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Different op amp open loop gains from analysis
Mar 20^th, 2013, 2:25pm

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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raja.cedt

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Re: Different op amp open loop gains from analysis
Reply #1 - Mar 20^th, 2013, 3:02pm

hello,
some thing wrong in your sim, please show the setup and results,
in most of the cases you would need small signal gain, so please connect in unity gain configuration and apply vdd/2(or some reasonable common mode) at the non-inverting terminal and find the loop gain.don't sweep input voltage for such a wide range in open loop it might saturate.

Keep in mind that large signal gain will always be grater than small signal gain.

Thanks,
Raj.

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batmanbeginz

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Re: Different op amp open loop gains from analysis
Reply #2 - Mar 20^th, 2013, 3:15pm

Thanks, yeah I connected in unity gain configuration and output is not following input exactly [the opamp configuration is folded cascode and I am using verilog A based models in cadence spectre].
I figured out that I was violating input common range, but as I am calculating gain by slope of vout vs vin transition within input common range, hence should it not be similar for transient and DC sweep ?

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raja.cedt

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Re: Different op amp open loop gains from analysis
Reply #3 - Mar 21^st, 2013, 1:22am

why verilog A????

1. Gain by slope of transfer curve...small signal.
2. Gain by dc sweep and find the difference (it is not equal to small signal slope, apply some common mode and find the o/p voltage and again change the common mode now find the difference) hence it is called large signal gain, which is slightly above than the above small signal gain.
3. Gain by transient will be some what different but will be more or less related.

Thanks,
Raj.

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