Vbe Matching. How much current?

Hi Group: I've built a test fixture for matching PNP and NPN medium power driver pairs. Right now, I'm measuring Vbe at 100uA collector current based on articles I've read. My question is should I also check higher currents?

500uA, 1mA, 10mA? My thinking is 100uA won't heat the DUT and will minimize self heating. I'm assuming if the Vbe is closely matched at 100uA it will remain matched at 20mA if the temp is the same.

Any suggestions? comments appreciated.

Thanks

Reply to
<r.laury
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You should match them at the current you intend to operate at... there is an emitter contact resistive variation as well as that of "IS".

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Why are you matching transistors? It's a lot easier to design circuits that always work, over the full range of actual parts.

John

Reply to
John Larkin

PNP and NPN BJTs? They are obsolete. CMOS is your friend.

Hardy

Reply to
HardySpicer

snipped-for-privacy@comcast.net

** Huh ??

You want PNP and NPN power devices to Vbe track EACH OTHER precisely ??

Getting devices of the same polarity to Vbe track is hard enough.

** Vbe matching is generally done for diff pairs and when power devices must operate in parallel.

WTF are you up to ?

..... Phil

Reply to
Phil Allison

It's basically a power opamp. I'm using the LM394 as the diff input pair. It's matched. I want to insure the output pair BD137 and BD138 are matched at an idle their current. If they are not matched in current gain, then there will be an offset at the output. All the app notes I've read on this subject recommend Vbe matching. Are you suggesting this is not required?

Thanks for your input!

Reply to
<r.laury

"Phil Allison"

** Unless you have an odd topology, matched idle current in an output pair is automatic.

Since devices in series pass the same current.

** So it is a Hfe or Vbe match you are wanting ??

Make up your mind.

** How about you post a schem somewhere.

Cos you a talking drivel at the moment.

.... Phil

Reply to
Phil Allison

There are two topologies you could be using, either emitters-connected together (a follower stage) or collectors-connected-together (a voltage gain stage).

In case of a follower, you want matching (roughly) of the transconductance, because roughly equal Vinput is the interesting input drive condition. If it's a voltage-gain stage, the more important quantity might be the current gain (beta), because typically matching drive current is fed to the transistors.

In neither case does the forward voltage have much relevance.

Reply to
whit3rd

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