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Design >> Analog Design >> why my sub-1v bandgap has upper temp curvature ?
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Message started by xwj623 on Jul 19^th, 2009, 11:38pm

Title: why my sub-1v bandgap has upper temp curvature ?
Post by xwj623 on Jul 19^th, 2009, 11:38pm

my bandgap is 1.2v, but has a upper temp curvature , it seems there is
something wrong with mirror Ma & Mb, but i am not understand why this happens and how to solve it? And is this curvature bandgap works well?

Title: Re: why my sub-1v bandgap has upper temp curvature ?
Post by xwj623 on Jul 19^th, 2009, 11:41pm

waveform

Title: Re: why my sub-1v bandgap has upper temp curvature ?
Post by thechopper on Jul 24^th, 2009, 8:44pm

Channel length modulation on Mb? What is the length for Ma and Mb?

Title: Re: why my sub-1v bandgap has upper temp curvature ?
Post by xwj623 on Jul 26^th, 2009, 9:51pm

HdrChopper wrote on Jul 24^th, 2009, 8:44pm:

Channel length modulation on Mb? What is the length for Ma and Mb?

5u.

Title: Re: why my sub-1v bandgap has upper temp curvature ?
Post by RobG on Jul 27^th, 2009, 7:37am

Who knows, but this often happens when you run the bipolars at too high a current density. The drop across the base resistance causes the upward curvature. You can fix it by using bipolars with larger area.

Opamp offset temperature dependence could also have similar effects. Since you divided down the "signal" the circuit will be very sensitive to opamp offset. Run a simulation with the difference between the two terminals of the opamp and see the variation. Most of bandgap design is choosing the proper current density and building a VERY good opamp. Most bandgaps fail because people underestimate the importance of these things.

rg

PS: I am assuming you are using the same resistors throughout the design. Different resistors mess everything up.

PS2, I came up with that circuit about 1 month before it was published in the red-rag! Isn't that funny how often that happens?

Title: Re: why my sub-1v bandgap has upper temp curvature ?
Post by xwj623 on Jul 27^th, 2009, 6:26pm

RobG wrote on Jul 27^th, 2009, 7:37am:

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?

1. i use the same res type whihc has a negative temp coefficient.
2. i am not quite understand what you are meaing. :(

Title: Re: why my sub-1v bandgap has upper temp curvature ?
Post by RobG on Jul 28^th, 2009, 7:30am

xwj623 wrote on Jul 27^th, 2009, 6:26pm:

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.

[/quote]
1. i use the same res type whihc has a negative temp coefficient.
[/quote]
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. :(

Title: Re: why my sub-1v bandgap has upper temp curvature ?
Post by RobG on Jul 28^th, 2009, 7:33am

xwj623 wrote on Jul 19^th, 2009, 11:38pm:

Did you mean 0.6v, not 1.2v?

In addition to what I said in the previous post, show the ratio of the current of Ma to Mb. Your mirror should be 1-to-1.

Title: Re: why my sub-1v bandgap has upper temp curvature ?
Post by xwj623 on Jul 28^th, 2009, 6:41pm

RobG wrote on Jul 28^th, 2009, 7:30am:

xwj623 wrote on Jul 27^th, 2009, 6:26pm:

1. i use the same res type whihc has a negative temp coefficient.
[/quote]
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.

Title: Re: why my sub-1v bandgap has upper temp curvature ?
Post by RobG on Jul 29^th, 2009, 4:44am

xwj623 wrote on Jul 28^th, 2009, 6:41pm:

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.

It isn't the negative temp-co of the resistor if it is only -145 ppm/C. It might be the bipolars, or the opamp, or the mosfets, or something else, but that can not be known without doing simulations that determine the effects of these items.

rg