OpAmp Non-inverting amplifier using transistor in feedback network

[ysba]

Hi,

I saw the following circuit in a guitar effect circuit which I'm studying.

I understand it is a active filter, but what's the purpose of the transistor circuit used at the feedback network? May I use the transistor model to obtain the Vout/Vin transfer function or it is used for something else?

Thanks!

 

[FvM]

It's gyrator used as simulated inductor, see https://en.wikipedia.org/wiki/Gyrator#Application:_a_simulated_inductor with design equations.

 

[AMS012]

How does it act as gyrator? Could you explain it in detail please...

 

[crutschow]

How does it act as gyrator? Could you explain it in detail please...

Did you read the Wikipedia article?