[SOLVED] Slew Rate of Subthreshold Opamp

[IntuitiveAnalog]

I am designing a two stage subthreshold opamp . When I am measuring its slew rate(Ibias=200nA and CL=0.1pF ,Vdd=1V) then output waveform is not reaching upto the amplitude of square wave. Another problem is that it is giving different values for different square wave(time period=2us) amplitudes.
1. +/- 750mV - 20V/ms
2. +/- 500mV - 12V/ms
3. +/- 250mV - 2V/ms

If anybody has idea what's wrong in it , then I will be thankful to him/her.

Thanks

 

[erikl]

Your stimulation time period=2µs is rather close to your output time constant CL/Ibias=0.5µs (and that's not the only limiting time constant of your OTA).

Try the slew rate measurement with a much longer stimulation time period.

 

[dick_freebird]

I=CdV/dt, no way around it. The "I" of interest being your
output stage maximum current-throw, if C is the external
load.

That input difference matters so much, indicates a gain
shortfall. Back end current must not be fully steered by
250mV simulus' applied input difference. Although there
could be input/feedback network related problems (like
an output strength insufficient to drive the feedback
leg, leading to some nonlinearities of gain vs drive or
network values) - have seen these even in higher current
OTAs before.

 

[IntuitiveAnalog]

Thanks for reply erikl & dick_freebird.
So dick_freebird how can I rectify it?

 

[D.A.(Tony)Stewart]

Can you increase Iq per channel or gm to 50mA/V?

 

[IntuitiveAnalog]

SunnySkyguy I am done with design , so I cannot change any parameter.

Thanks erikl, I increased time period to 20us and it worked.