Vbe Matching. How much current?

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

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

John

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

Hardy

snipped-for-privacy@comcast.net

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.

WTF are you up to ?

..... Phil

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!

"Phil Allison"

Since devices in series pass the same current.

Make up your mind.

Cos you a talking drivel at the moment.

.... Phil

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.