query regarding two stage telescopic amplifier

[nisha gupta]

i m designing transimpedance amplifier... for high gain i have chosen telescopic structure. i have designed two stage cascode telescopic amplifier. it is working fine in open loop with common mode feedback. ac gain and traient both response are fine. but as i connecting RC feedback transient response in not correct. i m attaching images also... plz let me know what could be the problem in design

 

[erikl]

Images missing ...

 

[nisha gupta]

i m designing transimpedance amplifier... for high gain i have chosen telescopic structure. i have designed two stage cascode telescopic amplifier. it is working fine in open loop with common mode feedback. ac gain and traient both response are fine. but as i connecting RC feedback transient response in not correct. i m attaching images also... plz let me know what could be the problem in design

 

[nitishn5]

What is the value of the R used in the feedback? What is the feedback you are using? Can you show the circuit?

 

[nisha gupta]

HI
I HAVE ATTACHED IMAGE OF two stage telescopic amplifier. feedback resistance is 3K i have used RC feedback...

 

[nitishn5]

Which nodes are the RC feedback connected to?

3k seems a small value. How is it compared to the output impedance of the Op-Amp? How much current is the R drawing? Can the Op-Amp support it?

 

[nisha gupta]

output impedance of 2nd stage is around 1K and current in 2nd stage is 2.47mA in each lag

 

[nitishn5]

Hi nisha,

output impedance of 2nd stage is around 1K and current in 2nd stage is 2.47mA in each lag

Is that value with the feedback in place or in open loop? What is it in open loop?

I feel that the RC is shunting the output impedance of the Op-Amp and hence reducing your gain. The R you use has to be selected in accordance to the output impedance of your Op-Amp. Try with some massive R and appropriate value of C and see whether you get the intended result.