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Steve Eddy
July 14th 03, 01:18 AM
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

Steve Eddy
July 14th 03, 05:55 PM
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