JPS6239564B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION An object of the present invention is to provide a differential amplifier in which the DC level of the output signal does not change even if the gain of the output signal is changed by changing the value of the load resistance.

A load resistor is connected to the collectors of the transistors constituting the differential amplifier, and by changing the value of this load resistor, the gain of the output signal can be changed. However, when the value of the load resistance changes, the magnitude of the DC current flowing through the transistor also changes, and therefore the DC level of the output signal also changes. Depending on the circuit used, it may be desirable to change only the signal gain without changing the DC level, such as when a plurality of circuits are DC-coupled and connected continuously.

The present invention aims to provide a differential amplifier that can meet such demands, and one embodiment of the present invention will be described below with reference to the drawings.

As shown in FIG. 1, the emitters of a first transistor 1 and a second transistor 2 are connected together.
Transistor 3 is connected between this connection point and the reference potential point.
A series circuit of the transistor 3 and the resistor 4 is connected, and the voltage of the DC power supply terminal 5 is connected to the base of the transistor 3 through the resistor 6,
7. A constant current circuit is constructed by applying the DC voltage obtained by dividing the voltage with the diode 8. A DC bias is applied to the bases of the transistors 1 and 2 by resistors 9, 10, 11, and 12.

Although the signal input terminal 13 is provided at the base of transistor 1 in FIG. 1, it may be provided at the base of transistor 2, or may be provided at both to apply input signals of opposite polarity to each other.

A load resistor 14 is connected between the collector of the transistor 1 and the DC power supply terminal 5, and the collector of the transistor 2 is directly connected to the DC power supply terminal 5. The collector of transistor 2 may also be connected to a DC power supply via a resistor.

A feature of this embodiment is that a transistor 15 is connected between the collector of the transistor 1 and the DC power supply terminal 5, and a DC voltage obtained from a diode 16, a transistor 17, and a resistor 18 is applied to its base to form a constant current circuit. Configure. A signal output terminal 19 is provided at the collector of the transistor 1.

A constant current circuit including a load resistor 14, a transistor 15, and a signal output terminal 19 are connected to a transistor 2.
It may be provided on the collector side.

In FIG. 1, the constant current flowing through the transistor 3 is designated as I. Normally, the base voltages of transistors 1 and 2 are set equal, so when no signal is input, the current flowing to the collector of transistor 1 and the current flowing to the collector of transistor 2 are equal.
It will be I/Z each.

Now, if an input signal voltage is applied to the signal input terminal 13 and the base voltage of transistor 1 becomes higher than the base voltage of transistor 2, the collector current of transistor 1 increases and the collector current of transistor 2 increases by the same amount. Decrease. Let this increased current of transistor 1 be ΔI _C .
At this time, assuming that there is no transistor 15, the initial resistance value of the load resistor 14 is R, the resistance value after the change is (R+ΔR), and the DC power supply voltage is _Vcc , the output voltage of the signal output terminal 19 is V _OUT is expressed by the following formula.

V _OUT = V _cc - (1/2I+ΔI) (R+ΔR) = V _cc -1/2I (R+ΔR) - ΔI (R+ΔR) The term ΔI・(R+ΔR) is the signal component, and the remaining term represents the DC component. .

As is clear from the above equation, by changing the resistance value R of the load resistor 14 by ΔR, ΔI(R+
The gain of the output signal can be changed by changing the magnitude of ΔR), but at the same time, the magnitude of 1/2I (R + ΔR) also changes by 1/2I・ΔR, so the output DC voltage level also changes. It will change.

Therefore, in the differential amplifier of FIG. 1, by adding a constant current including the transistor 15, the change in the output DC voltage when the resistance value R of the load resistor is changed and the gain is changed is compensated for. That's what I do. The operation will be explained in detail below.

Even if transistor 15 is added, transistor 1
Since the current value flowing through the transistor 15 is constant, if the constant current flowing through the transistor 15 is I _D , then the load resistance 14
The current flowing through the transistor 15 is reduced by I _D compared to the case without the transistor 15. Therefore, the output voltage V' _OUT output from transistor 1 to signal output terminal 19
is as follows.

V′ _OUT =V _cc −(1/2I−I _D +ΔI)(R+ΔR
) =V _cc -1/2I (R+ΔR)+I _D (R+ΔR)
- ΔR (R + ΔR) = V _cc -1/2I・R-1/2I・ΔR+I _D (R
+ΔR)-ΔI(R+ΔR) Therefore, the output DC voltage level is I _D (R+ΔR) compared to the case without transistor 15.
only can be compensated.

At this time, since the transistor 15 is a constant current circuit, the current change ΔI _C caused by the input signal flows only through the load resistor 14, and the gain of the output signal is not affected by the presence or absence of the transistor 15.

Furthermore, from the above formula, the values of the resistor 18 etc. are selected so that −1/2I・ΔR+ _ID (R+ΔR)=O, that is, I _D =1/2I(ΔR/R+ΔR), and the transistor 15 is adjusted. By setting the current _ID , it is possible to completely compensate for changes in the output DC voltage level when the resistance value of the load resistor 14 is changed.

The embodiment shown in FIG. 2 applies input signals of opposite polarity to the bases of both transistors 1 and 2,
Load resistance 1 is placed on both collectors of transistors 1 and 2.
Transistors 15, 21 are connected in parallel to each load resistor 14, 20 to form a constant current circuit.
, and signal output terminals 19 and 22 are provided respectively. 23 is a signal input terminal. In this case as well, compensation can be provided in the same way.

As described above, according to the present invention, even when the gain of the output signal is changed by changing the value of the load resistance, it is possible to compensate so that the output DC voltage level does not change.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a differential amplifier in one embodiment of the present invention, and FIG. 2 is a circuit diagram of a differential amplifier in another embodiment. 1... First transistor, 2... Second transistor, 3, 15, 21... Constant current circuit transistor, 14, 20... Load resistor, 13, 2
3...Signal input terminal, 19, 22...Signal output terminal.

Claims (1)

[Claims] 1 Connect the emitters of the first transistor and the second transistor, provide a constant current circuit between this connection point and a reference potential point, and connect the collector of at least one of the first transistor and the second transistor. and a DC power source, a signal input terminal is provided at the base of at least one of the first transistor and the second transistor, and a signal is output to the collector of the transistor to which the load resistor is connected. A terminal is provided between the collector of the transistor provided with this signal output terminal and the DC power supply, the size of which compensates for the change in the current flowing into this transistor that occurs when the resistance value of the load resistor is changed. A differential amplifier characterized by having a constant current circuit that flows a constant current.