JPH01233813A - Variable gain amplifier - Google Patents

Abstract

PURPOSE:To make the output DC level stable by increasing/decreasing differentially the DC compensation current supplied to the collector of a differential amplifier with respect to the increase/decrease in the gain control current supplied to a variable impedance means. CONSTITUTION:A control voltage is applied from a variable DC voltage source between bases of differential transistors(TRs) 16, 17 to vary the collector current of the TRs 16, 17. That is, the total current flowing through the collector of the differential TRs 16, 17 from the current source 20 is constant and when the control current from the collector of the TR 17 is increased, the collector current of the TR 16 is decreased by the DC current being a half the increment. That is, the DC compensation current fed to each emitter of a common base circuit 11 is decreased by a half the increment of the control current. Thus, when the control current is increased to increase the gain, even when the collector current of the differential TRs 1, 2 is going to increase, the DC current flowing to load resistors 5, 6 is kept constant by the decrease in the DC compensation current and the output DC level is made constant.

Description

Detailed Description of the Invention [Object of the Invention] (Industrial Application Field) 2. The present invention relates to a variable gain amplifier that has a stable output DC level and good frequency characteristics.

(Prior Art) The one shown in FIG. 7 is known as a conventional variable gain amplifier. This is (IEEE Transaction
B, T, RJ 1972, August issue, pages 158-162.

The amplifier shown in FIG. 7 includes a transistor 1.2 to which an input signal is supplied between the bases, an emitter resistor 3.4 connected between each of its emitters and a reference potential line VS, and an emitter resistor 3.4 connected between each collector and a voltage source line VD. The differential i! with a load resistor 5.6 connected to A variable impedance circuit 9 is connected between the emitters of the transistors 1 and 2 and is composed of a transistor 7.8 having a common base and a common collector.
A variable current source 10 is connected to a common base of this circuit 9. and an input terminal a that connects the input signal V in to the base of the transistor 1.2.

Output J terminal C1d input to b and connected to its collector
An output signal V is obtained.

In the above amplifier, the impedance of the variable impedance circuit 9 is controlled by changing the amount of current of the variable current source 10, and as a result, the gain changes. Therefore, the values of emitter resistance 3.4, load resistance 5.6 and variable current source 1
By appropriately setting the zero current change, desired gain change characteristics can be obtained. Another advantage is that good characteristics can be achieved even at relatively high frequencies.

However, as is clear from the configuration, in the above amplifier, the gain is controlled by changing the amount of current of the variable current source 10, so the amount of DC current flowing through the load resistor 5.6 changes, and as a result, the output DC voltage It has the disadvantage that it changes.

This variation in the output DC voltage makes DC coupling with other circuits difficult. In addition, if AC coupling is performed with other circuits using capacitance, it will naturally make it difficult to transmit low-frequency signals, and if integrated into an IC, capacitive coupling will require a capacitor with a relatively large area. This results in an increase in area, which is disadvantageous in terms of space and cost.

On the other hand, if a DC compensation current source is provided that differentially increases or decreases with respect to changes in the current ■ of the variable current source 10, and is dimensioned to compensate for changes in the DC current ■ of the load resistors 5 and 6, the output DC level However, it is difficult to maintain the output impedance of the DC compensated current source sufficiently high even in the high frequency range, and therefore there is a problem that good high frequency characteristics cannot be obtained.

(Problems to be Solved by the Invention) As described above, with the conventional circuit configuration, the output DC voltage changes when the gain is changed, it is not suitable for IC implementation, and it is not possible to obtain good high frequency characteristics. There was a problem that sometimes it was not possible.

Therefore, the present invention is intended to eliminate the above problems, and aims to provide a variable gain amplifier that has a stable output DC voltage, provides good characteristics over a wide frequency range, and is suitable for IC implementation. That is.

