Hello, I wanted to build a two stage amplifier, but in my case only first stage is amplifying the signal whereas the second stage is just acting as a buffer and not amplifying the output obtained from first stage. The input to first stage is 20mV and its output is -1.92V and the same thing is fed to second stage but giving output that is -1.92v. I have attached my circuit below. Kindly help.

 

hi 123,
Inputting -1.92v to the 2nd stage , which has a gain of 100, what are you expecting from the 2nd output.
It must be hard into saturation.
E

 

hi 123,
This is what LTSpice shows.
E

 

Guru, the feedback of the second stage is incorrectly configured. The R4/5 tap should connect to the inverting input, as Eric shows..

 

Guru, the feedback of the second stage is incorrectly configured.

Looks correct to me.

 

This is what LTSpice shows.

Interesting. I get -2.02 on V1 but only -7.95 volt on Vo. Breadboarded using a 4558.

 

hi sg,
The LTSpice models for the 741 are not ideal, so the Vout's will be suspect.
E

4558 data, with +/-V 15v supplies, Vout limits early. [ the TS has only +/-10V]

 

I agree sir.

 

"The second stage is a buffer and not amplifying". Because R4 is in the wrong location.

 

Why is the text of the first post wrong:

He means it's acting like a buffer and not amplifying.

 

Looks correct to me.

He has R5 connected to the output of the second opamp, not to its negative input pin. The second opamp is only acting as an (inverter) "Correction buffer" with a 100K between input and output!

 

He has R5 connected to the output of the second opamp, not to its negative input pin

Absolutely correct. Man I can't believe I missed that. I was focused on the using the inverting input vs the positive.
Anyway that dose confirm the problem.

 

I also can't believe I missed R4 in the wrong location.

 

Perhaps ypu can explain why you think you need a second stage with a gain of 1?

Bob

 

hi 123,
This is what LTSpice shows.
E

Thank you!!!