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How to get an I-V curve like this?

Negative impedance converters implemented with OpAmps have an I-V curve as shown below: an input voltage may correspond to multiple currents. This is because the state is stable whether the OpAmp is saturated or not. I'm trying to investigate this circuit and would like to get an I-V curve shown as below. I've tried to give an initial condition (0V) to the output of the OpAmp, but it doesn't help in the DC simulation - the OpAmp still saturates to VDD anyway. Is it possible to achieve this in Cadence with Spectre?

Frank Wiedmann over 1 year ago

Have you tried giving the initial condition at the input of the opamp?
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Doronzzz over 1 year ago in reply to Frank Wiedmann

Yes, and it didn't make much difference.
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Frank Wiedmann over 1 year ago in reply to Doronzzz

Is there any capacitance between the nodes where you are setting the initial condition?
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Doronzzz over 1 year ago in reply to Frank Wiedmann

The input stage of the opamp is a differential pair, so there might be some parasitic capacitance between them? but not an explicit capacitor.

I tried to make V+(0)=0, and V-(0)=VDD, but in the DC sweep simulation, the output still saturated to VDD. I confirmed that there are multiple states indeed, because if I don't do the sweep (just plot the DC operating point at a single point), the result is different and the opamp doesn't saturate at all.
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Frank Wiedmann over 1 year ago in reply to Doronzzz

I was asking about the capacitance because it would stabilize the initial condition. Now that you are saying that you are getting the intended results with a single dc analysis but not in a dc sweep, I'm thinking of the possibility that the sweep might make the initial condition ineffective. I suggest that you take a look at support.cadence.com/.../ArticleAttachmentPortal
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