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Design >> Analog Design >> question on PTAT current source
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Message started by zychang on Feb 3^rd, 2006, 6:55am

Title: question on PTAT current source
Post by zychang on Feb 3^rd, 2006, 6:55am

Hi all,

I have a quick question here on PTAT current source. The circuit I use is the simple PTAT current generator on Razavi's book(P391). It has a bipolar core with a supply-independent biasing scheme on top of it (please refer to the attachment). According to the book, if the NMOS and PMOS pairs are identical, the output current is going to be (VtlnN)/R. In my case, Vt equals to 0.68, N=5 and I want the output current to be 25uA. So it seems that I need a very large resistor in the middle (something like 50k). However, the simulation shows a 7k resistor is more than enough. I cannot figure out what is going on here. any suggestions will be appreciated.

Mike

Title: Re: question on PTAT current source
Post by ACWWong on Feb 3^rd, 2006, 3:07pm

Vt means kT/q. ie 26mV @ room temp.... not threshold voltage....

Title: Re: question on PTAT current source
Post by wjx197733 on Feb 5^th, 2006, 7:50pm

I think Vt is 26mV other than 0.5v

Title: Re: question on PTAT current source
Post by jjsnail on Mar 9^th, 2006, 2:52am

ACWWong wrote on Feb 3^rd, 2006, 3:07pm:

Vt means kT/q. ie 26mV @ room temp.... not threshold voltage....

iAnd 70k seems not very large...?