Thread Starter

UrielML96

Joined Nov 28, 2019
1
hello, I have to build an automatic gain control using an FET and an Operational Amplifier, for a phototransistor (SFH314) so that when the signal of an IR LED is weak the AGC maintains the signal, and when the signal is strong reduces it, the gate of the FET I have to control it with the phototransistor detection signal in DC once converted, and the AC signal without converting it into the Operational Amplifier and see its output with the gain control, I hope you understand, thank you very much.
 

MisterBill2

Joined Jan 23, 2018
4,579
In the link the amplitude control is based only on feedback, which means that the output must increase a bit as the input is increased. That is because the gain is reduced based on the output. Adjusting the output amplitude based on the input can be added to keep the output exactly constant with the input changing.
Thus the very first step is to find exactly what the requirements are, how much variation in the output is acceptable, and what the range of input signal amplitudes will be. All of those will affect how the control is implemented.
 

AnalogKid

Joined Aug 1, 2013
8,229
Is this school work? Job-related? Hobby?

Whatever, I suggest you do some research, search the web for AGC circuits, think about your circuit requirements (power, input and out signal ranges, frequencies, etc.), draw a schematic, and post it here for us to review and comment.

ak
 

LvW

Joined Jun 13, 2013
887
Is this school work? Job-related? Hobby?

Whatever, I suggest you do some research, search the web for AGC circuits, think about your circuit requirements (power, input and out signal ranges, frequencies, etc.), draw a schematic, and post it here for us to review and comment.
ak
Yes - I fully agree because there some severe restrictions for using a FET as a controllable resistance.
 

ronsimpson

Joined Oct 7, 2019
349
20 years ago I used a FM radio IC to do this project. The IF amplifiers in the IC had a good AGC function.
That IC was not in production and hard to find then.
Crazy idea. …….
 

MisterBill2

Joined Jan 23, 2018
4,579
Rather than put the FET in series with the input path, run the controlled stage as a non-inverting stage and then put the FET between the inverting input and the signal common line., so that as the fet conduction increases the effective gain increases. The large benefits are easier controlling the voltages on the FET, lower noise because it is grounded, and the input source impedance will have no effect on the control circuit. AND it requires fewer components.
 
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