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

I think Vt is 26mV other than 0.5v