The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> [Razavi] CMFB circuits: a couple questions
https://designers-guide.org/forum/YaBB.pl?num=1452222928

Message started by zummingsky on Jan 7^th, 2016, 7:15pm

Title: [Razavi] CMFB circuits: a couple questions
Post by zummingsky on Jan 7^th, 2016, 7:15pm

Hello, I'm studying CMFB on Razavi's "Analog CMOS IC Design", I have a couple questions and I would really appreciate some help.

At page 323, the author proposes the two following CMFB circuits:

1) The author says that if we neglect channel-length modulation and choose (W/L)15 = (W/L)9, (W/L)16 = (W/L)7 + (W/L)8, then for Vout,CM = VREF ID9 must track ID15 (in both circuits).
I think that ID9=ID15 only if the sources of M15 and M9 are at the same potential, but how does the circuit force Vsource15=Vsource9?

2) In what sense the second circuit gives a more accurate definition of output MC level?

At page 324, a bit different circuit is proposed.

Razavi says: "Transistors M17 and M18 reproduce at the drain of M15 a voltage equal to the
source voltage M1 and M2, ensuring that VDS15 = VDS9."
How do the two conditions (W/L)17,18 = (W/L)1,2 and Vgate17,18 = Vgate1,2 guarantee Vsource17-18=Vsource1-2?

Thanks in advance for your help.

Title: Re: [Razavi] CMFB circuits: a couple questions
Post by AnilReddy on Jan 9^th, 2016, 7:06am

Hi,

1) The author says that if we neglect channel-length modulation and choose (W/L)15 = (W/L)9, (W/L)16 = (W/L)7 + (W/L)8, then for Vout,CM = VREF ID9 must track ID15 (in both circuits).

That is the equilibrium point for the circuit because what else can be!. Just write down the source voltages and see and they should be the same. That's why the currents will track more or less.

I think that ID9=ID15 only if the sources of M15 and M9 are at the same potential, but how does the circuit force Vsource15=Vsource9?

Just take a little bit of difference between Vout1 & Vout2 and see they will finally come to VREF voltage.

How do the two conditions (W/L)17,18 = (W/L)1,2 and Vgate17,18 = Vgate1,2 guarantee Vsource17-18=Vsource1-2?

what is the exact confusion in this question?

Regards,
Anil Reddy

Title: Re: [Razavi] CMFB circuits: a couple questions
Post by zummingsky on Jan 12^th, 2016, 8:47am

I followed your suggestions and I solved my doubts. Thanks :)