# Query: Gain Control Circuit

#### SaurinM

Joined Jun 20, 2020
6
Good evening, they recommended this forum to try to answer my question.
I wanted to know if anyone can give me a hand in the analysis of the following circuit. Because she can't find her way back.
I hope you can help me.
Regards.

#### Papabravo

Joined Feb 24, 2006
19,266
It looks like a non-functional circuit, considering that Q1 can never be on.

#### SaurinM

Joined Jun 20, 2020
6
The circuit if it works, at the output we have an attenuated signal with respect to the input. But I cannot find the logic of its operation.

#### AlbertHall

Joined Jun 4, 2014
12,156
Maybe: as the input signal gets bigger, the rectified input across C4 increases turning on Q2 more and sending more current through D3 and D4. Thus the impedance of D4 falls attenuating the output signal more. So as the input rises, the output gets more attenuated. That would make it a compressor or automatic volume control or automatic level control.

#### SaurinM

Joined Jun 20, 2020
6
And Q1 never goes into operation?

#### Papabravo

Joined Feb 24, 2006
19,266
And Q1 never goes into operation?
With the base shorted to GND, Q1 can never do anything. R2 is also useless as it conducts no current and there is no voltage drop across it.

#### AlbertHall

Joined Jun 4, 2014
12,156
Yes, Q1 and R2 are just there to fill an empty spot on the PCB but that doesn't prevent the voltage doubler rectifier working.
Any comment on using a variably forward biased as an attenuator?

#### Audioguru again

Joined Oct 21, 2019
5,162
Can't you see (and hear) the severe distortion on the sinewave? The attenuator is not linear.

A Jfet is usually used as an attenuator since it produces low distortion when its signal level is 100mV or less, especially when half the signal voltage is fed to its gate.