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13th March 2019, 19:51 #1
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BGAP circuit with opamp
Hi all, I'm having troubles designing this circuit which supposed to be a BandGap circuit with opamp topology (instead of current mirrors).
in theory, the opamp supposed to force V2=Vd using a negative feedback. in simulation you can see that Vd is not equal to V2 (~70mV difference) and if I continue to simulate IR3,IR2 which supposed to be equal as well (I0 in theory) there is also some mismatch and off course V_ref is no const. over temperatures...
This circuit is rather simple in his theory and I think that the opamp parameters are wrong. I've used it from ahdllib (which I am not really familiar with) and configure it as an ideal opamp (high Rin, low Rout, high Gain).
appreciate any help.

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13th March 2019, 21:12 #2
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Re: BGAP circuit with opamp
The opamp output is sitting on the positive rail. It try to raise the top node voltage, but supply is not enough high. The ratio of the r1,r2 resistors are not good I think. r2 should be much smaller, or r1 should be bigger.
"Try SCE to AUX." /John Aaron/

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14th March 2019, 02:15 #3
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Re: BGAP circuit with opamp
I think you want a lot more than a 1:2 ratio of transdiodes'
area. I like 816X on the big side. 2:1 doesn't give you much
"input signal" to work with. And then with the headroom
problem and the inevitable gain reduction when soslammed
you can't close the loop.

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15th March 2019, 20:01 #4
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Re: BGAP circuit with opamp
regarding 
The opamp output is sitting on the positive rail. It try to raise the top node voltage, but supply is not enough high. The ratio of the r1,r2 resistors are not good I think.
R1 determine the nodes current and I have calculated it is sufficient enough (I=Vt*ln(N)\R1 where N number of diodes in parallel). are my calculations wrong ?
I think you want a lot more than a 1:2 ratio of transdiodes'
area. I like 816X on the big side.
I do agree that designing with 8,16,32 diodes will decrease any mismatch but the BGAP circuit should work even with 2 diodes.
As I said, the only "new" thing I have used here is the opamp from ahdlib and I have "configured" it manually, can this be the issue here?
thanks

16th March 2019, 13:24 #5
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Re: BGAP circuit with opamp
There are two things that can be wrong with your circuit:
1. Your loop is clearly not stable which is causing your output voltage to sit near Vdd (3.3V). The opamp output cannot go any higher than this because that is your Vdd! Increasing Vdd will not help. Flip your opamp and see if you get the correct voltage.
2. Your operating point for the output of the opamp is higher than 3.3V in which if you increase Vdd it will help...
I also see your betadc ~ 2 which is quite low! Clearly your BJTs are not biased properly!

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Yesterday, 13:37 #6
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Re: BGAP circuit with opamp
If you still have problems with the OPAmp you should try to add VCVS from analogLib.
"Try SCE to AUX." /John Aaron/
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