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Different op amp gains using different spectre analysis

OneNewBoy over 12 years ago

Hi all,

I am new to analog design. I have created an op-amp schematic and tried to get open loop gain by 2 ways:

(Simulation uses cadence virtuoso)

1) Transient analysis: applied a ramp pulse of 0->VDD to +ve input [with a dc value of VDD/2] and kept -ve input to Vdd/2, then plotted Vout vs V+ and took its derivative, this gave me 1300

2) DC analysis: applied a volt of 0->VDD at +ve volt and kept -ve input to VDD/2. Processed Vout as in step 1 and got ~ 50

I cannot believe what makes the 2 gains so different and which is correct ? Voltage supply is from 0->VDD.

Can someone help in this regard ?

Thanks a lot

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Andrew Beckett over 12 years ago

Using skipdc is not a good idea - it means that the transient will have a much harder time converging.
The behaviour you are seeing suggests that your circuit has more than one stable operating point.
Andrew.
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Andrew Beckett over 12 years ago

Using skipdc is not a good idea - it means that the transient will have a much harder time converging.
The behaviour you are seeing suggests that your circuit has more than one stable operating point.
Andrew.
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Vote Up 0 Vote Down

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