor Rf between the inverting input terminal and the output terminal of the operational amplifier 1 of the first embodiment described above are provided. A parallel circuit of the capacitor Cf is connected.

In addition, one variable capacitor C _SENS M of the variable capacitance sensor 60 having a differential structure in which one variable capacitor decreases at the inverting input terminal and the non-inverting input terminal of the operational amplifier 1 and the other variable capacitor decreases, A variable capacitor C _SENS P is connected.
The variable capacitance sensor 60 is a sensor that detects physical quantities such as acceleration and vibration using a MEMS (Micro Electro Mechanical System) structure, and the variable capacitances C _SENS M and C _SENS P are both 1 pF and a small capacitance, for example. ing. A capacitor Cpp of 10 pF, for example, is connected between the variable capacitor C _SENS M and the inverting input terminal of the operational amplifier 1 and the ground. Similarly, the variable capacitor C _SENS P and the non-inverting input terminal of the operational amplifier 1 are connected. For example, a capacitor Cpm of 10 pF is connected between the capacitor and the ground.

An AC oscillator 61 that outputs an AC carrier signal of ± 8 V at 100 kHz, for example, is connected to the electrode on the opposite side of the operational amplifier 1 of each variable capacitor C _SENS M and C _SENS P.
Further, a multiplier 62 as a demodulation circuit is connected to the output side of the operational amplifier 1, and the AC carrier signal of the AC oscillator 61 is input to the multiplier 62. The capacitance detection signal demodulated by the multiplier 62 is subjected to noise removal by a low-pass filter 63 including a resistor R1 and a capacitor C1, and is output from an output terminal Tout.

Further, a parallel circuit of an adjustment trimmer capacitor Cpin and a resistor Rpin is connected between the variable capacitor C _SENSP of the variable capacitor 60 and the non-inverting input terminal of the operational amplifier 1 and the ground. Here, the trimmer capacitor Cpin is adjusted so that the 100 kHz carrier signal included in the input signal of the multiplier 62 which is the output of the charge amplifier 50 is minimized.
When the charge amplifier 50 is configured in this way, it is possible to measure a change in capacitance of the low-pass filter 63 connected to the output terminal Tout below a cutoff frequency (for example, 72.3 Hz).

In the circuit configuration of the third embodiment, the operational amplifier 1 is shown as the power supply voltage dependency with and without the bipolar transistors Q16 and Q26 cascode-connected to the junction field effect transistors Q15 and Q25 constituting the operational amplifier 1. The output voltage was measured by changing the voltage of Vdd, which is the operating power supply supplied to the power supply. The variable capacitance sensor 60 was kept stationary without applying an external force. The measurement results are shown in FIG.
As is apparent from FIG. 11, when there is a cascode-connected transistor, the output value does not change even when the voltage of the operating power supply Vdd is changed, and “0” V is maintained. In the absence, an output change of about 1.5 mV was observed with a change of 1 V in the voltage of the operating power supply Vdd.

This phenomenon occurs due to the difference between the positive and negative input capacities of the differential charge amplifier. It can be confirmed by circuit simulation that such a phenomenon does not occur when the capacitance balance is perfect, but in an actual circuit, the capacitance variation of the variable capacitance sensor 60 and the plus and minus gains are determined. There are variations in capacity. The zero point is determined by adjusting the gain of this variation with the trimmer capacitor Cpin. However, since the input capacitance of the operational amplifier 1 depends on the source-drain voltages of the n-channel junction field effect transistors Q15 and Q25, the cascode connection In the absence of such a transistor, when the power supply voltage is changed, the capacity balance, that is, the gain balance is lost, and the above-described output change phenomenon occurs.
As a result of examining the capacitance balance by simulation, it was found that there is a capacitance imbalance of about 0.1 pF in this embodiment. This level of imbalance can easily occur in manufacturing.

On the other hand, in the case of a high-resolution variable capacitance sensor, a resolution of about 5 uV is required. Therefore, when there is no cascode-connected transistor, the allowable power supply voltage fluctuation is about 5 mV in this embodiment. Such a power supply can be realized by, for example, a lead storage battery, but it is difficult to reduce the size of the sensor device.
Therefore, when applied to a high-resolution sensor, as in the first embodiment described above, transistors having the same polarity are cascaded on the drain side of the n-channel junction field effect transistors Q15 and Q25, that is, on the positive power supply line Lp side. By connecting, it is desirable to suppress the influence of fluctuations in the power supply voltage.

DESCRIPTION OF SYMBOLS 1 ... Operational amplifier, Lp ... Positive electrode side power supply line, Ln ... Negative electrode side power supply line, 11 ... 1st buffer, 12 ... 2nd buffer, 13 ... 1st current-voltage conversion circuit, 14 ... 2nd current voltage Conversion circuit, 15 ... output stage, Q11 to Q14 ... bipolar transistor, Q15 ... junction field effect transistor, Q16 ... cascode-connected bipolar transistor, Q21-Q24 ... bipolar transistor, 21,24 ... bias circuit, 22 ... current mirror circuit , Re ... Emitter connection resistance, Q25 ... Junction field effect transistor, Q26 ... Cascode-connected bipolar transistor, + tin ... Positive side input terminal, -tin ... Negative side input terminal, Q31-Q37 ... Bipolar transistor, 41, 42 ... Constant Current circuit, Q41 to Q44 ... bipolar transistor, 0 ... charge _{amplifier, C SENS ...} variable capacitor 60 ... variable capacitance sensor, 61 ... AC generator, 62 ... multiplier, 63 ... low-pass filter

