whether these two amps are stable?

[fengluan]

look at two figs. the PM=90 ,whether they all are stable?

 

[DenisMark]

I think both is unstable. If u have point where gain is 0 dB and phase shift >= 180 deg than in closed loop configuration that circuit will be not stable.

 

[kivvzhou]

I can tell you some rule :
1.IN many cmos circuit design book tell us ,at the UGBW,the phase margin will at least 45deg,but in our practice circuit ,the phase margin usually 60deg ,in some A/D circuit ,the Phase margin will 70~80 deg ;
2.AS a amp circuit ,you have to see the closed AC characteristics ,this will give you the meaning.
3. gain margin :when the AC Frequence to 0deg or -180(the same ),the gain must below a certain value,usually -10db(this can vary as you circuit design requirement),and ensure the gain curve
drop
4.The transition of the ac frequence will 3 multiple of the UGBW ,this ensure the stable

Added after 2 minutes:

so the two amp are unsuccessful design ,they will appear stable problem

Added after 13 seconds:

so the two amp are unsuccessful design ,they will appear stable problem

Added after 10 seconds:

so the two amp are unsuccessful design ,they will appear stable problem

Added after 1 seconds:

so the two amp are unsuccessful design ,they will appear stable problem

 

[flatulent]

Plot these in polar coordinates and see if the trace circles the -1 +j0 point.

 

[fengluan]

look at this fig.

for the bode diagrame like this ,i think it's stable. but if i use the Nyquist Criterion, it seems alse encircle the point (-1,0).

whether i understand "encircle (-1,0) " is uncorrect?

 

[renwl]

look at two figs. the PM=90 ,whether they all are stable?

is it simulation results or theoretical drawing?

 

[fengluan]

it's all the simulation result .but drawed myself, may have some shift from actually result.