Please explain me this opamp design

[vistapoint]

opamp design question

I am a newbie coming across a opamp designed by a guru:



Are those caps on the right-top corner used for cancelling zeros?
And C0-C3 for Miller compensation?
What are those transistors connected by 1.15V used for? they connect cpp/cnp to the input transistors' source.

 

[Vamsi Mocherla]

Re: opamp design question

Well, this circuit is a simple two stage differential amplifier with the first stage being folded cascode. The caps C0 - C3 are indeed for compensation but not miller. They are for Cascode compensation technique.

Coming the transistors connected to 1.15 V, I am not sure where the sources are connected. Mostly they will be for providing the lead compensation and hence might act as resistors.

 

[vistapoint]

Re: opamp design question

The output class-AB control transistors M25-M28 have the same size. Is it a design flaw? It seems to me that the PMOS & NMOS should equally devide the current in quiescent?

 

[Vamsi Mocherla]

Re: opamp design question

You are right, the CLASS AB control should be ratioed as per the current flowing. Then the PMOS and NMOS have to be sized as per the current, giving similar VGS values

 

[vistapoint]

Re: opamp design question

The output transistors are in right ratio. I was talking about the two pairs of NMOS/PMOS connected in parallel before the gates of the output transistors. They are used to control the voltage difference between the output gates. If you still feel like to answer the question, you may need to check the schematic. Thanks.

 

[srivatsan]

Re: opamp design question

Somehow I am not able to see a clear picture of the circuit. Could you post it again? Thanks.
srivats

 

[vistapoint]

Re: opamp design question

You can download if it's still not clear.

btw, I stongly recommend imageshack.us/ for sharing schematics. I guess it's quite necessary to have a picture when we talk about circuits.

 

[layes2]

Re: opamp design question

ok
it is for common mode feedback

 

[alvays]

opamp design question

M25~28 and M40~43 with controlling signal pc and pd cooperate to function as turnon/off. Is it?

when pc and pd equal to be vdda and gnda, system will work on or down.

 

[vistapoint]

Re: opamp design question

The right hand side circuits are for CMFD.

M25-28 are not for power down/on. They are connected to bias. They should be for output stage voltage level shift.

BTW, the bias current of the current mirror is 200nA. Does it mean the circuit works in sub-threshold region?

 

[alvays]

opamp design question

I missed some transistors just now.
yeah, I also think M25-28 is for level shift, because the complementary amplifier of second stage

and M16, 17 function as CMFD

but under control signal of M44~45, node opn&onn can be gnda. Also node opp&onp can be vdda. So that the all circuit can be turn off. Is right?

where did node ss connect to? The function of c0~c3 is unknown to me.

 

[vistapoint]

Re: opamp design question

M25-28 are for level control. I just read from some books. It's suggested pmos/nmos should be equal to the output transistor ratio. But in the schematic it's not.


C0-C3 they call Cascode Miller Compensation.

I have doubts with the caps on the up-right corner. They are connected to some switches. I doubt they are like resistors with caps for zero cancellation. But I am not sure.

ss is the source of the two input nmos transistor. Check out the sch. I am still not clear why it's connected to 1.15V through transistors.