hello,
it works in active region, becaz when you have Vbc=0, collector base junction still reverse bias. You need some finite Vbc to drive the transistor into saturation. regarding your 2nd Question most of the times it depends the modeling, so now a days Quite common model is gummel poon model.

Thanks,
Raj.

If you have a quality Bipolar transistor,
why not use one of the classic biplolar BG's?

These devices will be less of an issue with
offsets and mismatch issues.

If this is the lateral PNP available on CMOS, then
the diode approach is what you are stuck with.

If you got a Beta around 60-120 thats a good bipolar,'
if its Beta 5-20 thats one of the junky ones.

you got access to C,B,E  of the transistor, or is the collector tied to ground?

collector tied is generally a lateral junk PNP avaialable on CMOS.

Dan Clement wrote on Jan 10^th, 2012, 6:41pm:


Dan,

I would suspect that you didn't get good models. I haven't made so many bandgaps (maybe 2-3 spread over a long period of time), but we made some "corrections" to the manner in which our models were being fitted and then we suddenly started getting good fits whereas we were condemned to having to trim the bandgaps earlier in every single case.

Of course, it also depends on the kind of precision you are trying to get but I would say that 1% accuracy should be possible without trimming if your models are accurate.

Vivek

mixed_signal wrote on Jan 8^th, 2012, 7:30pm:


I don't understand what your "doubt" is. The simulator will work fine if Vbc = 0 V.

Maybe the problem is with the "saturation" term. Everything was great in the world in the old days when bipolars ruled. All you had to remember was saturation is bad. Do not operate in saturation! Then came MOS and the dummies called the normal operating mode "saturation."

So don't get confused:
Bipolar: Saturation is bad!!! In saturation BC collector diode is forward biased and the device no longer acts like a transistor.

MOSFET: Saturation is good! In this region MOS acts like a transistor!

one more point I forgot to mention which often causes much grief is not just the modelling of BJTs but that of resistors.

If the resistor tempco is not properly characterized or if you are using dimensions vastly different from those used in model extraction, then you are going to be way off.

Another point where I have seen some careless data extraction from silicon is when there is more than one resistor type available in the process. It may happen that the different resistor types were not all characterized thoroughly on their own and that the results were bunched together (SHOCKING), especially for more expensive measurements like tempco. In that case, you may do all you want and end up tuning your bandgap perfectly at the wrong point. This is quite hard to catch but you may be able to figure out this if you see same tempco quoted for all types of resistors.

Unfortunately, the last of these errors is almost impossible to detect for you as a designer. So, a good solution might be to make a copy of your bandgap with probe pads and suitable strucures for accurate measurement of resistances so as to be able to measure out critical parameters such as resistance, Vbe, tempco and check how well they fit to your simulations. Obviously, you need to do a fair bit of tricky measurements in this case.

Vivek

duplicate post...