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Design >> Analog Design >> Question about slew rate of fully differential amp!!
https://designers-guide.org/forum/YaBB.pl?num=1240563730

Message started by Yutao Liu on Apr 24^th, 2009, 2:02am

Title: Question about slew rate of fully differential amp!!
Post by Yutao Liu on Apr 24^th, 2009, 2:02am

Hi everyone,
I have a schematic of fully differential amp shown below. I am sure that the negative slew rate is I9/Cc. But I am confused about the positive slew rate.

In my opinion, M1 and M3 is shunt down when a large step signal is stimulated in vin+, so only the current from M11 would charge the compensation capacitor Cc and CL. Therefore, the positive slew rate seems to be I11/(Cc+CL).

However, I don't understand whether the current of M12 affects the slew rate and how it affect the circuit.

Can anybody help?

Thanks a lot!

Title: Re: Question about slew rate of fully differential amp!!
Post by subgold on Apr 24^th, 2009, 3:09am

based on what you have described, you still treat the amp in a single-ended manner. for fully differential signal, M11 and M12 are playing the same role on slewing, one for positive slewing and the other for negative slewing.

Title: Re: Question about slew rate of fully differential amp!!
Post by Vikram C on Apr 24^th, 2009, 3:42am

Hi, what i am getting from the schematic is that if you use it in fully diff configuration, when V0+ rises, V0- falls. So, M12 current will definitely affect the slew rate, when Vo- is rising & V0+ is falling.

If u r not taking, differential output and taking single ended output, them M12 current ain't very imp for slew rate.

Please tell me if i missed something.

Title: Re: Question about slew rate of fully differential amp!!
Post by Yutao Liu on Apr 24^th, 2009, 4:08am

so, what is the slew rate of this circuit when treating it as fully differential amplifier?

Title: Re: Question about slew rate of fully differential amp!!
Post by thechopper on Apr 25^th, 2009, 6:41pm

Hi,

I would say that, since you are driving a capacitve load M9,10,11 and 12 will have larger bias currents than M3,4,5,6. Therefore the -ve slew rate will be given by I3/CC and I6/CC while the +ve slew rate will be I4/CC and I5/CC. In this case I3=I6 under -ve slew rate condition and I4=I5 under +ve slew rate conditions assuming a balanced amplifier.

Hope this helps
Tosei