JPH03147414A - Amplifier circuit - Google Patents

Abstract

PURPOSE:To obtain an amplifier circuit capable of securing the dynamic range effectively by comparing the output voltage of an amplifier section with a reference voltage and supplying a current to the impedance output terminal at a high gain stage of the amplifier section when the output voltage is lower than a clip voltage. CONSTITUTION:When an output voltage Vout is lower than a clip potential VCL, a current is supplied to the output terminal at the high gain stage A of an operational amplifier and the level is increased. Thus, the operational amplifier loses its own control capability and voltage feedback is produced to apply clipping so that the output level is not lower than the clip potential VCL. Since the breakdown voltage of transistors(TRs) does not limit the circuit operation the output voltage Vout of an output stage B is a waveform output limited by the clip voltage VCL. Moreover, since the output of the clip circuit is shaped independently of emitter clip of TRs, the circuit is simplified and not affected with an error of the clip level or a temperature drift. Thus, the dynamic range is effectively ensured.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to an amplifier circuit that is used, for example, in an amplifier circuit in the input stage of an FM modulator and has a waveform clipping function that can prevent overmodulation. Regarding.

(Prior Art) An amplifier circuit having a waveform clipping function as shown in FIG. 4 is used in the input stage of a conventional FM modulator. This amplifier circuit is composed of a gain stage A, an output stage B, and a clip stage C.

Gain stage A consists of a pair of NPN transistors Ql+Q2, with each collector connected to a pair of NPN transistors Q3 . Q4 and resistors R, , R2 are connected to the VCC power supply line through a current mirror circuit, and each emitter is commonly connected to form an NPN) transistor Q.

and a differential amplifier circuit connected to a GND power supply line via a current RI1 made up of a resistor R3 and a resistor R3.

In addition, the output stage B is a current source consisting of an NPN transistor Q6 whose collector is connected to the VCC power supply line, and whose emitter is an NPN transistor Qy and a resistor R4. It consists of an emitter follower connected to the GND power supply line via 2.

The gain stage A and the output stage B constitute a negative feedback operational amplifier, and the voltage level of the input signal Sin supplied to the input terminal 11 and the output signal S out to the output terminal 13 in the differential amplifier circuit of the gain stage A are determined. This voltage difference is amplified and outputted by the emitter follower of the output stage B. The respective gains of gain stage A and output stage B are controlled by transistor Q, by a gain control signal supplied to control terminal 12.

By changing the base bias of Q7, each transistor Q5
．． Current source constituted by Q7 1. , J2 can be set appropriately by controlling the amount of current.

The above-mentioned negative feedback operational amplifier is composed of a gain stage A having a high voltage gain and an output stage B converting it to a low impedance to obtain an output, and a clip circuit that limits the output signal within a certain range. In C, a series circuit of resistors R, , R6, and R is inserted between the VCC power line and the GND'J source line, and the voltage Vllll generated at the connection point a of the resistors R6 and R6 is applied to the base of the PNP transistor Q8. The voltage VIL generated at the connection point between resistors R6 and R7 is supplied to the base of an NPN transistor Q9, and the emitter of transistor Q8 is connected to the output terminal of gain stage A (collector of Q2), and the collector is connected to GND. The collector of the transistor Q9 is connected to the VCC power line, and the emitter is connected to the output terminal 13.

This clipping circuit C was designed based on the following considerations: it is easier to clip the signal to a high impedance point, and it is easier to convert the output to a low impedance point for coupling with other circuits. It consists of Now, gain stage A
Assuming that the amplifier circuit by output stage B has 100% negative feedback, the clip level at this time is R6 + R7 Vl) l - R6 + R6 + R7''vCC + VBE8 Vn
Ee −(1)7 VIL-R5+R6+R7゜Vcc Vcc9 −
It is expressed as (2). However, "B2O is transistor Q8
BH3 is the base-emitter voltage of transistor Q. The potential Vin of the signal input to the input terminal 11 is V, L<Vin<
When it is in the range of v1□, both transistors Q, , Q in the clip circuit C are in the off state, the gain stage A and the output stage B of the amplifier circuit operate as voltage followers of the operational amplifier, and the output terminal 13 is in the OFF state. of the potential Vout-Vin,
A signal equal to the input signal is output. However, when the input signal becomes large in amplitude and reaches a state of Vin≧VIH, transistor Q8 turns on and transistor Q
The collector current of Q4 flows out to the emitter of Q8. The voltage follower operation of the operational amplifier increases the collector current of transistor Q4, but all of it is absorbed into the emitter of Q8, so the emitter potential of Q8 does not increase any further. In this way, the potential of the output terminal 13 is fixed at vlH as shown in the waveform diagram in the figure.

