For the curious, please look over at

alt.binaries.pictures.radio

I will now get out of the way!!

Cheers, John Stewart

Hi flipper and John

A link to this article has already been posted on the thread "On the
absence of feedback in triodes". Note the summary. It's a fiction.

cheers, Ian

"flipper" wrote in message
...
On Wed, 11 Jan 2006 09:20:48 -0500, John Stewart
wrote:

For the curious, please look over at

alt.binaries.pictures.radio

I will now get out of the way!!

Cheers, John Stewart



I haven't been abused lately so I'll take a shot at it.

It strikes me that the article "inherent feedback in triodes"
should
be called, more precisely, feedback when a triode is driving a
resistive load (or any load that alters plate voltage) because
that's
what's sampling the output and causing the feedback.

The output of the triode itself is current, a function of grid and
plate voltage. The external plate resistor samples that current
and
converts it to a voltage that is then subtracted from B+.

I.E. ra-Anode-Zb is the feedback summing junction.

The feedback is influenced by ra so there is an 'internal'
component,
which shows in his "equivalent control voltage" (1/u)dVb term. He
calls 1/u the "feedback transmission coefficient" but, clearly, if
there is no Zb there is no dVb and no feedback.

One should be able to operate a triode close to the no feedback
condition with a solid state bipolar current mirror in the plate
circuit since voltage change across Vbe would be negligible
compared
to B+.

The analysis is valid for the traditional triode amplifier
circuit,
however, as there will be "feedback" from ra-Anode-Zb.