JPH0457505A - Composite differential amplifier circuit - Google Patents

Abstract

PURPOSE:To facilitate the constitution of an application system for an external control voltage and to reduce number of components by feeding back a DC output of a differential amplifier through an operational amplifier to the differential amplifier. CONSTITUTION:A common connecting point is connected to a 1st input terminal of an operational amplifier 6, an external control voltage is applied to a 2nd input terminal of the operational amplifier 6 and an output of the operational amplifier 6 is fed back to a differential amplifier 3 as a power supply voltage. Thus, the feedback control is attained, in which the voltage at the common connecting point of 1st - 3rd resistors 21-23 is obtained by dividing the potential difference between a DC output potential of the differential amplifier 3 and a constant potential and is lower than the DC output potential, a low voltage as an external control voltage is enough for the purpose. Thus, the constitution of the application system of the external control voltage is facilitated and number of components is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a composite differential amplifier circuit that controls the DC output of a differential amplifier to be constant by feeding back the DC output of the differential amplifier via an operational amplifier.

[Prior Art] Conventionally, a circuit shown in FIG. 4 has been known as a composite differential amplifier circuit that applies feedback so that the DC output of a differential amplifier is constant.

That is, the input signal generated by the bias voltage source 1 and the input signal source 2 is input to the differential amplifier 3. The differential amplifier 3 includes an NPN transistor 11.12 constituting a differential transistor pair whose emitters are commonly connected, a constant current source 13 connected between the common emitter end and a ground terminal, and a transistor 11.12. and resistors 14 and 15 connected between the collector and the power supply terminal, respectively, and differentially amplify the input signal.

The output of this differential amplifier 3 is amplified by an emitter follower circuit 4. The emitter follower circuit 4 includes NPN transistors 16 each having a base inputted with the differential output of the differential amplifier 3 and a collector connected to a power supply terminal.
．． 17, and constant current sources 18 and 19 connected between the emitters of these transistors 16 and 17 and the ground terminal, respectively.

The positive phase output of the emitter follower circuit 4 is input to the inverting input terminal of the operational amplifier 6 via a low pass filter consisting of a resistor 5 and a capacitor 9. Therefore, only the DC component of the positive phase output from the emitter follower circuit 4 is applied to the inverting input terminal of the operational amplifier 60.

An external control power source 7 is connected to a non-inverting input terminal of the operational amplifier 6. The output terminal of the operational amplifier 6 is fed back to the power supply terminals of the differential amplifier 3 and the emitter follower circuit 4.

Next, the operation of the conventional composite differential amplifier circuit configured as described above will be explained.

The input signal output from the input signal source 2 is sent to the differential amplifier 3.
and is differentially amplified by the emitter follower circuit 4. When this differentially amplified output passes through a low-pass filter consisting of a resistor 5 and a capacitor 9, only its DC component is extracted.

Here, the DC component of the differentially amplified output, that is, the DC output voltage, is Vo1.The external control voltage from the external control power supply 7 applied to the non-inverting input terminal of the operational amplifier 6 is V. , where the open gain of the operational amplifier 6 is Ao, the output voltage of the operational amplifier 6 is V. , can be expressed as the following equation.

Vop"Ao (Vc Vo) "' (1
)Vc-Vo=Vop/Ao...'
(2) Since the open gain of the operational amplifier 6 is extremely large, assuming that it is infinite, from the above equation (2),
The following equations (3) and (4) can be obtained.

Vc -VO'-FO・ (3) Vo →Vc ... (4)
As is clear from this equation (4), the DC output voltage V of the emitter follower circuit 4. ■ External control voltage. Since it matches, the external control voltage V. By keeping constant the DC output voltage V from the emitter follower circuit 4. can be kept constant.