[Structure of the Invention] (Means for Solving the Problems) A variable gain amplifier of the present invention has first and second transistors forming a differential pair, each emitter of which is connected to the first and second transistors. The respective collectors are connected to the reference potential point via the second emitter resistor. a differential amplifier connected to a voltage source via a second load resistor, supplying an input signal between its bases and obtaining an output signal between its collectors; variable impedance means, which is provided between the emitters of the second transistor and which varies the impedance by controlling the aS current to control the gain of the differential amplifier; each collector of the second transistor and the first . The third load resistor is connected to the second load resistor. a common base circuit configured by cascode-connecting a fourth transistor; and supplying the control current to the variable impedance means, while differentially increasing or decreasing the control current to each emitter of the common base circuit; and current control means for supplying a DC compensation current.

(Function) In the present invention, the DC compensation current supplied to the collector of the differential amplifier differentially (complementarily) increases or decreases in response to increases or decreases in the gain/1q control current supplied to the variable impedance means. Even if the control current changes, the DC current flowing through the load resistance is kept constant, and the output DC level can be stabilized.

Furthermore, since the input impedance of the grounded base circuit to which the DC compensation current is supplied is sufficiently low, it is less affected by the output impedance of the DC compensation current source connected to the grounded base circuit, and the characteristics against high frequencies are also good.

(Example) The present invention will be described below based on the example shown in the drawings.

FIG. 1 is a circuit diagram showing a variable gain amplifier according to an embodiment of the present invention. The same components as in FIG. 7 will be described with the same reference numerals.

The difference from the conventional variable gain amplifier shown in FIG. 7 is that a common base circuit 11 is provided in cascode connection between the collectors of transistors 1 and 2 constituting the differential amplifier and the load resistor 5.6. On the other hand, a current control circuit 12 is provided in place of the variable current source 10 in FIG. The common base circuit 11 has a load resistance of 5°6 between the collectors of transistors 1 and 2, and the collectors of transistors 13 and 14.
The emitter path is connected and the DC voltage source 15 is connected to the common base of the transistors 13 and 14.

The current control circuit 12 has a function of supplying a control current corresponding to the variable current source 10 to the variable impedance circuit 9, and a grounded base circuit that supplies a DC compensation current that differentially changes (increases or decreases) with respect to this control current. It has a function of supplying to each of the 11 emitters.

The input signal @Van is input to the input terminals a and b connected to the base of the transistor 1.2, and the output signal Vm is input to the output terminal c connected to the collector of the transistor 13.14.
, get to d. The other circuit configurations are the same as those shown in FIG.

FIG. 2 shows an embodiment of the current control circuit 12, in which one side is a differential transistor 16 consisting of a multi-collector.
．． Connect the emitters of 17 with two resistors 18 and 19,
A current 120 is connected to the connection point of the two resistors, a series circuit of a variable DC voltage source 21 and a DC voltage source 22 is connected to the differential transistor 170, and a DC voltage 'm22 is connected to the base of the differential transistor 16. Connected. Therefore,
A controlled voltage is applied between the bases of the differential transistors 16 and 17 by the variable DC voltage source 21, so that the transistors 16 and 17
．． 17 collector current is changed.

The collectors of each of the multi-collector differential transistors 16 are connected to the emitters of each of the common base circuits 11 of FIG. do.

In the circuit of FIG. 2, the total current flowing from current source 20 through each collector of differential transistors 16, 17 is constant;
When the lJ III DC current is added, each collector current of the differential transistor 16 decreases by half of the DC current increase. That is, the DC compensation current supplied to each emitter of the common base circuit 11 is reduced by half the increase in control current. Therefore, when the control current is increased to increase the gain, even if the collector current of the differential transistor 1.2 tries to increase, the DC current flowing through the load resistor 5.6 is kept constant due to the decrease in the DC compensation current, and the output DC The level remains constant. Also, in order to lower the gain, control 1
1, even if the collector current of the differential transistor 1.2 attempts to decrease, the DC current flowing through the load resistors 5 and 6 is kept constant due to the increase in the DC compensation current.
The output DC level remains constant.

