nus_lin wrote on Jan 15th, 2011, 2:31am:dear rg, it is a textbook structure, with two pmos on the top, two bjt in the bottom, a resistor connects to on of the bjt with bigger size, and an ideal amplifer.
if you do derivation to the current output, you will see how good is your ptat.
Most probably the circuit in the text book neglected, among other things, the effect of the bjt base current to simplify the calculations.
This, on top of the pmos not being ideal as avlsi suggested, might be the sources of your circuitīs non-ideal behavior over temp.
Best
Tosei