In order to achieve the above object, one aspect of an operational amplifier according to the present invention includes a diode inserted in series so that a current flows in a forward direction between a positive power supply line and a negative power supply line, and a mutual emitter. First and second buffers having an npn bipolar transistor having a base connected to an anode of a diode on the control power line side and a pnp bipolar transistor having a base connected to a cathode of a diode on the negative power line side A resistor interposed between connection points between the emitters of the npn bipolar transistor and the pnp bipolar transistor in the first and second buffers, the npn bipolar transistor in the first and second buffers, and the High potential among pnp bipolar transistor collectors A first current-voltage conversion circuit connected to the positive power supply line connecting the first current-voltage conversion circuit, a second current-voltage conversion circuit connected to the negative power supply line connecting the lower potential side of the collector, the first and first N-channel first and second junctions for suppressing voltage noise density in which a source is connected to a connection point of the anode of the diode and a base of the npn bipolar transistor in the second buffer and a drain is connected to the positive power supply line side Type field effect transistor, and an output stage electrically connected to the outputs of the first and second current-voltage conversion circuits and controlled by the first and second current-voltage conversion circuits . The gates of the first and second junction field effect transistors are used as input terminals, and the output of the output stage is electrically connected to the output terminal to form a voltage feedback differential amplifier configuration.
In one embodiment of the charge amplifier according to the present invention, at least the operational amplifier is applied as an operational amplifier constituting the integrating circuit.

Claims (10)

A diode connected in series so that a forward current flows between the positive power supply line and the negative power supply line, and the emitters connected to each other, and a base connected to the anode of the diode on the positive power supply line side First and second buffers each having a bipolar transistor and a pnp bipolar transistor having a base connected to a cathode of a diode on the negative power supply line side;
A resistor connected between connection points between the emitters of the npn bipolar transistor and the pnp bipolar transistor in the first and second buffers;
A first current-voltage conversion circuit connected to the positive power supply line connecting a higher potential side of collectors of the npn bipolar transistor and the pnp bipolar transistor in the first and second buffers, and a lower potential of the collectors; A second current-voltage conversion circuit connected to the negative power line connecting the side;
The n-channel first and second junction types have a source connected to a connection point between the anode of the diode and the base of the npn bipolar transistor in the first and second buffers, and a drain connected to the positive power supply line side. A field effect transistor;
An output stage electrically connected to the outputs of the first and second current-voltage conversion circuits and controlled by the first and second current-voltage conversion circuits;
The gates of the first and second junction field effect transistors are used as input terminals, and the output of the output stage is electrically connected to the output terminal to form a voltage feedback differential amplifier configuration. Operational amplifier.   A transistor having the same polarity as the first and second junction field effect transistors is cascode-connected between the drains of the first and second junction field effect transistors and the positive power supply line, and the control terminal of the transistor is connected 2. The operational amplifier according to claim 1, wherein the potential is fixed to an intermediate potential between a ground potential and a power supply potential.   3. The operational amplifier according to claim 1, wherein the diode is composed of a diode-connected bipolar transistor. A diode inserted in series so that a current flows in a forward direction between the positive power supply line and the negative power supply line, and the emitters connected to each other, and a base connected to the anode of the diode on the positive power supply line side first and second buffers having an npn bipolar transistor and a pnp bipolar transistor having a base connected to the cathode of the diode on the negative power supply line side;
A resistor interposed between connection points between the emitters of the npn bipolar transistor and the pnp bipolar transistor in the first and second buffers;
A first current-voltage conversion circuit connected to the positive power supply line connecting a higher potential side of collectors of the npn bipolar transistor and the pnp bipolar transistor in the first and second buffers, and a lower potential of the collectors; A second current-voltage conversion circuit connected to the negative power line connecting the side;
First and second junction types of p-channel having a source connected to a connection point between the cathode of the diode and the base of the npn bipolar transistor in the first and second buffers and a drain connected to the negative power supply line side. A field effect transistor;
An output stage electrically connected to the outputs of the first and second current-voltage conversion circuits and controlled by the first and second current-voltage conversion circuits;
The gates of the first and second junction field effect transistors are used as input terminals, and the output of the output stage is electrically connected to the output terminal to form a voltage feedback differential amplifier configuration. Operational amplifier.   A transistor having the same polarity as that of the first and second junction field effect transistors is cascode-connected between the drains of the first and second junction field effect transistors and the negative power supply line, and a control terminal of the transistor is connected. 5. The operational amplifier according to claim 4, wherein the potential is fixed to an intermediate potential between the ground potential and the power supply potential.   6. The operational amplifier according to claim 4, wherein the diode is a diode-connected bipolar transistor.   The current flowing through each bipolar transistor in the first and second buffers is set to a noise suppression current value in a range of 1/10 to 1 of the current flowing through the first and second junction field effect transistors. The operational amplifier according to claim 1, wherein:   The operational amplifier according to claim 1, wherein each of the first and second current-voltage conversion circuits includes a current mirror circuit.   9. A charge amplifier characterized by applying the operational amplifier according to claim 1 as an operational amplifier constituting an integrating circuit.   A physical quantity sensor having a differential structure having a pair of electrode parts composed of a movable electrode and a fixed electrode that generates a change in capacitance according to a change in physical quantity, and supplying the physical quantity sensor to one of the movable electrode and the fixed electrode in the pair of electrode parts A bias voltage generation circuit that generates a bias voltage, and the other of the movable electrode and the fixed electrode in the pair of electrode portions is input to an input terminal, and the minute capacitance between the movable electrode and the fixed electrode in the pair of electrode portions The charge amplifier according to claim 9, wherein the difference is input to gates of the first and second junction field effect transistors of the operational amplifier.

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