According to the above embodiment, since the DC current flowing through the load resistor 5.6 is kept constant, the output DC level can be stabilized. In addition, since the DC compensation current source terminal of the DC IJ I11 circuit 12 is connected to the emitter of the common base circuit 11 whose impedance is sufficiently low, the amplifier is not easily affected by the output impedance of the DC compensation current source, and can be used at high frequencies. Good gain IJ characteristics can be obtained up to

FIG. 3 shows another embodiment of the current control circuit 12, and the difference from FIG. 2 is that PNP transistors 16, 17
is changed to NPN transistors 23, 24, and current sources 25, 26, 27 are connected to the respective collectors of transistors 23, 24.

However, the transistor 24 is a multi-collector transistor. Then, the output obtained by subtracting the collector current of the transistor 23 from the current source 25 is used as a control current to the variable impedance circuit 9, and the output obtained by subtracting each collector current of the transistor 24 from the current source 26 and 27 is used as the control current for the common base circuit 11. This is the DC compensation current to the emitter.

FIG. 4 shows another embodiment of the base grounding circuit 11, in which the voltage source line VD and the reference potential are connected to the current source 28.
Diode 29. A DC voltage source 15 is connected in series, and a connection point between a current source 28 and a diode 29 is connected to a transistor 13.
14 common bases. At this time, since the base-emitter voltage of the transistors 13 and 14 and the forward voltage drop of the diode 29 are approximately equal, the emitter voltage of the transistors 13 and 14 can be considered to be approximately equal to the voltage of the DC voltage source 15. . In this way, if the emitter voltages of the transistors 13 and 14 can be considered to be approximately constant, the current source 2 of the current control circuit 12 shown in FIG.
5.26.27 is replaced with resistor 30.31.32 and the fifth
The circuit configuration shown in the figure can be used.

FIG. 6 shows another embodiment of the present invention, which differs from FIG. 1 in that the variable impedance circuit 9 is constructed with PNP transistors 33 and 34 instead of NPNt-Langistaph 8. In this case, it goes without saying that the output level from the upstream control circuit 12 to the variable impedance circuit 9 is changed in a predetermined manner.

Note that the variable impedance circuit 9 includes a transistor 7.
8 may be replaced by a diode, or the transistors 7 and 8 may be diode-connected.

[Effects of the Invention] As described above, according to the present invention, the output DC level is stable even when gain control is performed, good characteristics are obtained over a wide frequency range, and the variable gain amplifier is suitable for IC implementation. can be realized.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a variable gain amplifier according to an embodiment of the present invention, Fig. 2 is a circuit diagram showing an embodiment of the current control circuit of Fig. 1, and Fig. 3 is a circuit diagram showing an embodiment of the current control circuit of Fig. 1. 4 is a circuit diagram showing another embodiment of the base grounding circuit in FIG. 1, and FIG. 5 is a circuit diagram showing another embodiment of the current control circuit in FIG. 1. FIG. 6 is a circuit diagram 1 showing another embodiment of the present invention, and FIG. 7 is a circuit diagram showing a conventional variable gain amplifier. 1.2.7.8.13.14...transistor, 3.
4,5.6...Resistor, 9...Variable impedance circuit, 11...Base grounding circuit, 12...Current control circuit,
15... Voltage source, VD... Voltage source line, S...
Reference potential point line, a... Input terminal, Fig. 5 Fig. 6

Claims (1)

[Claims] It has first and second transistors forming a differential pair, each emitter of which is connected to a reference potential point via a first and second emitter resistor, and each collector of which is connected to a reference potential point via a first and second emitter resistor. , a differential amplifier connected to a voltage source via a second load resistor, supplying an input signal between the bases and obtaining an output signal between the collectors; and a differential amplifier provided between the emitters of the first and second transistors. , variable impedance means for controlling the gain of the differential amplifier by varying impedance with a control current; collectors of the first and second transistors;
, a common base circuit configured by connecting third and fourth transistors in cascode between a second load resistor; and supplying the control current to the variable impedance means, while supplying the control current to each emitter of the common base circuit. A variable gain amplifier characterized in that it comprises current control means for supplying a DC compensation current that increases or decreases differentially with the control current.