# AGC Circuit

Discussion in 'Homework Help' started by Robert.Adams, May 25, 2010.

Feb 16, 2010
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I have an extra credit problem where I need to find an AGC circuit online and modify it to meet certain design parameters. I need to show that it meets these requirements in MultiSim.

So far I have been using http://www.longrope.com/circuits/new_page_2.htm (circuit II) with all the +12V changed to +9 V.

From this I get my input sine wave with about half peak to peak voltage floating up around 3.5V.

Is there a better AGC circuit out there that I should use (such as one where there is no DC offset on the output)? Otherwise, how would I go about modifying this one?

Apr 5, 2008
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3. ### PRS Well-Known Member

Aug 24, 2008
989
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If better means simpler, then there is a simpler circuit. AGC implies a variation in the amplitude of the input signal as per radio signals. And so you use AGC with your IF strip. The usual way of dealing with variant amplitude of input signals is to make your IF strip (1 or 2 or even 3 discreet amplifier stages) is to take a quantitative sample of the output of that strip and use it as negative feedback at the first stage of the IF strip. This sounds more complicated than it really is. In practive, it makes use of the voltage produced at a simple diode-resistor detector and feeding it back through a resistor to the first amplifier of the IF strip as a manner of changing it's gain. The upshot is that the higher the output of the last amplifier in the IF strip, the more the overall IF strip decreases the gain of that strip at its input. The idea is to keep strong stations from blasting and allow weak stations to be heard.

4. ### Wendy Moderator

Mar 24, 2008
20,772
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Ever look at a light bulb on a wien bridge oscillator? This is a form of AGC. Collins Radio also used something similar for 70Mhz IF circuits during the mid 70's.