JPH0354429Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a voltage controlled variable gain circuit, and more particularly to a voltage controlled variable gain circuit using a differential amplifier.

FIG. 1 shows a circuit diagram of a conventional example of a voltage controlled variable gain circuit (hereinafter abbreviated as VCA). In the figure,
A pair of transistors with their emitters connected in common
A differential amplifier 1 is configured by Q ₁ , Q ₂ and a load resistor R ₁ , and a current source 2 consisting of a transistor Q ₃ and a resistor R ₂ is connected to the common emitter connection point _of transistors Q ₁ and Q 2 . There is. This current source 2 supplies a current according to a predetermined input signal ei to the transistors Q ₁ ,
Pour into each emitter of _Q2 . A control voltage from a variable voltage source 3 is connected between the bases of transistors Q ₁ and Q ₂ .
Vc is applied, and by changing this control voltage Vc, the output derived from the collector of the transistor _Q2 is controlled.

The collector output of the transistor _Q2 is connected to the fixed contact of the Mute switch 5 via the buffer amplifier 4.
Applied to 5a. The movable contact 5b of the switch 5 is connected to the fixed contact 5a via a resistor _R3 and becomes the output end of the VCA, and the fixed contact 5c is grounded via a resistor _R4 .
When 5b is on the fixed contact 5a side, the mute is OFF, and when it is on the fixed contact 5c side, the mute is ON.

When the MIUTO is OFF, the collector output of the transistor Q ₂ becomes the VCA output as it is because the resistor R ₃ is short-circuited, but when the MIUTO is ON, the collector output of the transistor Q ₂ is connected to the resistors R ₃ and R _4.
Muting characteristics are obtained by dividing the Note that the amount of muting is R ₃ /R ₃ +R ₄ .

However, in such a conventional configuration, since the mute switch 5 is inserted into the signal system, distortion and
It has the disadvantage of having an adverse effect on the signal due to characteristics such as noise. Furthermore, there is a drawback that the voltage dividing resistors R ₃ and R ₄ become noise resistances of the amplifier connected to the next stage, resulting in a deterioration of the S/N ratio.

An object of the present invention is to provide a voltage-controlled variable gain circuit that can eliminate adverse effects on signals and electronically attenuate the output by a certain amount by performing muting control in a control system.

A voltage controlled variable gain circuit having a muting function according to the present invention includes a pair of transistors whose emitters are commonly connected to perform differential operation, and a current source that flows a current according to a predetermined input signal to the emitters of the pair of transistors. A voltage controlled variable gain circuit comprising a control voltage source that applies a variable control voltage between each base of the pair of transistors to change the level of an output signal derived from one collector of the pair of transistors. , comprising a constant voltage source and superimposing means for superimposing the output voltage of the constant voltage source on the control voltage when the mute is turned on, and the output voltage of the constant voltage source is a voltage at a level that attenuates the output signal by a certain amount. It is characterized by

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 is a circuit diagram showing an embodiment of the present invention, in which parts equivalent to those in FIG. 1 are designated by the same symbols. In this example, the transistor
The base of Q ₁ is connected to the movable contact 5b of the mute switch 5 via the variable voltage source 3, and the base of the transistor Q ₂ is grounded. A fixed contact 5a of the miute switch 5 is directly grounded, and a fixed contact 5c is grounded via a constant voltage source 6. The Mute switch 5 turns OFF when the movable contact 5b is on the side of the fixed contact 5a, and turns ON when the movable contact 5b is on the side of the fixed contact 5c, and a known electronic switch such as a transistor can be used as the switch.

