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Design >> Analog Design >> a question about overdrive voltage&sub-threshold
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Message started by cetacean on Jun 27^th, 2008, 4:28am

Title: a question about overdrive voltage&sub-threshold
Post by cetacean on Jun 27^th, 2008, 4:28am

In the CMOS Op-Amp Design, it often mention that ensure MOS transistors operate far away from the subthreshold region,describe to operate the transistors wiht a minimum gate overdrive voltage.
Vgs - Vt = sqrt(I*L/uCox*2W) >= Voverdrive,min

but when the current is very low,base this equ ,the W/L would be too large.
so i want to know when it's low current ,how to fix the W/L?

Title: Re: a question about overdrive voltage&sub-threshold
Post by aaron_do on Jun 27^th, 2008, 5:36am

Hi,

why do you say that when the current is small, W/L is large?

If you want the same V_gs-V_t and you reduce I, then you must reduce W/L...

cheers,
Aaron

Title: Re: a question about overdrive voltage&sub-threshold
Post by cetacean on Jun 29^th, 2008, 6:14pm

i'm so sorry to make a stupid mistake. Actually i wanna to say the W/L would be very very small.i.e. i=5uA,and the Vgs-Vth=0.2v in 0.6um CMOS process

Title: Re: a question about overdrive voltage&sub-threshold
Post by aaron_do on Jun 30^th, 2008, 1:20am

Good question. Apart from increasing L, operating in subthreshold, or using some kind of source degeneration...i'm not sure.

cheers,
Aaron

Title: Re: a question about overdrive voltage&sub-threshold
Post by cetacean on Jun 30^th, 2008, 2:50am

Thanks for your answer!
but the MOS transistors turn into sub-threshold region,i/(W/L)shold be less than about 100nA.then the w/l would large enough,in fact too large i think.

how to fix the sizing when the current is small have been haunting me for a long time. :(

Title: Re: a question about overdrive voltage&sub-threshold
Post by Berti on Jun 30^th, 2008, 4:25am

Hi,

I think that sub-threshold is usually avoided in high-speed design because large transistors
are necessary. But an OTA with 100nA bias current is probably not high-speed - and you can operate transistors in sub-threshold region.

Regards

Title: Re: a question about overdrive voltage&sub-threshold
Post by cetacean on Jun 30^th, 2008, 4:41am

I should correct that:no the bias current 100nA,but the current of per W/L is 100nA