JPS6325765Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an amplification circuit that is suitable for IC implementation and that reduces high frequency noise.

First, prior to explaining the present invention, an example of an amplification circuit will be explained with reference to FIG. That is, the first
In the figure, the emitters of transistors Q ₁ and Q ₂ are connected to the collector of transistor Q ₃ forming a constant current circuit to form a differential amplification circuit DA, and the base of transistor Q ₁ is connected to resistor R ₁ , Connected to R ₂ and constant current source 1. A current mirror circuit KM ₁ consisting of a diode D ₁ , a transistor Q ₄ , etc. is connected to the collector of one transistor Q ₁ with the power supply terminal T as a reference potential point, and the other transistor
A current mirror circuit KM ₂ consisting of a diode D ₂ and a phase inversion transistor Q ₅ on the collector of Q ₂ .
is connected to the collector of the transistor _Q5 , and a current mirror circuit _KM3 consisting of a diode _D3 , a transistor _Q6, etc. is connected to the collector of the transistor Q5. The collector of the transistor Q ₄ is connected to the base and collector of the transistor Q ₇ , and the transistors Q ₇ and Q ₈ constitute a current mirror circuit KM ₄ with the terminal T as a reference potential point, and the transistor Q ₈
The collector of is connected to the base of one output transistor _Q9 . Furthermore, the collector of transistor Q ₄ is connected to the base and collector of transistor Q ₁₀ , and a current mirror circuit is formed by transistors Q ₁₁ and Q ₁₂ with terminal T as a reference potential point.
KM ₅ is configured, and the collector of transistor Q ₁₁ is connected to the base of the other output transistor Q ₁₂ .

The transistors _Q9 and _Q12 are connected in a complementary push-pull configuration, and the speaker Sp is connected to the collector of the transistor Q9. A negative feedback resistor _R5 is connected between the collector and the base of the transistor _Q2 . A resistor _R6 and a capacitor _C2 are connected between the connection point between the resistor _R5 and the base of the transistor _Q2 and the ground.
A series circuit of these is connected.

Furthermore, a transistor _Q13 with a common emitter is provided, the base and emitter of a transistor _Q14 are connected to the collector and base of the transistor _Q13 , and the collector of the transistor Q14 is connected to the collector and the base of the transistor _Q13 .
Connected to the base and collector of Q ₁₅ ,
Transistors Q ₃ , Q ₁₅ , with the ground as the reference potential point
Q ₁₆ constitutes a current mirror circuit KM ₆ . Diodes D ₄ and D ₅ are connected to the collector of transistor Q ₁₆ , and these diodes D _{4 and} D 5 are connected to the collector of transistor Q 16 .
A constant voltage circuit is configured with transistors Q ₁₇ and Q ₁₈ and diode D ₆ using D ₅ as a reference voltage source.
The output terminal thereof is used as the reference potential point of the current mirror circuit KM ₅ of the transistors Q ₁₀ and Q ₁₁ .

According to such a configuration, a change in the collector current of transistor _Q13 is negatively fed back to the base of transistor _Q13 by transistor _Q14 , so that
The collector current of transistor Q ₁₃ becomes constant, and as a result, the collector current of transistor Q ₁₄ also becomes constant. And transistor Q ₁₅ ,
Since Q ₁₆ constitutes a current mirror circuit KM ₆ , if the collector current of transistor Q ₁₄ is constant, the collector current of transistor Q ₁₆ is also constant, and diodes D ₄ and D ₅ have a constant reference voltage. is obtained.

Then, in transistor Q ₁₇ , the voltage between both terminals of resistor R ₇ is compared with the reference voltage of diodes D ₄ and D ₅ , and the comparison output is supplied to transistor Q ₁₈ to determine the emitter-collector impedance of transistor Q ₁₈ . is controlled, the emitters of transistors Q ₁₀ and Q ₁₁ are kept at a constant potential with respect to the potential of terminal T +Vcc. Furthermore, the transistor
Since the voltage between the base and emitter of Q ₁₇ and the voltage between both terminals of diode D ₆ are equal, the transistor
The emitter potential of Q ₁₀ and Q ₁₁ is equal to the base potential of transistor Q ₁₇ .

Furthermore, since the transistors Q ₃ and Q ₁₅ are part of the current mirror circuit KM ₆ and the collector current of the transistor Q ₁₆ is constant, the collector current I ₃ of the transistor Q ₃ is also constant, and the transistor Q ₃ is a sink type. Acts as a constant current source.

When an input signal Si as shown in FIG. 2A is supplied from the constant current source 1, the current of this signal Si is amplified by the transistors Q ₁ and Q ₂ , and the output current is supplied to the transistor Q ₅ . , transistor
A collector current _I5 as shown in FIG. 2B flows through _Q5 . However, of this current _I5 , Id is the DC component, and Ia
is the alternating current component due to the input signal Si.