In this configuration, when the mute is OFF, only the control voltage Vc is applied between the bases of the transistors Q ₁ and Q ₂ , and the attenuation characteristic of the VCA at this time is such that the current of the current source 2 corresponding to the input signal ei is reduced to I ₀ , when the collector current of transistor Q ₂ is I ₁ , it is expressed as I ₁ =I ₀ /1+exp(q/KTVc) (1). Here, K is Boltzmann's constant, T
is the absolute temperature and q is the electron charge. Further, the output voltage Vo is expressed as Vo=I ₀ R ₁ /1+exp(q/KTVc) (2). It is clear from equation (2) that the control voltage Vc
It can be seen that the output voltage Vo changes exponentially with changes in . The attenuation characteristic of this VCA is shown by a solid line a in FIG. 3, and the amount of attenuation changes approximately linearly at -20 dB/60 mV.

On the other hand, when the mute is ON, the constant voltage V from the constant voltage source 6 is superimposed on the control voltage Vc (VC+V),
Since it is applied between the bases of transistors Q ₁ and Q ₂ , the output voltage Vo is Vo=I ₀ R ₁ /1+exp {q/KT (Vc+V)} (3). Therefore, the output voltage Vo is attenuated by a certain amount in proportion to the constant voltage V, as shown by the dashed line b in FIG. Note that FIG. 3 shows a case where V=60 mV and the amount of muting is -20 dB.

FIG. 4 is a circuit diagram showing another embodiment of the present invention, in which parts equivalent to those in FIG. 2 are designated by the same reference numerals. In this example, the transistor
The base of Q ₁ is grounded via a variable voltage source 3,
The base of transistor Q ₂ is Miyuto switch 5
is connected to the movable contact 5b. The fixed contact 5a of the Miyuto switch 5 is directly grounded.
5c is grounded via a constant voltage source 6.

In this configuration, when the mute is OFF, the base of the transistor _Q2 is grounded by the mute switch 5, so that only the control voltage Vc is applied between the bases of the transistors _{Q1 and Q2} . On the other hand, when the mute is ON, the constant voltage -V is applied from the constant voltage source 6 to the base of the transistor Q ₂ , so the voltage {Vc-
(-V)} will be applied between the bases of transistors Q ₁ and Q ₂ . As a result, the same attenuation characteristics (shown in the third diagram) as in the embodiment described above can be obtained.

As detailed above, the voltage controlled variable gain circuit (VCA) with muting function according to the present invention
According to , the configuration is such that a constant voltage is selectively superimposed on the variable control voltage applied between the bases of the differential transistor, so by superimposing the constant voltage, the attenuation characteristics can be shifted by a certain amount. However, it is possible to obtain a constant amount of attenuation over the entire control range using the variable control voltage, and because the control system performs muting control, the S/N is low.
A good output signal can be obtained. By using an electronic switch as the mute switch and using a control circuit such as a microcomputer, it is easy to implement electronics, and the output can be electronically attenuated by a certain amount. Furthermore, when such a VCA is applied to an audio amplifier, it can be used as an audio amplifying circuit.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a conventional example of a voltage controlled variable gain circuit, Fig. 2 is a circuit diagram showing an embodiment of the voltage controlled variable gain circuit according to the present invention, and Fig. 3 shows the attenuation characteristics of Fig. 2. 4 are circuit diagrams showing other embodiments of the voltage controlled variable gain circuit according to the present invention. Explanation of symbols of main parts 1...Differential amplifier, 2
...Current source, 5...Myuto switch, 6...Constant voltage source.

Claims (1)

[Scope of utility model registration request] a pair of transistors whose emitters are commonly connected to perform differential operation; a current source that flows a current according to a predetermined input signal to the emitters of the pair of transistors; and a variable control voltage between the bases of the pair of transistors. A voltage controlled variable gain circuit comprising a control voltage source applied to change the level of an output signal derived from the collector of one of the pair of transistors, a constant voltage source, and an output of the constant voltage source when the mute is turned on. a superimposing means for superimposing a voltage on the control voltage, the output voltage of the constant voltage source being a voltage at a level that attenuates the output signal by a certain amount, a voltage controlled variable gain circuit having a muting function. .