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About OP AMP compensation

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fengluan

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In a two stage op amp,use a resistor in series with compensation capicator .As the resistir increase,the zero will move into the left-plane.sometimes ,this LPZ shift the phase posetive and the phase margin large than 90, then is it still stability?

thanks
 

yes i think , but i also think that the loop settling will be slow
 

You bet! For all kinds of negative feedback on Bode plot it's of open loop amp it's should prevent situation when simultaineously gain cross 0 dB and phase equal 180 or more then 180. Thus ever for closed loop configuration with strong negative feedback (unit gain) the circuit remain stable. The phase margin may be more that 90, it correspond stability condition.
Keep in mind that position of poles and zeros may change during large transients. Always verify stability for input voltage step.
 

btw what is magical about 90 , why didnt u say above 80 or 110 , the whole idea about stability is as Denis said
 

Y'd better not to introduce a zero before the GBW
 

fengluan said:
In a two stage op amp,use a resistor in series with compensation capicator .As the resistir increase,the zero will move into the left-plane.sometimes ,this LPZ shift the phase posetive and the phase margin large than 90, then is it still stability?
thanks
Click to expand...

Have a look at the gain margin as well. Usually when the PM>90deg, gain margin will be very less. It's preferable to have a gain margin of atleast 10db (this is a rough figure, which someone told me.)

Also, I agree with pfd001
 

It can be stable, but speed is too low
 

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