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opamp design

1234v

hai all,  

i hav designed an "opamp" with Vdd= 5v; Vss= -5v; V+ = 500mv (ac magnitude); V- =-500mv ( ac magnitude);   with these specifications , when i tried to plot Vout vs frequency :  the output magnitude is 40Kv upto some freuency and then starts falling down to zero volts( linear scale).  help me in interpreting  the results. 

 

thank you

  • An ac analysis in a circuit simulator (assuming that's what you did; you didn't say) is a small signal analysis. This means that it is (generally) linearizing the circuit equations about the DC operating point - and hence assuming that everything is linear - an assumption that holds for a small signal. Given that you have a differential input of 1V, the output is about 40kV which means you have a gain of 40,000 at low frequency, rolling off at higher frequencies. Of course you wouldn't really get 40kV out of the opamp - you're only seeing this because it's a linear model and hence no clipping of the signal is happening. It's quite common for people to put 1V into an ac analysis and see what comes out and treat the result as gain.

    This is pretty basic stuff - I'd have thought you ought to be able to read this in a book somewhere or consult a colleague.

    Regards,

    Andrew.

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