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# How to simulate the output resistance of op-amp by hspice?

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#### cqmyg5

Hello All,

I want to know the output resistance of a cascode op-amp, I need this output resistance for farther active filter design. How to get it by hspice simulation?

I tried the ".tf" command, it gave me a very small value, about 0.01.

Re: How to simulte the output resistance of op-amp by hspice

Add ac source at the output, then run ac analysis

out_____||(C1)___+ (Vsource)

gnd_____________-

rout=v(out)/i(c1)

Re: How to simulte the output resistance of op-amp by hspice

cqmyg5 said:
Hello All,

I want to know the output resistance of a cascode op-amp, I need this output resistance for farther active filter design. How to get it by hspice simulation?

I tried the ".tf" command, it gave me a very small value, about 0.01.

1. you put a current source at the output ( from the output to gnd) and put AC=1.
2. Run AC analysis across frequecy.
3. Plot graph v(out) vs frequecy. You will be able to get output impedance.

How to simulte the output resistance of op-amp by hspice?

I have tried this method, I think it's right. You can reference:
1. Connecting your OPAMP to buffer structure.
Run .TF, you can find the output impedance.
It's very small, because it's close loop output impedance.
2. Changing structure to run AC simulation.
You can get low frequency gain.
3. So, your open loop output impedance is:
Ro_closeloop*Avdc
You also can run .OP to prove this method.
I think it is right.

Re: How to simulte the output resistance of op-amp by hspice

In allen's book ,chapter 6,open-loop configuration, table 6.6-2, in the .lis file ,there is a "output impedance",

Then ,what's the relationship between this value and that others have discussed?

thx.

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