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buddypoor

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Bremen, Germany

Re: Few questions
Reply #15 - May 22^nd, 2009, 1:16am

raja.cedt wrote on May 21^st, 2009, 12:19pm:

hi buddy poor,
in the first what i mean to say for finding dc gain both methods has to give same answer, how capacitive coupling will effect? or could you please explain how to find dc gain?
for second question i derived expression for output impedance and substituted all small signal parameters (gm,gds) from .op result and i have same operation point in both cases?

Thanks,
Rajasekhar.

During dc analysis all capacitors are neglected (set to zero Farad).
Therefore, if caps are there, results of dc and ac analyses are different.
Regarding the output impedance, I would trust the simulation results.
Probably your calculations using formulas are not correct.
Which formula did you use ? Is the circuit a simple common source amp without any feedback?

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LvW (buddypoor: In memory of the great late Buddy Rich)

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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: Few questions Reply #16 - May 22^nd, 2009, 2:49am hi buddypoor, it is a single stage differential op amp with unity gain feedback.I used r=(rp//rn)/(1+A) where a=gmn*(rp//rn). thanks, rajsekhar.
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buddypoor

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Posts: 529
Bremen, Germany

Re: Few questions
Reply #17 - May 22^nd, 2009, 5:15am

raja.cedt wrote on May 22^nd, 2009, 2:49am:

hi buddypoor,
it is a single stage differential op amp with unity gain feedback.I used
r=(rp//rn)/(1+A) where a=gmn*(rp//rn).

thanks,
rajsekhar.

On May 20th you spoke about a common source amplifier; now it is an opamp ?
Can you, please provide a circuit diagram ?
I cannot follow you, because an opamp with unity gain feedback
does not need any resistors. What is rp and rn and gmn ???

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LvW (buddypoor: In memory of the great late Buddy Rich)

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