i have to make an automatic gain control amplifier using an op-amp LM741 as the basic amplifier and then using a differential error amplifier and a jfet for the freedback path.the output has to be kept constant at 5V and the gain of op-amp amplifier has to be 150 without feedback provided.Please help with the design of the feedback path and the error amplifier.

 

Hi Abhishek,
Your requirement sounds very peculiar. And you know, without giving a feed back its not possible tocontrol the gain of any system, be it an Op-amp. It will remain in its natuaral open loop gain.
What is that you meant by keeping the out put voltage constant to 5V? is that the offset voltage? Elaborate.
bipin

 

Hi
The basic aim of the circuit is to make the outout voltage level constant at 5V(peak to peak) even if the input voltage is changed to few decibels.This has to be achieved by using a JFET as a voltage controlled resistor.This circuit has applications in audio and video amolfiers where u have to keep the output constant even if there is some attenuation of signal at the input.
Hoping that someone will find me an answer.
Abhishek

 

hi,

the answer lies in how an audio/video amplifier is employed and it's usually found in any chip manufacturer of audio/video amps. they have demo circuits for their product.

try to check with ANALOG DEVICE, TEXAS INSTRUMENT, MAXIM IC, PHILIPS