[Problem to be solved by the invention] However, the conventional composite differential amplifier circuit configured in this way requires a low-pass filter to extract the DC component from the differential output signal, and it is difficult to extract the stable DC component. Therefore, the time constant of this low-pass filter must be set large. In other words, specifically, it is necessary to increase the capacitance of the capacitor 9, so when trying to convert a conventional composite differential amplifier circuit into an IC, it is necessary to increase the capacitance of the capacitor 9.
However, there is a problem in that it must be externally connected, and an external terminal 8 is also required for this purpose.

In addition, in this composite differential amplifier circuit, the external control voltage V. By keeping constant, the DC output voltage ■o is controlled to be constant. Therefore, the external control voltage V. A regulator voltage is often used to obtain a stable voltage. In this case, the regulator voltage and the DC output voltage of the differential amplifier have the same voltage value, so if the regulator voltage is low, an amplifier is required to amplify the DC current. Therefore, there is a problem that the number of elements increases.

The present invention has been made in view of such problems, and includes:
When implemented as an IC, no external parts are required, and the number of elements in the external control voltage supply system can be significantly reduced.
The purpose of the present invention is to provide a composite differential amplifier circuit suitable for IC implementation.

[Means for Solving the Problems] A composite differential amplifier circuit according to the present invention includes a differential amplifier that differentially amplifies an input signal, and a differential output terminal of the differential amplifier that is connected in series with the same amplifier. a first and second resistor having a resistance value, a third resistor connected between a common connection point of these first and second resistors and a constant potential point, and a first input terminal connected to the common and an operational amplifier connected to the connection point, having a second input terminal to which an external control voltage is applied, and whose output is supplied as the power supply voltage of the differential amplifier.

[Function] According to the present invention, signals having mutually opposite phases with respect to the DC output voltage are output to the differential output terminal of the differential amplifier.
If the third resistor is not connected, the first and second resistors of the same resistance value are connected in series to this differential output terminal.
At the common connection point of the resistors, an output with the AC components of the differential output signal canceled out, that is, a DC output voltage appears.

On the other hand, since a third resistor is connected between the common connection point of the first and second resistors and a constant potential point (for example, a ground terminal), the potential of the common connection point is actually does not match the DC output voltage of the differential amplifier, and is a voltage obtained by dividing the potential difference between the DC output voltage and the constant potential point by the first to third resistors.

Therefore, the common connection point is connected to the first input terminal of the operational amplifier, an external control voltage is applied to the second input terminal of the operational amplifier, and the output of the operational amplifier is fed back as the power supply voltage of the differential amplifier. As a result, feedback control is performed such that the common connection point of the first to third resistors matches the external control voltage.

According to the present invention, since the DC component of the differential output is taken out by the first and second resistors, no external capacitor is required. Therefore, the circuit can be easily integrated into an IC.

Further, according to the present invention, since the potential at the common connection point of the first to third resistors is obtained by dividing the potential difference between the DC output voltage of the differential amplifier and the constant potential point, the DC output potential The potential is lower than that. Therefore, since a low voltage can be supplied as the external control voltage, the configuration of the external control voltage supply system is facilitated, and the number of elements can be reduced.

[Embodiments] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a circuit diagram showing the configuration of a composite differential amplifier circuit according to a first embodiment of the present invention.

In this FIG. 1, the same parts as those of the conventional circuit shown in FIG. 4 are given the same reference numerals, and the explanation of the overlapping parts will be omitted.

The circuit shown in FIG. 1 differs from the conventional circuit shown in FIG. 4 in the configuration between the emitter follower circuit 4 and the operational amplifier 6.

That is, in FIG. 1, resistors 21 and 22 having the same resistance value are connected in series between the emitters of NPN transistors 16 and 17, which are the differential output terminals of the emitter follower circuit 4. A resistor 23 is connected between the common connection point of these resistors 21 and 22 and the ground terminal. The common connection point is connected to the inverting input terminal of the operational amplifier 6.

Next, the operation of the composite differential amplifier circuit according to this embodiment configured as described above will be explained.

The input signal output from the input signal source 2 is sent to the differential amplifier 3.
and is differentially amplified by the emitter follower circuit 4.