Next, when Vln≦VIL, the base potential of the transistor Q6 becomes lower than the base potential of the transistor Q, and Q is turned on.

Therefore, the collector current of transistor Q7 is all Q
It will flow to 9. The voltage follower operation of the operational amplifier increases the collector current of Q2, but since feedback by Q6 is no longer applied, the collector potential of Q2 decreases, leading to a state where Q2 is saturated and Q6 is cut off. In this way, as shown in the waveform diagram in the figure, the output terminal 13
The potential of is fixed at VIL.

Here, the clip levels V and lI are determined by the NPN transistor Q and the PNP transistor Q, as shown in equation (1).
Since each vBR of 8 cancels each other out, although some offset (approximately 0.2 V or less) remains, temperature drift and the like almost completely cancel each other out. However, since (2) and L are directly affected by the VBR of the NPN laser resistor Q9 as shown in equation (2), there is a drawback that the clip level error voltage and temperature drift are large.

Therefore, FIG. 5 shows an example of an amplifier circuit that is a modification of the amplifier circuit shown in FIG. 4 and configured to eliminate the drawbacks thereof. This circuit constitutes an emitter follower as an output stage B not only by the NPN transistor Q6 and current source 2 shown in FIG. 4, but also by the PNP transistor Q10 and current source I, Q6. Each emitter output of Q+o is amplified in class AB by an NPN transistor Q1r and a PNP transistor Q1°, and is led to the output terminal 13 and the base of the transistor Q2. A clip circuit C in such an amplifier circuit connects the emitter of the transistor Q8 to a transistor Q.

It is configured by connecting to the emitter of

That is, the clip level V2H of the clip circuit C
V2L is represented by . The potential Vi of the signal input to the input terminal 11
When n is in the range V 2L<V 1n<V 2H, transistor Q8. Q and are both in an off state, and a signal equal to the input signal with a potential Vout=Vln is output to the output terminal 13. In the state of Vin≧V2H, Q
t.

The base potential of Q8 becomes higher than that of Q8, and QIo
is turned off. In this way, feedback as an operational amplifier is no longer applied, and the emitter potential of Q +o becomes Q8
The potential of the output terminal 13 is fixed at a constant potential while the emitter of is activated, and the potential of the output terminal 13 is fixed at v2)l. Further, in a state where VIn≦v2L, the base potential of Q6 becomes lower than the base potential of Q9, and Q6 is turned off. In this way, feedback as an operational amplifier is no longer applied, the emitter potential of Q6 is fixed at a constant potential with the emitter of Q9 activated, and the potential of the output terminal 13 is fixed at v2L.

In this example, compared to the example in FIG. 4, V2)1 is the same as vll, but v2L is VBE
is NPN) transistor Q6 and PNP transistor Q
1o cancels each other out, making it easier to suppress errors and fluctuations in the clip level.

Furthermore, in the circuit shown in FIG. 4, the clip circuit C is modified as shown in FIG. 6. This circuit connects the emitters of a pair of NPN transistors QIIIQ+2 in common and connects them to the VCC power line via a current source I4, connects the base of Ql to the above point, and connects its collector to the GND power line. The base of Ql2 is connected to the output terminal 13, and the collector is a diode-connected NP.
N Current source I consisting of transistor Q13 and resistor R8
, and connect to the GND power line via
) Connect to the base of transistor Q +4. and,
The collector of this transistor Q+a is connected to the VCC power supply line, and its emitter is connected to the output terminal 13.

In the amplifier circuit with the above configuration, the clip potential V on the high level side
31I is the same as the example in FIG.
The voltage feedback type is determined by R6+R7. That is,
When the base potential of Q1□, which is given the same potential as the output terminal 13, becomes lower than the base potential of Qs+,
The collector current of Q1□ increases, the base potential of Q14 becomes higher than the base potential of Q6, and Q6 turns off. In this way, feedback as an operational amplifier is no longer applied, and the output terminal 13 is fixed at the base potential of Q++, that is, V3L, by the voltage feedback loop constituted by the differential circuit of Q11Q12 and 014. In this way, by using feedback control in the clipping circuit C, errors and fluctuations in the clipping level can be suppressed to the smallest level than in any of the previous examples.

