Dear all,
I also have tried to build this circuit. Its not working. I have used IRF 9540 which is p-channel MOSFET in place of J176. And instead of LM358 I have used TLV2374 which is a R-R opamp. Also I have not used a POT, instead I have fixed the resistor value 10K. Rest of the circuit is all same. Can any one help me out in correcting the circuit and make it work.

 

Dear all,
I also have tried to build this circuit. Its not working. I have used IRF 9540 which is p-channel MOSFET in place of J176. .............................. Can any one help me out in correcting the circuit and make it work.

The J176 is a depletion mode P-Channel J-FET.
You can't substitute an enhancement mode P-MOSFET in that circuit.
Why did you think you could?

 

My bad... IRF 9540 was a wrong choice. I replaced it with J176.... and now its working fine...Thanks crutschow.

Now I am facing issues when trying to integrate this circuit with my already existing circuit. In my design, the AGC circuit is feeding an op-amp, again a TLV2374. Op-amp is configured as a inverting amplifier with Rin = 22Kohm and Rfeedback = 10Kohm, and the non-inverting terminal of the Op-amp is connected to a voltage divider (R1=R2=47Kohm). what I observed was, the AGC output is reduced to around 200mV once I connect it with the op-amp input (through Rin). And also its (output) not remaining constant...its varying (it was giving a constant output of 1.2 V before integrating with Op-amp). What count be the reason..?

 

Since I'm not good at visualizing what you have, can you post a circuit diagram?

 

Sorry,
Please find the attachment.

 

Still don't have the complete picture.
What is "AGC input" connected to on the AGC circuit?

 

Let me start afresh... I have this AGC circuit (see attachment) implemented with TLV2374 in place of LM358, and was working fine (giving a constant 1.2Vpp output ). Now I wanted to integrate this circuit with another circuit (see attachment in previous post). In my previous post , AGC output is the Audio output signal of AGC circuit mentioned in the attachment.

When I probe at the AGC output I was seeing a drop in voltage level from 1.2Vpp to 200mVpp. And also it is not remaining constant as it should be. what could be the reason. Hope this time I am more clear.

Please guide me if I am missing on any thing while clubbing both the circuits.

correction: In my previous post I mistakenly typed (in attachment) TLV3274 instead of TLV2374. My apologies.

 

Crutschow,
The AGC input is connected to a function generator through which I am generating a simple sine wave and feeding the AGC circuit. AGC was giving constant 1.2Vpp for various levels of input sine wave. This completely changes when I integrate it with my circuit. The AGC output is not remaining constant for varying input.

 

hello guys...
any one there to help me out...?

 

Sorry I still can't picture how you have everything connected. If you can't show me a complete diagram of your hookup, I'm unable to help.

 

This thread was split off and moved from here.

 

Sorry I still can't picture how you have everything connected. If you can't show me a complete diagram of your hookup, I'm unable to help.

Sorry for discomforting you. Here is the exact circuit for your perusal. Here the input is from a function generator and I am probing the signal at the output with a scope. I observed that output is not remaining constant for a varying input. why...?

 

I simulated your circuit and did not see any drop in output voltage amplitude or change in the AGC action when I connected the output opamp load nor do I see any reason for a noticeable change.
Thus I suspect there's an error in your circuit construction.
Have you double-checked all connections and component values?

 

Maybe you could post the scope pictures with an input of .1 volts. The + of the op amp the output of the op amp and the collector of the transistor.