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howtotest if amplifier has PM 70 degree

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Member level 3
Sep 27, 2009
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PM 70 degree

I have designed the 2 stage miller opamp. I am trying to test its performance. I have to meet 70 degree PM. I dont know how to test that. Plz help

connect one of the input to some dc voltage....then connect the other input to an ac source with the same dc bias and give it a little swing...

For example:
fix Vin+=1.5V
Vin-=0.1V swing with dc bias of 1.5V

now you get the output
plot the magnitude and phase response....
now observe the phase at gain of 0dB.....this gives you the phase margin.....

If your phase margin is not 70 degrees ,adjust the miller capacitor....

Hope this helps you....

adding some more details here;

as suggested add a dc bias to vin+ node and similarly add to vin-
v1 vin+ 0 1.5
v2 vin- 0 1.5 ac 1
** ac source of 1v added to perform ac analysis.
** 1v magnitude is preferred as it gives direct magnitude in db's and u need not ** do any further calcultaion.

perform .ac simulation using .ac statement, plot vdb(out) and vp(out)
check phase at vdb(out) = 0 and calculate phase margin.

do put expected load on output node as per your specifications...

DC bias: does it mean common mode input..?

how did you choose it as 1.5V..?

PS: i am working in 0.5 micron and 5V process.

really sorry for writing 1.5v there.
yes ,it is common mode input voltage in which u are expecting ur desing to work.

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