BJT buffers

[edvard]

I'm talkin' emitter followers here.

Why BJTs, you ask?
I have been messing around with FET switching, which necessitates some sort of buffer.
Most pro stuff I've seen schematics for use BJT buffers on the output, if not both.
A FET source-follower was making my circuit oscillate (high-gain 3-stage CMOS inverter stuff).

So-
PROS:
Decent input/output impedance.
Smaller PCB footprint than a Opamp.
More consistent than FET.
Cheaper than both.

CONS:
Lower input impedance than both FET or Opamp.

QUESTIONS:
Any other Pros/Cons?
Why would the FET source-follower cause the circuit to oscillate?
Why would a PNP buffer cause noise from my volume pot?
Since most FET-switch circuits require a buffer on the input AND output, what's the impact on clean tone with two buffers in series?

[merlinb]

QUESTIONS:
Any other Pros/Cons?

Single transistor buffers of any type look pretty pathetic by comparison with an opamp buffer. They can't source current in both directions and gain might be only ~0.9.

Why would the FET source-follower cause the circuit to oscillate?

Possibly because its high input impedance is more sensitive to stray capacitive coupling through the air, from some other part of the circuit, than a BJT. A simple BJT emitter follower actually has a rather pathetic input impedance (at least the variety we use does).

Since most FET-switch circuits require a buffer on the input AND output, what's the impact on clean tone with two buffers in series?

If they're good buffers like opamps, then none at all. Simple emitter followers, however, do produce some distortion, volume loss and possible slew-limiting, depending on usage. Such things could indeed become audible, cumulatively.

[Gus]

Look at the bootstrapped EF s in some of my schematics.

Yes you need to be mindful of oscillations in followers.  Simple ways to control this look at the EF in some wha circuits for one.

Also note you can offset the emitter from 1/2 depending on what the EF is driving

Google, ASK, look in books etc  EF stability.