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Design >> Analog Design >> questions about PTAT
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Message started by nus_lin on Jan 13^th, 2011, 11:40pm

Title: questions about PTAT
Post by nus_lin on Jan 13^th, 2011, 11:40pm

i simulated a basic PTAT core based on SMIC18MIX technology.

the simulated ptat output (based on ideal resistor) is actually proportional to T-(2E-4)*(T-300)^2, and is nothing to do with the area ratio and the current density (except for very low/high current ). somebody tell me why there a nonlinearity exists?

Title: Re: questions about PTAT
Post by RobG on Jan 14^th, 2011, 6:04am

A proper PTAT source should not have any curvature so you have something non-ideal kicking in. It is impossible to say what it is unless you show us the circuit that you are using.

rg

Title: Re: questions about PTAT
Post by nus_lin on Jan 15^th, 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.

Title: Re: questions about PTAT
Post by avlsi on Jan 15^th, 2011, 9:03am

Hi,

In reality, the pmos are non-ideal, BJT itself is non-ideal.
Please post the simulated response and test circuit to be more clear in terms of non-linearity.

Title: Re: questions about PTAT
Post by thechopper on Jan 17^th, 2011, 4:49pm

nus_lin wrote on Jan 15^th, 2011, 2:31am:

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 circuit´s non-ideal behavior over temp.

Best
Tosei