The three conventional examples described above all perform waveform clipping at the emitter of the transistor, but when implementing this as an integrated circuit, the problem is that the emitter and base of the transistor EB breakdown (hereinafter referred to as EB breakdown). Recently, linear ICs have also become smaller and more highly integrated, and the EB breakdown voltage has also tended to decrease more and more. In normal analog signal processing LSIs, in most cases there are vertical (NPN) transistors and horizontal (lateral) PN transistors.
P) A transistor is used, but while the latter can have a relatively high breakdown voltage, the former has a low breakdown voltage, and in order to maintain high performance even when miniaturized, the concentration is lowered and the base width is narrowed. Therefore, it is becoming increasingly disadvantageous in terms of pressure resistance.

For example, in the case of an IC with a power supply voltage VCC of 9cm, the typical value is that the EB breakdown voltage of a PNP transistor is greater than IOV (the emitter and base of a lateral PNP transistor are vertical NPN), which corresponds to the base and collector of a transistor. Therefore, of course V. C
On the other hand, the EB breakdown voltage of NPN transistors has fallen from about 5V to about 3V recently, and is likely to fall further in the future.

In the example of FIG. 6, Vcc is -9V and the signal level is 4.
Assume that the waveform swings around 5V (=Vcc/2) with a maximum amplitude of 3.5V, the clip level is 3Il or 7V, and V and L are 2V. At this time, a maximum reverse bias of about 7V and about 8V is applied between the base and emitter of Q81Q14, respectively, and the lateral PNP transistor Q8
There is no problem, but vertical NPN) transistor Q1
4 is ・but・rekuda, wake up the back. In this way, due to the EB breakdown of the NPN transistor QI4, the output waveform is distorted and the transistor Q14 also deteriorates, making the circuit unusable in terms of its functionality and reliability. Exactly the same thing happened in the fourth
This also applies to the NPN transistor Q shown in the figure and the NPN transistor Q shown in FIG.

In other words, if we consider the equivalent circuits of the above three examples shown in FIG. 7, the lower clip circuit based on the emitter follower of the NPN transistor Q+s will
Since an excessive reverse bias is applied between the base and emitter of the N-transistor QI5, there is a problem that the circuit becomes unusable due to insufficient withstand voltage, especially in recent processes with advanced microfabrication. In addition, V in the figure.

L is the clip voltage.

To avoid such problems, the cheapest method is to connect a diode in cascade to the emitter of the clip transistor. An example in which this is applied to the circuit shown in FIG. 5 and 9 is shown in FIG. In FIG. 8, Dl and D2 are diodes that compensate for the insufficient breakdown voltage of transistor Q. However, if this is done alone, the variation in VBH of the diodes D, D2 and its temperature drift will cause the clip level potential to fluctuate, so as shown in the figure, the compensating diode D
, ,D4 must be inserted, and diodes D, ,D6 must also be inserted to prevent Q2 from being saturated due to these insertions.

On the other hand, when the above compensation method is applied to the circuit of FIG. 6, the result is as shown in FIG. 9. In FIG. 9, D7 and D8 are diodes that compensate for the insufficient withstand voltage of the transistor Q+a. Diode D0. This is for compensating for variations in VBH of D, O beams, and D8 and their temperature drifts.

The drawback of this method of cascading a diode to the emitter of the clip transistor is that there is a voltage loss corresponding to that voltage. For example, in the example shown in Figure 8, output terminal 1
The potential of 3 cannot be lowered below 3VBE. This is because the current source (2) will no longer operate. This is because in the example shown in Fig. 5, the lowest potential of the output terminal I3 was VBE.
This means that the dynamic range is narrowed by 2VBE.

If the center potential of the signal is fixed at Vcc/2, this influence extends to the dynamic range on the Vce side, resulting in a total loss of 4 VaE. In addition, errors in the clip level due to VBE mismatch will also increase.

In the case of the example shown in FIG. 9 as well, when the potential of the output terminal 13 is lowered to 3Vo or less, Q10 enters the saturation region, so the dynamic range becomes narrow as in the previous example. In this example, Q
You can add a Darlington input circuit to the differential input of II+012 or add a level shift circuit to prevent saturation of Q and □ and suppress the loss of dynamic range, but the clip level error still remains. Furthermore, the number of elements increases considerably due to the additional circuit.

(Problems to be Solved by the Invention) As described above, in the conventional amplifier circuit having a waveform clipping function, when an analog integrated circuit is configured, the NPN (NPN) transistor emitter
Increasingly, they are unable to withstand the drop in base-to-base breakdown voltage and become unusable. Conventional countermeasures against this problem have drawbacks such as increased clip level error and temperature drift, and the need for many elements to compensate for the error.

