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# How to do ac simulation in cadence for an opamp?

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#### rockycheng

##### Member level 5

I heard about some tricky ways, using large resistance and capacitance to form a feedback. But I'm not quite sure. Could anyone tell me the exact way? Thanks a lot.

Regards
rockycheng

how to do ac simulaton in cadence?

Hi,

Can you brief in detail what AC analysis you want to run. If you say large resistor and caps, that could be to have lower cutoff frequency, some kind of filter termination. So, im not sure what you are talking about. Please be specific.

Thanks,
Suria

post the circuit out, maybe we can help.

ho to do ac simulation in cadence

Thank you for the reply! It is just a normal cascode OTA. I've tried the following test circuit to see the gain vs. frequency transfer function. The result is just the same without the feedback. Would anyone tell me why and when to use the circuit like this? Or it is useless at all?

You can maybe refer to the following PDF. It's a general testbench method.
Hope it helps.

diegodeotti

Points: 2

### diegodeotti

Points: 2
simulation of op amp in cadence

when calculating the operating point, the R connect output and negetive input,
when doing AC analusis, the C short the negetive inout to ground.
This is the method to simulate open loop characters.

rockycheng said:
Thank you for the reply! It is just a normal cascode OTA. I've tried the following test circuit to see the gain vs. frequency transfer function. The result is just the same without the feedback. Would anyone tell me why and when to use the circuit like this? Or it is useless at all?

op amp ac simulation

You can find some guideline in the book of " the designer's guide of spice and spectre"

You can do step responce simulation.

Hi

You can try this.. I found it and it helps me to understand how to test Op Amp

pouya444

### pouya444

Points: 2
how to do ac simulation of op amp in cadence

How to do ac simulation in cadence for an opamp?
Answer: Analog Artist -> Analysys > AC > Enable.
You can try different values for diffeent responses.
Hope it helps!

how to do ac simulation

you can find the reason in the book ' CMOS analog circuit design' by Allen(chapter 6)

I need such kind document.

T=RC must vrey large

search google with keywords: rinco mora, he has a few pdf pages explainging how to do loop breaking ac simulation. extremely good. you will get full understanding from it.

spectre op amp ac simulation

rockycheng said:
I heard about some tricky ways, using large resistance and capacitance to form a feedback. But I'm not quite sure. Could anyone tell me the exact way? Thanks a lot.

Regards
rockycheng

Using spectre

guide to spice & spectre

designer's guide spice and spectre.you will google some lab notes.

you could use iprobe and stb

the stb is interesting! thanks a lot

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