Here, the positive phase output and negative phase output of the signal differentially amplified by the differential amplifier 3 and the emitter follower circuit 4 are respectively V O
Pt V OMN The DC component of these outputs, that is, the DC output voltage, is V. Similarly, assuming that the AC output voltage is VO, the output VOP+VOM of the emitter follower circuit 4 can be expressed as shown in the following equations.

V op: +-vo + Vo
-(5) VOM= VO+VO-(6) Also, let the resistance value of resistor 21.22 be R, and the resistance value of resistor 23 be R2, and set the voltage at the common connection point of resistors 21 and 22.23 to Vat resistor 21. Assuming that the current that flows is i□ and the current that flows through the resistor 22 is 12, the following equation is obtained.

Va”R2(!,+i2)−(9)
Substituting equations (7) and (8) into equation (9), we get the following (10
) formula is found.

(10) By substituting equations (5) and (6) into equation (10), the following equations (11) and (12) can be obtained.

1+2R2/R1 R1+2-R2 Thus, the potential VB at the common connection point of the resistors 21 to 23 is the DC output voltage V of the emitter follower circuit 4. The resistance 2
It is a value obtained by dividing the voltage by resistance values R1 and R2 of 1 to 23.

On the other hand, an external control voltage ■ is applied to the non-inverting input terminal of the operational amplifier 6. is given, and the output of the operational amplifier 6 is fed back as the power supply voltage of the differential amplifier 3 and emitter follower circuit 4, so the control system acts so that the potential VB at the common connection point becomes equal to the external control voltage Vc. . Therefore, the DC output voltage V. becomes a constant voltage expressed by the following equation (13).

As described above, according to the composite differential amplifier circuit according to this embodiment, a voltage dividing circuit including resistors 21 to 23 is used to extract the DC component of the differential amplification output without using a low-pass filter. Therefore, no external parts are required, which is extremely advantageous for IC implementation.

Furthermore, in equation (13), if R1 and R2 are set so that R1=2R2, for example, the DC output voltage V. can be rewritten as the following equation (14).

Vo "2Vc - (14) That is, according to this circuit, the external control voltage V to be applied to the non-inverting input terminal of the operational amplifier 6. Therefore, it is sufficient to apply only half the voltage of the DC output voltage V. Therefore, For example, if 6V is required as the DC output voltage Vo of a differential amplifier, a regulator that outputs a voltage of 6V was conventionally required, but according to the circuit of this embodiment, a voltage of 3V is output. This has the advantage that the design of the regulator becomes very simple.

Also, the DC output voltage Vo of the differential amplifier is set to the external control voltage V
. For example, when varying in the range of 6 to IOV,
Conventionally, if there was only a 6V regulator, the regulator voltage had to be further amplified using an operational amplifier, etc.
Although there was a problem of an increase in the number of elements, according to the circuit of this embodiment, the external control voltage ■. What is necessary is to set the variable range of 3 to 5V. Therefore, it is possible to deal with this problem with only a 6V regulator, and the design margin can be greatly increased.

FIG. 2 is a circuit diagram showing the configuration of the operational amplifier 6 in the embodiment of FIG. 1 in more detail. The operational amplifier 6 is configured as follows.

In FIG. 2, an NPN transistor 31 whose base is connected to the common connection point of the resistors 21 to 23 and an NPN transistor 32 whose base is connected to the external control power source 7 are as follows.
Their emitters are commonly connected to form a differential transistor pair. A constant current source 33 is connected between the common emitters of the transistors 31 and 32 and the ground terminal.

The collector of the transistor 31 is connected to an external power supply 39 via a PNP transistor 34 whose base and collector are connected, and the collector of the transistor 32 is directly connected to the external power supply 39.

Further, the emitter of a PNP transistor 35 is connected to the external power supply 39. This PNP) transistor 3
5 has its base connected to the base of a PNP transistor 34, and forms a current mirror circuit together with the transistor 34. Further, a capacitor 36 is connected between the base and collector of the PNP transistor 35 in order to stabilize the circuit.

