tempus fugit said:

Hey all;

I don't know if there's a general answer for this question, but here goes.

I'm reading up on voltage followers (noninverting opamp), and it says that

the gain of a voltage follower is approximately 1. Is the gain a little less

than 1 or a little more than 1, or is there no straight answer for this

question?

The reason I ask is that in the same textbook it says that for instability

to occur, the gain must be greater than 1. I'm looking at using a high speed

opamp to buffer the output of a signal generator, and I don't know if it

will oscillate or become otherwise unstable if I configure it as a voltage

follower.

Thanks

The thing with voltage followers is that:

1) They are low output impedance - damping of oscillations requires

resistance which can imply high impedance, so there can be times when

low impedance could pose a problem. This is the first clue.

2) You do need *total loop* gain to be greater than one *and* a

multiple of 360 degrees of phase shift to get oscillation. The

voltage follower's "unity gain", which is actually <1, is only a

*forward gain* but it pretty close. Further it nearly meets the other

criteria due to *forward* phase by being 0 degrees shift. All you

need is a little *reverse* feedback.

3) They can have non-ideal, non-trivial output impedance. Not just a

simple Thevenin source with a resistor, but an impedance. For

example, emitter followers (bipolar transistors operating as voltage

followers) can have net inductive output impedances at high frequency,

which can cancel Miller capacitances that normally band-limit voltage

feedback *and* if loaded improperly, can create an additional low

impedance current feedback path. These two feedback paths could give

enough total loop gain for oscillation.

Op Amps, however, are a bit easier to deal with. To determine if your

Op Amp will be unstable you need to look at its Bode Plot and see what

the gain margin and phase margin are. That's the quick, cheap

analytic method. The other is the empirical method - pick something,

try it, if it oscillates add compensation to tweak the phase back to

stability as per the manufacturer's app note.