Problem with the gain plot of a charge amplifier

[taik]

Charge Amplifier

hi, hope someone can advise me on the problem i am facing.

below shows a circuit which the gain is c1/c2. however, i am unable to get this gain. the gain plot is shown. it should be an all-pass tf with gain C1/C2 but i am not getting anything close.

can anyone tell me what's the problem?

c1=50p, c2=1p, open-loop gain of op-amp=60dB

 

[safwatonline]

Charge Amplifier

how are you biasing the input node

 

[taik]

Charge Amplifier

one dc source at pos and neg node each...

 

[drDOC]

Re: Charge Amplifier

I think you just don't have a DC operating point. Check the voltage on the negative input.
Maybe you can put a big resistor parallel to C2.

 

[safwatonline]

Charge Amplifier

try posting ur schematc/test_bench

 

[ricklin]

Charge Amplifier

Looks like an operating point problem.
And what is DC source at pos and neg each? This is usually AC short to ground.
I think at least a detailed diagram is needed to show how you bias the amplifier.

 

[LvW]

Re: Charge Amplifier

........ it should be an all-pass tf with gain C1/C2 but i am not getting anything close

Besides the bias problems already mentioned such a circuit never will work as an allpass filter. An allpass is a device with constant gain and frequency dependent phase.