JPH0145766B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a voltage gain control amplifier whose gain changes depending on the magnitude of an applied DC power supply voltage.

FIG. 1 is a circuit configuration diagram showing an example of a conventional voltage gain control amplifier, in which 1 and 2 are NPN transistors forming a differential pair, and 3 and 4 are first and second transistors connected to the bases of transistors 1 and 2, respectively. Second
The emitters of transistors 1 and 2 are connected to each other via series-connected resistors 5 and 6, and a current source 7 is connected between the connection point of resistors 5 and 6 and the ground point. has been done. 8 and 9
are npn transistors forming a differential pair, and their emitters are commonly connected to the collector of transistor 1. 10 and 11 are NPN transistors forming a differential pair, and their emitters are commonly connected to the collector of transistor 2. The collectors of transistors 8, 9, and 10 are all connected to the power supply terminal 12.
The collector of the transistor 11 is connected to the power supply terminal 12 via a resistor 13. The bases of transistors 8 and 11 are commonly connected to the first control voltage terminal 14, and the bases of transistors 9 and 10 are connected in common to the first control voltage terminal 14.
The bases of the two are commonly connected to the second control voltage terminal 15. An output terminal 16 is drawn out from the collector of the transistor 11.

Next, the operation of this conventional example will be explained. Let g _n1 be the mutual conductance of the differential amplification section composed of transistors 1 and 2, resistors 5 and 6, and current source 7, and let V 1 be the direct current (DC) voltage applied to the first control voltage terminal 14, and let V ₁ be the direct current (DC) voltage applied to the first control voltage terminal . If the DC voltage applied to the control terminal 15 of FIG. 2 is V ₂ and the resistance value of the resistor 13 is R _L , then the gain of the voltage gain control amplifier shown in FIG. 1 depends on the value of V ₁ - V ₂ . The change is shown by the solid line in FIG. On the other hand, if the current value of the current source 7 is I _O and the DC power supply voltage applied to the power supply terminal 12 is V _CC , then the output DC at the output terminal 16 is
The potential changes as shown by the broken line in FIG. 2 depending on the value of V ₁ -V ₂ .

The conventional voltage gain control amplifier is configured as described above, so when the gain is changed by the value of V ₁ - _{V 2} , the DC potential of the output also changes at the same time.
Connection to the next amplification stage may be difficult, or V ₁ −V ₂
This has the disadvantage that the change in the output signal is superimposed on the output signal and has an adverse effect.

This invention was made to eliminate the drawbacks of the conventional ones as described above, and the control voltage V ₁ −
It is an object of the present invention to provide a voltage gain control amplifier in which the output DC potential does not change when the gain is changed by _V2 .

FIG. 3 is a circuit configuration diagram showing an embodiment of the present invention, in which parts equivalent to those of the conventional example shown in FIG. In this embodiment, the collector circuit of the transistor 11 includes a power supply terminal 12 and a resistor 1.
A transistor 17 is inserted between the transistor 3 and the transistor 17, the collector of which is connected to the power supply terminal 12, and the emitter of which is connected to the resistor 13. The collectors of the transistors 9 and 10 are commonly connected, but not directly connected to the power supply terminal 12, but are connected to the power supply terminal 12 via a resistor 18, and this commonly connected collector is connected to the base of the transistor 17. has been done.

In the voltage gain control amplifier of this embodiment configured as described above, the change in gain with respect to the value of V ₁ -V ₂ is completely the same as in the conventional example, as shown by the solid line in FIG. Then the output for the value of V ₁ − V ₂
We will discuss changes in DC potential. If the DC bias voltages of the first signal input terminal 3 and the second signal input terminal 4 are equal, the collector current of the transistor 1 and the collector current of the transistor 2 are equal, and each becomes I _O /2. The collector current of transistor 8 (I _C8 ) and the collector current of transistor 9 (I _C9 ) change depending on the value of V ₁ - V ₂ .
If I _C8 = α・I _O /2, then I _C9 = (1-α)・I _O /2
(However, 0≦α≦1). Similarly, the collector current of transistor 10 (I _C10 ) and transistor 1
The collector current (I _C11 ) of 1 is I _C10 = (1-α)・
I _O /2, I _C11 = α・I _O /2. If the resistance value of the resistor 18 is R ₁ , the base potential V _B of the transistor 17 is V _B = V _CC −R ₁ (I _C9 + _C10 ) = V _CC −R ₁ (1−
α) I _O , so the output DC potential V _O at the output terminal 16 is V _O = V _B − V _BE −R _L I _C11 = V _CC −R ₁ (1−α) I _O − V _BE −R _L
αI _O /2=V _CC −V _BE −{R ₁ (1−α)+R _L α/2}I _O =V _CC −V _BE −[R ₁ +
{(R _L /2)−R ₁ }α]I _O. However, V _BE is the base-emitter voltage of the transistor 17. Therefore, R _L /2=
If R ₁ , then V _O = V _CC −V _BE −R ₁ I _O , which is independent of α, that is, the output DC potential depends on the control voltage.
V _O will no longer change. This state is shown by the broken line in FIG.

Note that when an AC signal voltage is applied between input terminals 3 and 4, the collector current of transistor 9 I _C9
The amount of change in is the collector current I _C10 of transistor 10
Since the phase is 180° different from the amount of change in and the magnitude is the same, the total current does not change and does not appear as a change in the base potential V _B of transistor 17, so the output
It also does not appear at the DC potential _VO .

In addition, although the above embodiment shows a voltage gain control amplifier device composed of npn transistors,
It may also be configured with an npn transistor.

Further, in the above embodiment, the control voltage is limited to a DC voltage, but an AC voltage may be used. Furthermore, if the output signal of this amplifier is controlled by a rectified signal, it can also be an automatic gain control (AGC) amplifier.

As described above, according to the present invention, since a circuit is added to eliminate fluctuations in the output DC potential due to the control voltage, there is an effect that connection to the next stage is facilitated.

[Brief explanation of drawings]

Fig. 1 is a circuit configuration diagram showing an example of a conventional voltage gain control amplifier, Fig. 2 is a characteristic diagram showing the relationship between the gain control voltage, gain, and output DC potential of this conventional example, and Fig. 3 is a diagram of the present invention. FIG. 4 is a circuit configuration diagram showing one embodiment of the present invention, and FIG. 4 is a characteristic diagram showing the relationship between the gain control voltage, gain, and output DC potential of this embodiment. In the figure, 1 is a first transistor, 2 is a second transistor, 3 and 4 are signal input terminals, 8 is a third transistor, 9 is a fourth transistor,
10 is a fifth transistor, 11 is a sixth transistor, 13 is a collector resistor, 14 and 15 are gain control voltage terminals, 17 is a seventh transistor, 18
is the common collector resistance. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A first and second transistor that performs a differential amplification operation with respect to a signal input constitutes an emitter-coupled first differential circuit whose current distribution ratio is changed by a gain control voltage, and the emitter-coupled portion third and fourth transistors connected to the collector of the first transistor and supplying the collector current of the first transistor; an emitter-coupled second differential transistor whose current distribution ratio is changed by the gain control voltage; fifth and sixth transistors constituting a circuit and whose emitter coupling portions are connected to the collector of the second transistor and supplying the collector current of the second transistor, operating in phase by the gain control voltage; a common collector resistor commonly connected to the collectors of the fourth and fifth transistors;
a seventh transistor whose base is connected to the commonly connected collectors of the transistors and whose emitter is connected to the collector of the sixth transistor via a collector resistor to constitute a collector circuit of the sixth transistor. A voltage gain control amplifier characterized by: