Does hfe matter much for simple bipolar transistor buffer?

[chumbox]

Notice a lot use 2N5088s etc but I only have 2N2222As kicking around.  My guess from reading some AMZ articles (brilliant as usual) is I won't even notice the slightly under unity gain difference but just wondering others experiences?  Or even whether someone can explain how hfe even effects a buffer?

[R.G.]

Current gain affects buffer input impedance.

As a crude approximation, the input impedance at a simple NPN buffer's base is hfe times the unbypassed emitter resistance. If the emitter resistor is, for instance, 10K, and you have an hfe (at the selected operating current and temperature...  ) of 100, then the input impedance is (crudely) 100*10K = 1M.

That's the simplistic view of just the input impedance at the base in isolation. In fact, any loading from bias networks for the transistor appear in parallel with that, and any loading on the output interacts with the effective value of the emitter resistor, because it appears in parallel with the emitter resistor, too.

So you won't get 1M by using a 10K and an NPN with hfe=100.

But using an NPN with an hfe of 500... that helps a lot. Notice that it helps more than raising the emitter resistor. And sometimes you *can't* raise the load it's driving - that's why you wanted a buffer to start with.

So no, it doesn't matter much, unless you have one of those situations where what drives it and what it drives makes it matter.

The devil is always in the details.

But if some buffer is better than no buffer at all, stick what you have in the NPN position, and order super-whiz high gain ones to go in when you can put them in.

[chumbox]

Thanks R.G.  Once again a great explanation.  I'll socket it for now and play with it when I get some parts.

[PRR]

Use the '2222s for now, they ARE a great part; but order a baggie of 2N5089 the next time you order parts.

I was taught to "assume hFE>50" (so that '2222 was overkill), but sometimes knowing that you paid for hFE>400 makes a design work a bit better. And today the cost difference is about zero (in our under-30V world).

[chumbox]

Thanks Paul.  Yeah the reason I have so many 2222s is because I love them.  But a restock of everything is due