PNP, ) The collector of the transistor 35 is a constant current source 37
and the NPN of the emitter follower.
) is connected to the base of transistor 38. and,
The emitter of this transistor 38 is connected to the power supply line of the differential amplifier 3 and emitter follower circuit 4.

According to the above configuration, the operational amplifier 6 has the resistors 21 to 23
Feedback is applied to the power supply lines of the differential amplifier 3 and the emitter follower circuit so that the potential at the common connection point is equal to the voltage of the external control power supply 7, so the DC output voltage of the emitter follower circuit 4 is always a constant value. controlled by.

FIG. 3 is a circuit diagram showing a composite differential amplifier circuit according to a second embodiment of the present invention.

This circuit differs from the circuit shown in FIG. 2 in the configuration of the operational amplifier 6.

That is, in FIG. 3, an NPN transistor 41 whose base is connected to the common connection point of the resistors 21 to 23, and an NPN transistor 42 whose base is connected to the external control power supply 7.
and whose emitters are commonly connected to form a differential transistor pair.

A constant current source 43 is connected between the common emitters of the transistors 41 and 42 and the ground terminal.

The collector of the transistor 41 is connected to the external power supply 39 via a PNP transistor 44 whose base and collector are connected, and the collector of the transistor 42 is connected to the external power supply 39 through a PNP transistor 44 whose base and collector are connected.
It is connected to external power supply 39 via transistor 45 . The bases of the PNP transistors 44 and 45 are connected to each other to form a current mirror circuit. Further, a capacitor 46 is connected between the collectors of the PNP transistors 44 and 45 to stabilize the circuit.

The collector of the PNP transistor 45 is a PNP transistor 47 whose collector is connected to the ground by Darlington.
It is maintained by 48 bases. The emitter of transistor 48 is connected to external power supply 39 via constant current source 49 . The emitter of this transistor 48 is connected to the power supply line of the differential amplifier 3 and emitter follower circuit 4.

In this circuit as well, by the same effect as in the circuit of FIG. 2, the differential amplifier 3 and the emitter Since feedback is applied to the power supply line of the follower circuit, the DC output voltage of the emitter follower circuit 4 is always controlled to a constant value.

[Effects of the Invention] As described above, according to the present invention, since the DC component of the differential output is taken out by the first to third resistors, no external capacitor is required. For this reason,
The circuit can be easily integrated into an IC.

Further, according to the present invention, since the potential at the common connection point of the first to third resistors is obtained by dividing the potential difference between the DC output voltage of the differential amplifier and the constant potential point, the DC output potential The potential is lower than that. Therefore, since a low voltage can be supplied as the external control voltage, the configuration of the external control voltage supply system becomes easy and the number of elements can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a composite differential amplifier circuit according to a first embodiment of the present invention, FIG. 2 is a circuit diagram showing a more detailed configuration of the same circuit, and FIG. 3 is a circuit diagram of a second embodiment of the present invention. A circuit diagram of a composite differential amplifier circuit according to an example, FIG. 4 is a circuit diagram of a conventional composite differential amplifier circuit. 1; Bias voltage source, 2; Input signal source, 3; Differential amplifier, 4; Emitter follower circuit, 5.14゜15.21~
23; Resistor, 6; Operational amplifier, 7; External control power supply, 8;
External terminal, 9.36.46; Capacitor, 11, 12,
16, 17, 31, 32.38, 41.42; NPN)
Ransistor, 13.18, 19, 33, 37.43.4
9; constant current source, 34, 35, 44, 45, 47.48;
PNP) transistor, 39; external power supply

Claims (1)

[Claims] (1) A differential amplifier that differentially amplifies an input signal, first and second resistors connected in series between the differential output terminals of the differential amplifier and having the same resistance value; a third resistor connected between the common connection point of the second resistor and the constant potential point, and a third resistor whose first input terminal is connected to the common connection point and
an operational amplifier to which an external control voltage is applied and whose output is supplied as a power supply voltage of the differential amplifier.