Is this an AVC (automatic volume control)?

[Boncuk]

Hello friends,

I stumbled over the design of an intercom these days. There is a circuit at the input of the amplifier consisting of a transistor, three resistors and five capacitors (three of them electrolytic).

I simulated the circuit with a function generator at its input. At high "volume" nothing changes. If the volume is reduced drastically there is no voltage at the output capacitor. After a while it increases continuously to the same level as before.

Can somebody shed some light on the functions? Also the waveform changes considerably, from sine to something similar to a square wave.

Regards

Hans

 

[Boncuk]

Is this an AVC?

Oops,

no way to add an attachment when editing. A case for the super moderator!

Here it is.

 

[HS3]

no way to add an attachment when editing.

Yes, you can add and change attachments when editing. If you are at the "post quick reply" interface then go to the "advanced" interface and you can do it there.

Yes, that looks like an ingenious AVC based on the rectifying non-linearity of the B-E of the transistor which changes the quiescent work point of the transistor and changes the input impedance and amplification. We would have to see the response as it might be too slow. It is ingenious in its simplicity. Most I have seen were more complex.

 

[Nigel Goodwin]

I'd suggest it's more probably a very poorly designed common-base amplifier?.

It's purpose is to better match the impedance of the speaker used as a microphone, except it's actually much too low at 2.7 ohms. You're getting bizzare simulations because it's got such strange values on the base.