1. srikanth.remudala

    Thread Starter New Member

    Nov 30, 2014
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    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.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    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?
     
  3. srikanth.remudala

    Thread Starter New Member

    Nov 30, 2014
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    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..?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Since I'm not good at visualizing what you have, can you post a circuit diagram?
     
  5. srikanth.remudala

    Thread Starter New Member

    Nov 30, 2014
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    Sorry,
    Please find the attachment.
     
    • AGC.pdf
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  6. crutschow

    Expert

    Mar 14, 2008
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    Still don't have the complete picture.
    What is "AGC input" connected to on the AGC circuit?
     
  7. srikanth.remudala

    Thread Starter New Member

    Nov 30, 2014
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    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.
     
  8. srikanth.remudala

    Thread Starter New Member

    Nov 30, 2014
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    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.
     
  9. srikanth.remudala

    Thread Starter New Member

    Nov 30, 2014
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    0
    hello guys...
    any one there to help me out...?
     
  10. crutschow

    Expert

    Mar 14, 2008
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    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. :rolleyes:
     
  11. MrChips

    Moderator

    Oct 2, 2009
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    This thread was split off and moved from here.
     
  12. srikanth.remudala

    Thread Starter New Member

    Nov 30, 2014
    9
    0
    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...?
     
  13. crutschow

    Expert

    Mar 14, 2008
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    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?
     
    Last edited: Dec 9, 2014
  14. ronv

    AAC Fanatic!

    Nov 12, 2008
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    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.
     
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