RobG wrote on Jul 28th, 2009, 7:30am:xwj623 wrote on Jul 27th, 2009, 6:26pm:maybe the bipolars at too high a current density,because it is 3.2um*3.2um device whihc fab support, while flow a 4uA current.
the pnp dimension can't change according PDK,i use 1:8 structure,
is it a good methord to use 4:32 considering layout and process?
There is nothing wrong with using 4:32 or 1:8, but too high or too low a current density will cause errors like you have shown. A proper test would be to bias both the 1x and the 8x transistor with the same current and check the difference in voltage over temperature. The difference in voltage between the two bipolars should not depend on the current (it should be nkT/q*ln(8)). So start at a low current (like 0.5uA) and keep increasing it until the difference in voltage starts to change from nkT/q*ln(8). Bias the transistors somewhere close to where the voltage starts to change.
1. i use the same res type whihc has a negative temp coefficient.
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a very large negative temp-co resistor will turn the curvature upward. To test this, try setting the temp-co to zero or use a different resistor type
Quote:2. i am not quite understand what you are meaing.
Plot the difference in voltage between the input terminals of the opamp over temperature and post it here. Also tell me what the ratio of R2a to R2b is. If you do that I might be able to explain it to you.
rg [/quote]
Power supply is 1.2v ,vref is 0.6v.
R1 is about 28k,
R2a=R2b is about 140k.
Ma:Mb=1:1.
i want to know the upward curvature whether due to the high current density of PNP or due to negetave resistor?
the negetave resistor coefficient is about -145 ppm/C.
thank you.