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Design >> Analog Design >> what is the role of this capacitance?
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Message started by wjx197733 on Feb 8^th, 2006, 5:08pm

Title: what is the role of this capacitance?
Post by wjx197733 on Feb 8^th, 2006, 5:08pm

Hi, all: please look at this circuit. In this circuit, I think capacitance C2 is miller capacitance, but what is the role of the capacitance C1? How to confirm the value of C1?

Help me, please!!!

Title: Re: what is the role of this capacitance?
Post by steven on Feb 8^th, 2006, 6:31pm

Isn't it still a Miller cap? Because the stacked PMOS pair is cascoded with the NMOS pair. So the cap is connected from the output node to the previous stage.

Title: Re: what is the role of this capacitance?
Post by Paul on Feb 8^th, 2006, 7:43pm

Hi,

As Steven says, both are Miller capacitors. I have seen schematics using C2 in the past, but I haven't seen a circuit using both capacitors at the same time. Notice that both nodes don't have the same conductance, so the pole expression in presence of both capacitors may be somewhat different. Would be interesting if someone had time to write the equations...

Paul

Title: Re: what is the role of this capacitance?
Post by steven on Feb 8^th, 2006, 8:15pm

Paul wrote on Feb 8^th, 2006, 7:43pm:

Hi Paul,

I think the circuit would introduce zeros in the response. If considering the Thevenin impedance looking from the drains of M15 and M19, the impedance will be similar to common-gate topology. At high-frequency, the feedback network becomes inductative. I guess in this respect, either of them is redundant.

Regards,
Steven

Title: Re: what is the role of this capacitance?
Post by panditabupesh on Feb 11^th, 2006, 9:48am

It is cascode compensation. Not sure why both, miller creates a zero (because of the feedforwad), and cascode by eliminating the feedforward path is supposed to get rid of the zero.

-bp