It's my understanding that followers such as emitter followers operate
within what basically amounts to a 100% negative feedback loop and
that paired followers such as the Darlington and Sziklai (CFP) have
the device pairs tied together in what also basically amounts to a
100% negative feedback loop between devices with the pair's output
ultimately being fed back to the input. In other words, the single
device follower utilizes a single negative feedback loop and the
Darlington and Sziklai pairs utilize two negative feedback loops.

I've been told this is incorrect. That a single device follower is an
"open loop" follower without any negative feedback and that a
Darlington pair is simply a pair of open loop followers tied together
such that there is also no negative feedback involved. But that the
Sziklai operates with 100% negative feedback.

I've researched numerous sources and haven't been able to find
anything which confims the latter position. Everything seems to point
to the former.

Am I missing something?

se

On Mon, 14 Jul 2003 08:23:49 GMT, Kevin McMurtrie
wrote:


There is negative feedback in an emitter follower but the open loop gain
is much too low for NFB to control the large amount of distortion.

The Art of Electronics has some equations related to this. Look up
"feedback" in the index and check the sections "with finite loop gain"
and "emitter resistor as."

Thanks. Yes, I realize that. By 100% feedback I mean that effectively
100% of the output is fed back to the input.

Ultimately what I'm trying to determine is whether or not the
following assertions are true:

a. There is no such thing (well, in any practical sense) as an "open
loop" follower. A follower follows because its output is fed back to
its input.

b. There is basically no more feedback between device pairs in a
Sziklai pair than a Darlington pair.

se