what's the function of this circuit.

[walker5678]

Attached file is a circuit which is the output part of a full differential op amps. It is one side of the output stage.
M29 and M30 is the push-pull common source output stage. The gate signal of M29 and M30 is from the output of the floating bias circuit of the folded cascode input stage.
But i don't know what is the function of the circuit from M1 to M28. Seems like the over current protection circuit? Anybody can help me to understand it? Thanks a lot.

Best regards,
walker

 

[Fom]

Where is an attachment?

 

[walker5678]

have added.

 

[Fom]

Next time use more compact format for yours picture. I think some people just don't want to lost their valuable points to see your picture.
For example GIF or PNG format. I convert your circuit into GIF format.

You are right. It is push-pull amplifier. But I didn't see any overcurrent protection.
Besides push-pull amplifier output current is limited itself by push-pull principle.
The most of transistors just biased current sources.
Simulation can help understand it in full detail, but you need to know sizes of transistors and resistors value.
As I understand NMOS M21, M10, R3, C1 and M21 and the similar mirrored PMOS components are DC negative feedback that help bias M29 and M30 to limit quiescent current. But this this feedback can reduce AC gain also. So this feedback suppressed for AC signal by C1. I.e. for AC signal this negative feedback doesn't work.
Another thing. I think the current of all mirrors is too low (because all this currents are included in quiescent current). The mirror current is not enough to control the gate of M29 or M30 efficiently. So there is another positive feedback that help to increase a slew rate of the gate signals of M29 and M30:
M21, R7, M22 and M28 (with mirrored PMOS component).
That's my idea at first glance.
I hope it help you.

 

[walker5678]

HI Fom,
Thanks for your kind explanation and converting my picture.
My confusion is that M27 is a negative feedback and M28 is a positive feedback, so when gate signal of M21 and M30 change the current of M27 and M28 will tend to change in different direction, but their current should be equal, so how they work together?
BTW, the joint terminal between R6 and R7 is connected to out+.

best regards,

 

[Fom]

If connection point of R6 and R7 is OUT+, that make the circuit more explainable.

the current of M27 and M28 will tend to change in different direction,

Yes. But gate of M27 will respondes with delay, so it will has no effect on AC signal.
But for DC signal... For example let suppose the Vcc is doubled or change to 2*Vcc. Vout+ is also changed from Vcc/2 to Vcc. As a result current of M28 should be increased. The current M25 also should be increased. So this increased current can flow only trough PMOS and NMOS bias transistors (left side of circuit - not numbered). As a result the gate-source bias of M30 and M29 will be decreased. But is it is happened the pair M21-M10 will return current of M27-M28 to normal operation current.
As to positive feedback sometimes that's called "gain booster".