Then, the DC component Id is equal to 1/2 of the collector current I3 of transistor _Q3 , and the DC component Id is equal to 1/2 of the collector current _I3 of transistor Q3 _.
operates in opposite phase to transistor Q ₅ , and the DC component of I ₆ is also Id, and this DC component Id and signal −Ia
flows through transistor _Q6 as current _I6 . Therefore, the remaining AC component 2Ia flows through the transistors Q ₇ and Q ₁₀ , and the positive half cycle of the AC component 2Ia becomes the collector current I ₁₀ of the transistor Q ₁₀ as shown in FIG. 2C, and the negative half The cycle begins with the collector current I ₇ of transistor Q ₇ as shown in Figure 2D.
becomes. That is, when I ₄ = Id, current I ₄ becomes current I ₆ and flows through transistor Q ₆ , but when I ₄ > Id, the increase (positive half cycle of AC component Ia) becomes current I ₁₀ . flows into transistor Q ₁₀ , and when I ₅ < Id, its decrease (negative half cycle of AC component Ia)
is made part of the current _I6 by the current _I7 from the transistor _Q7 .

Furthermore, since transistors Q ₁₀ and Q ₁₁ constitute a current mirror circuit KM ₅ ,
As shown in FIG. _2C , a current I ₁₁ equal to the current I _{10 flows through Q 11, and since this collector current I 11 is} also the base current of transistor _{Q 12 ,} the positive input signal Si A collector current corresponding to a half cycle of flows. Similarly, a current I ₇ is applied to the transistor Q ₈ as shown in FIG. 2D.
A collector current I ₈ equal to , therefore, flows through the transistor Q ₉ during a negative half cycle period of the input signal Si, and a collector current corresponding to the negative half cycle flows through the transistor Q 9 . Therefore, the transistors Q ₁₂ and Q ₉ perform class B push-pull amplification, and the amplified output of the input signal Si is supplied to the speaker Sp.

In addition, in the case of the above-mentioned class B push-pull amplifier, the impedance seen from the collectors of transistors Q ₄ and Q ₆ to the base side of transistors Q ₈ and Q ₁₁ is high impedance, and the impedance of transistors Q ₄ and Q ₆ is high.
It is driven by constant current.

In addition, class B push-pull amplifiers are usually composed of ICs, and high-frequency transistors are used as the transistors, and the low-frequency input signal Si (second
Figure A) has been enlarged.

By the way, this high-frequency transistor has good transient high-frequency characteristics, so when applied to a low-frequency amplification circuit as described above, it has wide-band characteristics that extend considerably to a high frequency range of several hundred kHz, that is, resistor R ₅ ，
When configuring a negative feedback circuit using _R6 etc., 1M
Hz, if not configured, it becomes an amplification circuit with an upper limit frequency of about 200KHz, and high frequency noise is generated from the speaker Sp lead wire, etc., and when a low frequency amplification circuit is used in a radio receiver. Since high frequency noise is fed back to the bar antenna etc., it becomes the main cause of noise in radio receivers,
There is a drawback that N deteriorates.

Therefore, in the past, a filter was inserted in the output stage to prevent high-frequency noise from being superimposed on the lead wire of the speaker Sp, but installing this filter was complicated, and radios using a live amplification circuit Receivers etc. become expensive.

In view of this, the present invention aims to provide an amplification circuit that reduces the above-mentioned high frequency noise by using a small capacitance capacitor that can be easily integrated into an IC.

Hereinafter, one embodiment of the amplifier circuit according to the present invention will be explained in detail with reference to the circuit diagram in FIG. 3 and the characteristic curve diagram in FIG. 4, but the parts corresponding to those in FIG. The same reference numerals are used to omit redundant explanation.

That is, the present invention uses a transistor as a transistor circuit with high input impedance in the final output stage.
In parallel to the transistors Q ₄ and Q ₆ as the first transistors for driving Q ₈ and Q ₁₁ , second transistors Q ₁₉ and Q ₂₀ similar to these first transistors are connected separately, and the second Transistors Q ₁₉ and Q ₂₀ as output terminals of transistors Q ₁₉ and Q ₂₀ are connected from the midpoint of their collectors through capacitor C to transistor Q ₂ as an opposite phase point of the amplifier stage in front of the amplifier.
By feeding back a part of the output signal to the base of the amplifier circuit, high frequency components, which are the main cause of noise in the amplifier circuit and cause S/N deterioration, are removed.

