how to analyse this circuit

[wjxcom]

Hi, all, please help me to analyse this circuit. This circuit is a OP, an I do not know what's the role that capacitance C1 and C2 play.

 

[rockycheng]

C2 is the compensation capacitance. It decreases the magnitude of the dominant pole, and thus the unity gain frequency. This helps increase the phase margin, and make the system more stable.

I think C1 which is connected to the source of the cascode transistor may be with the same use. It would be nice if someone could explain it more clearly. Thanks.

 

[bageduke]

The purpose of C1 is also for compensation. The difference between C1 and C2 is that C1 is to increase magnitude of output pole by factor of gm18*rds18. At the same time, C1 will create a LHP zero which will also help to compensate the circuit. If you check the output resistance of this diff amp, it became 1/(gm22*gm18*rds18) which is very small.

 

[rockycheng]

bageduke,

Could you give the final output pole with the extra compensation?

And I don't understand why the output resistance can be reduced. From your reasoning, it seems that without C1, the output resistance is 1/gm22. But I think it should be (rds22 // rds21).

Thanks a lot.

 

[tsanlee]

This is a two stage amplifier.
the C1 and c2 are compensation cap.
the Compensation Pole in C2 is small than it in C1
,because the impedance of C2 is more than it in C1.


Notelease take care the DC bias of M23 M18

 

[bageduke]

1)The output pole with C1 should be at same magnitude of gm18*rds18/(rds21||rds22)*CL, CL is opamp external load capacitor.

2)You can see M18 as single gate feedback with gain of gm18*rds18, now output resistance became (rds21||rds22)||1/(gm22*gm18*rds18)≈1/(gm22*gm18*rds18)