This invention was made to solve the above problems, and it realizes a clipping circuit that does not rely on transistor emitter clipping with a simple circuit configuration, eliminates clip level errors and its temperature drift, and increases the dynamic range. It is an object of the present invention to provide an amplifier circuit that can effectively secure power.

[Structure of the invention] (Means for solving the problem) In order to achieve the above object, an amplifier circuit according to the present invention has the following features:
A gain stage amplifies the input signal with a predetermined voltage gain and outputs it at high impedance, an output stage converts the output of this gain stage to low impedance and outputs it, and extracts the output voltage of this output stage and uses it as a reference voltage. The gain stage is configured to include a comparison means for comparison, and a current control means for forcibly adding or subtracting a current to the high impedance output terminal of the gain stage according to the comparison result of the comparison means.

(Function) In the amplifier circuit with the above configuration, the output voltage of the amplifier section, which is composed of a high gain gain stage and an output stage that converts to low impedance, is compared with a reference voltage that is a clip voltage, and if the output voltage is lower than the clip voltage level, The current is made to flow into the high impedance output terminal of the gain stage of the amplifier section.

This ensures that when the output voltage is lower than the clip voltage,
The potential at the output end of the gain stage of the amplifying section increases, and the amplifying section loses its own control ability, and voltage feedback is applied to perform clipping operation so as not to fall below the clipping voltage level. In this way, a waveform output whose voltage level is limited by the clip voltage is obtained at the output stage after impedance conversion.

(Example) The present invention will be described in detail below with reference to FIGS. 1 to 3.

FIG. 1 shows the basic configuration of an amplifier circuit according to the present invention. In FIG. 1, the same parts as in FIG. 7 are designated by the same reference numerals, and different parts will be explained here.

That is, this amplifier circuit serves as a clip circuit C for an operational amplifier consisting of a gain stage A using a high gain differential amplifier circuit and an output stage B that converts the high impedance output of the gain stage A into a low impedance output. The level comparator C1 compares the output level Vout of the operational amplifier with the clip voltage vcL, and
A current is forced to flow into the high impedance output terminal of the gain stage A through the current mirror circuit C2 in a state where t≦vcL.

According to the above configuration, when the output voltage v out becomes lower than the clip voltage VCL, current flows into the output terminal of the gain stage A of the operational amplifier and its potential increases, so that the operational amplifier loses its control ability. , voltage feedback is applied so that the voltage does not become lower than the clipping voltage level VCt.

That is, the operation is exactly the same as the emitter clip of a conventional NPN transistor.

In this case, the breakdown voltage of the transistor does not limit the circuit operation at all.

Therefore, the output voltage V out of the output stage B becomes a waveform output limited by the clip voltage vcL. In this case, the clip circuit is realized without using the emitter clip of the transistor, so even though the circuit is simple, there is no error in the clip level or the influence of its temperature drift, and the dynamic range can be effectively secured. be able to.

In the above embodiment, the case where the clip level is on the low level side has been described, but the same effect can be obtained with respect to the clip level on the high level side by using a similar circuit configuration. However, in this case, the emitter clip of a lateral PNP transistor can also be used, so there is often no need to use the circuit according to the present invention.

FIG. 2 shows a configuration in which the present invention is applied to the clip low level side of the amplifier circuit shown in FIG.

In FIG. 2, the same parts as in FIG. 5 are denoted by the same reference numerals.
NP) transistor Q, 6. It consists of a differential circuit with Q, □, current source I, and a current mirror circuit with NPN transistor QI8IQ+9 and resistor R9RIO,
Current mirror circuit C2 is transistor Q 201 0
21 and resistors R11 and R1□.

That is, in the clip circuit C having the above configuration, the output potential Vout applied to the base of the transistor Q16 is the clip potential Vout applied to the base of the transistor Q1□.
When it becomes lower than 4L, collector current flows through Q16. This collector current is turned back in order through the current mirror circuit formed by Q181Q+9 and the current mirror circuit formed by Q201Qz+, and then passed through the transistor Q2 of gain stage A.
is forced into the collector. Therefore, gain stage A
The operational amplifier having a voltage follower configuration using the output stage B becomes unable to perform its own negative feedback control, and the collector potential of the output transistor Q2 attempts to rise. Further, this potential increase is transmitted to the output terminal I3 through the emitter follower formed by the transistor Q6. Then, a negative feedback circuit is formed by the differential circuit of the transistor Q16°Q +7 of the clip circuit C and the output stage B, so the potential of the output terminal 13 is fixed at the clip low level (base potential of Q17) V4L. will be done.

