Automatic Gain Control conceptual understanding

Discussion in 'Homework Help' started by Dutchman, Oct 20, 2013.

  1. Dutchman

    Thread Starter New Member

    Oct 20, 2013
    This is my first time posting here and need help understanding how a circuit works. The circuit is for a RF electronics class, and is not a homework problem, but a circuit that consists of a couple circuits that we have studied. My goal here is to understand a couple of portions of the circuit that I do not quite understand. The circuit is an automatic gain control circuit comprised of a common emitter amplifier, voltage follower, peak detector and attenuating feedback circuit. I have attached the multisim schematic.
    The first questions I have are:

    1. The dual diodes used in the peak detector along with the two caps work to keep the voltage at this point the same as the input voltage, but how exactly? I understand the very basic concept of the single cap and diode at the far right of the circuit next to the 680 ohm resistor, but I dont understand how the first diode and cap function or why they are needed here.
    2. The first transistor in near the input that is used to attenuate the input signal by the amount of difference detected by the peak detector is a little confusing to me. Does this transistor stay on at all times? or is there a point at which it has equalized the circuit and drops below the required current to attenuate any current from the input?
    3. I understand the common emitter amp and the voltage follower, except why the voltage follower collector is not used as the out put instead of the common emitter collector. Isnt the voltage follower used to "clean" up the signal from the amp? the voltage follower in this circuit as most I think, is unity gain, right? so why not use the collector for the output?
    I basically need/want to understand how the circuit works as a whole with each component in the circuit. Any calculations or conceptual information you could provide so I can have a firm understanding of this circuit I would be greatly appreciative.

    Feel free to ask me any questions or have me do some work to help the conversation move along.
    • AGC.jpg
      File size:
      278.3 KB
  2. Dutchman

    Thread Starter New Member

    Oct 20, 2013
    Also, I just noticed that I left out the 47uF bypass cap on the emitter of the amp, parallel to R7.
  3. bertus


    Apr 5, 2008
    anhnha and Dutchman like this.
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
    AD1 - this circuit is not a peak detector it is a voltage doubler.
    Watch this video
    AD2 - For the most of the time Q1 will be OFF (cut-off). But when the input signal is large enough so that voltage doubler is large the 0.6V Q1 start attenuate the input signal.
    AD3 - Emitter follower can provide much more current to the load than "voltage follower collector " (such circuit don't exist, we can only build a common emitter amplifier with gain of -1 ).
    anhnha and Dutchman like this.