LM117-type bandgap: Better for matched currents or matched densities?

Question

I teach an electronics lab to undergraduates, but I'm a control engineer and so sometimes I feel a little out of my element. In one of our labs, we discuss several different voltage regulators, and I put together a document discussing the bandgap mechanism in the classic LM317:

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In that document, I discuss the method National Semiconductor uses -- it uses a current mirror to push two identical currents through transistors that have different emitter densities. I also mention that National *could* have made its mirror with different emitter densities to setup ratio-ed CURRENTS through transistors with the same emitter densities.

It seems like every example I've found uses the choice that National used for its LM117. Is there a reason for this choice? Is there a reason why everyone seems to pass a matched current through ratioed densities instead of ratioed currents through equal densities?

Thanks -- Ted

hahn.alan · Accepted Answer

Not sure Ted, probably ease of fabrication and better thermaltracking. Ask the chemical engineers.

miso · Answer

This is really an interesting question. Maybe a Monte Carlo analysis might yield a difference. Generally if I had to do a precision current multiplier, I would use some emitter degeneration due to VBE mismatch. Maybe the current mirror accuracy is more sensitive to component matching than the N:1 in the NPNs.

Jim Thompson · Answer

1:1 Matched currents are easy to obtain with accuracy and stability.1:N is not.Why do you talk of Q17 and Q19, but call them PNP's?  They're NPN.Do you know Todd J.... ?  We are collaborators.                                        ...Jim Thompson-- | James E.Thompson, P.E.                           |    mens     || Analog Innovations, Inc.                         |     et      || Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    || Phoenix, Arizona  85048    Skype: Contacts Only  |             || Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  || E-mail Icon at  |    1962     |              I love to cook with wine     Sometimes I even put it in the food

Ted Pavlic · Answer

Okay. But I'm guessing that there are similar 1:N-type problems with the delta_(VBE) generator. Right?

Is it the case that the problems you'd have with generating the matched currents are going to be more than the problems you'd have with generating the ratioed current densities?

That was a typo. I wrote the second part before the first, and I copied and pasted and evidently didn't catch everything. I've made the fix and updated the live docs. Thanks for pointing that out.

I don't think so. Is he a buckeye?

When I'm not teaching undergraduates about circuits or control, I'm usually collaborating with behavioral ecologists on bio-mimicry projects. Do any work with leaf cutter ants? :)

Thanks for your help -- Ted

Michael A. Terrell · Answer

According to Bill's rants, all you have is coconspirators.

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Jim Thompson · Answer

Yep ;-)                                        ...Jim Thompson-- | James E.Thompson, P.E.                           |    mens     || Analog Innovations, Inc.                         |     et      || Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    || Phoenix, Arizona  85048    Skype: Contacts Only  |             || Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  || E-mail Icon at  |    1962     |                  The truth is incontrovertible                     Democrats may attack it                 Ignorant Liberals may deride it                   But in the end, there it is                  - Winston Churchill (edited)

Michael A. Terrell · Answer

Now he can rant all he wants. I'm forward his whining to localhost. :).

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There are two kinds of people on this earth: The crazy, and the insane. The first sign of insanity is denying that you're crazy.

LM117-type bandgap: Better for matched currents or matched densities?

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