Now, assuming I, -21, when the potential of the output terminal 13 is sufficiently lower than the clip low level v4L, the collector current 1 of the transistor Q2 of the gain stage A is all supplied from the transistor Q21 of the clip circuit C. receive. If the mirror ratios of the current mirror circuit are all 1, then Q
10. When the collector currents of Q1□ are both equal to 11, the collector current of Q2+ becomes 11, and the negative feedback loop causes the potential of the output terminal 13 to reach the clip low level V4L.
When it becomes equal to , it becomes stable, and the output terminal 13 is fixed at exactly the same potential as the clip low level.

Next, a configuration in which the present invention is applied to the clip low level side of the amplifier circuit shown in FIG. 4 will be described with reference to FIG. 3.

However, in FIG. 3, the same parts as in FIG. 4 are given the same reference numerals, and their explanations will be omitted.

This amplifier circuit has a differential circuit composed of a transistor Q221023 and a current source I6, and a current source composed of a transistor Q24 and a resistor R13 is added. That is, here, to simplify the circuit, the active load Q3. Use Q4.

For this purpose, a current source consisting of a transistor Q24 and a resistor R13 is separately provided, and the transistor Q 221 Q 23
The differential pair lever current source is controlled.

Now, if the potential of the output terminal 13 applied to the base of the transistor Q2□ becomes lower than the clip low level V5L applied to the base of the transistor Q23,
The collector current that was flowing in Q23 stops flowing, and Q24
becomes active and transistor Q of gain stage A
A collector current is forced to flow through 3. This collector current is Q3. It is turned back by the current mirror circuit by Q4, and Q
Flows into the collector of 2. For this reason, the operational amplifier with the voltage follower configuration consisting of the gain stage A and the output stage B is unable to perform negative feedback control by itself, and the transistor Q2
The collector potential of will try to rise. This potential increase is transmitted to the output terminal 13 through the emitter follower formed by the transistor Q6. Then, the differential circuit consisting of transistors Q2□ and Q23 of clip circuit C and the output stage B
A negative feedback circuit is constituted by the output terminal 13.
The potential of is fixed at clip low level V5L.

As is clear from the above embodiments, the amplifier circuit having a clipping function according to the present invention does not use the emitter clip of the transistor unlike the conventional circuit, so when limiting the output lower limit level in particular, the emitter φ of the NPN transistor Problems such as breakdown caused by excessive reverse bias applied between the bases do not occur. Furthermore, in addition to this, since the clip circuit itself performs negative feedback operation, there is almost no clip level error or temperature drift, and sharp lip characteristics can be obtained. Therefore, it can be said that it has an extremely ideal circuit configuration when integrated into an IC.

[Effects of the Invention] As described above, according to the present invention, a clip circuit that does not rely on transistor emitter clips can be realized with a simple circuit configuration, and the dynamic range can be made effective without clip level errors or the effects of temperature drift. It is possible to provide an amplifier circuit that can secure the

[Brief explanation of the drawing]

FIG. 1 is a basic circuit diagram showing an embodiment of an amplifier circuit according to the present invention, FIGS. 2 and 3 are circuit diagrams showing specific embodiments of the invention, and FIG. 4 is a conventional clip circuit diagram. A circuit diagram showing the basic circuit configuration of a functional amplifier circuit,
5 and 6 are circuit diagrams each showing the configuration of a conventional improvement example that improves the problem of the circuit of FIG. 4, and FIG. 7 explains the problem of the circuit of FIGS. 4 to 6. 8 and 9 are circuit diagrams showing the configuration of a conventional improved example for improving the problem explained in FIG. 7, respectively. 11...Input terminal, 12...Control terminal, 13...
Output terminal, A...gain stage, B...output stage, C...
・Clip circuit, C1...Level comparator, C2...
current mirror circuit.

Claims (1)

[Claims] A gain stage amplifies the input signal with a predetermined voltage gain and outputs it at high impedance, an output stage converts the output of this gain stage to low impedance and outputs it, and extracts the output voltage of this output stage and uses it as a reference voltage. An amplifier circuit comprising a comparison means for comparison, and a current control means for forcibly adding or subtracting a current to a high impedance output terminal of the gain stage according to a comparison result of the comparison means.