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Re: Emitter follower stage as a feedback amplifier, Part 2 (with corrections)

Posted by mfc (A) on July 13, 2003 at 18:18:42

In Reply to: Re: Emitter follower stage as a feedback amplifier, Part 2 (with corrections) posted by kurt s on July 13, 2003 at 16:29:24:

Hi Kurt,

I think you have it right basically. Back to the books for
me. Ahh, here are some terms I've been using incorrectly. Beta
isn't feedback factor according to Langford-Smith. The quantity
(1-BA) is. They call "B" the transfer coefficient.

In general A/A'= 1 - BA where A = open loop gain and A'= closed
loop gain. For a follower (page 317-318) B = -1,
so A/A' = 1 + BA = 1 + A in all followers (ideally).

OK so far so good. Now to the Sziklai follower. Nothing in
Langford-Smith here. How to explain the large peak in frequency
response at the hi-end of the Sziklai? Sticking with your model
(and Langford-Smith's), at some frequency, Beta or "B" which is
normally -1 must flip and become +1 (so we get the large peak in
the frequency response).

Also in order to get the harmonics over the multiplication hump
and down as low as they are in the Sziklai, since feedback factor
must be large for this to occur and B = -1, this only leaves "A"
which must be very large in the Sziklai.

Thoughts anyone?

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