That is, according to the amplification circuit described above, a pair of resistors of tens of kΩ or more are connected with the point P as the midpoint of the connection.
The DC potential V _A of the output stage is the ratio of R ₁ and R ₂ , that is, point A
At the same time as determining the potential of , the input impedance of the amplifier circuit is determined. Furthermore, when a negative feedback loop is formed by resistors R ₅ , R ₆ and capacitor C ₂ , the collector currents of transistors Q ₄ , Q ₆ , Q ₁₉ , and Q ₂₀ are almost the same, and transistors Q ₁₉ , Q ₂₀ The collector potential of is determined by the ratio of R ₃ and R ₄ , and the load of transistors Q ₁₉ and Q ₂₀ is the parallel resistance of R ₃ and R ₄ . Further, the currents of the transistors Q ₁₉ and Q ₂₀ are the same as the currents of the driving transistors Q ₄ and Q ₆ , and a predetermined gain can be obtained in the transistors Q ₄ , Q ₆ , Q ₁₉ , and Q ₂₀ .

Then, from the midpoint of connection between resistors R ₃ and R ₄ and the midpoint of connection between the collectors of transistors Q ₁₉ and Q ₂₀ , a capacitor C of a small capacity, for example, about 6PF is connected to the previous amplification stage, that is, the differential amplification circuit. By applying negative feedback to the base of transistor _Q2 , which is the opposite phase point of the DA, high-frequency components of the entire amplifier circuit are cut. The cut-off condition for this high frequency range is the transistor Q ₁₉ ,
It is determined by the gain of _Q20 and the capacity of the negative feedback capacitor C. Also, this negative feedback is caused by the drive transistor.
It has almost no effect on the operation of Q ₄ and Q ₆ ,
The output impedance of transistors Q ₄ and Q ₆ remains high, and transistors Q ₄ and Q ₆ drive the next-stage class B push-pull amplifier stage with high input impedance consisting of transistors Q ₈ and Q ₁₁ with a constant current. are doing.

As a result, the amplifier circuit from transistors Q ₁ and Q ₂ to transistors Q ₄ and Q ₆ has a characteristic in which the upper limit frequency is approximately 50 kHz, and the high frequency components are equivalently cut, as shown in curve 2 in Figure 4. Therefore, the high frequency noise mentioned at the beginning is considerably cut out.
Incidentally, when the capacitor C is connected but a negative feedback circuit is not formed by resistors R ₅ , R ₆ etc., the upper limit frequency is about 10 kHz as shown in curve 3. On the other hand, as described in the conventional example, although a negative feedback circuit is configured with resistors _R5 , _R6, etc., if capacitor C is not connected, the upper limit frequency is 1MHz as shown in curve 4.
In addition, if a negative feedback circuit with resistors _R5 , _R6 , etc. is not configured and capacitor C is not connected, the upper limit frequency is about 200kHz, as shown in curve 5, and if capacitor C is not connected, It can also be seen from the characteristic curve diagram in FIG. 4 that high frequency noise is generated.

Thus, according to the amplifier circuit according to the present invention, a second transistor similar to the first transistor is connected in parallel to the first transistor for driving a transistor circuit having a high input impedance in the final output stage, High frequency components are removed by feeding back the signal from the output terminal of the second transistor, which provides an output current proportional to that of the first transistor for driving, through a capacitor to the opposite phase point of the preceding amplification stage. using high frequency transistors
Even if it is an IC, the high frequency characteristics will not be excessively good and high frequency noise will be generated from the speaker lead wire.When applied to a radio receiver, the high frequency noise will not be returned to the bar antenna etc. and noise will be generated. Deterioration of S/N is reliably avoided.

It is also possible to consider applying negative feedback from the final output stage to the previous amplification stage via capacitor C.
In this case, the capacitance of capacitor C becomes large and it cannot be configured within the IC and is attached externally, which is not preferable when implementing the amplifier circuit into an IC, and although it is used as the final output stage, In effect, negative feedback must be applied from the drive stage.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of an example of a conventional amplifier circuit;
1 is a waveform diagram for explaining the operation of FIG. 1, FIG. 3 is a circuit diagram of an embodiment of the amplifier circuit according to the present invention, and FIG.
The figure is a frequency characteristic curve diagram of the amplifier circuit of FIG. 3. Q ₄ and Q ₆ are first transistors, Q ₁₉ and Q ₂₀ are second transistors, C is a capacitor, and P is an opposite phase point.

Claims (1)

[Claims for Utility Model Registration] A differential amplifier as an input stage, first and second transistors whose bases are connected to the output terminal of the differential amplifier, and driven by the first transistor, an output stage with a high input impedance and a class B push-pull connection from which negative feedback is applied to the differential amplifier from the output end of the second transistor;
an amplifier circuit in which a capacitor is connected between the input terminal of the differential amplifier and the input terminal of the differential amplifier to provide negative feedback to the output of the transistor;