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On Mon, 14 Mar 2005 23:48:27 -0600, (John Byrns) wrote:

In article , Stewart Pinkerton
wrote:

John Byrns wrote:

It is obvious by inspection that the feedback voltage developed across R7,
by the load current flowing through Tr2 & R7, is in series with the input
voltage to Tr1, hence a feedback loop exists around two stages. This
feedback loop can be easily seen if you delete R4 from the circuit.

But R4 *is* in the circuit, which reduces your claimed 'feedback' to
utterly negligible proportions. Without R4, it wouldn't *be* an
emitter follower, it would be part of a Darlington pair, which I have
never seen described as a feedback loop.

You deleted the part where I specifically pointed out that R4 was present
in the circuit, and has a significant effect. My point was simply to
first consider the amplifier without R4 present, to make the basic
topology clearer, before considering the effect of R4.

I would describe a Darlington pair with an unbypassed emitter resistor as
a feedback pair. If you don't believe it try this simple experiment with
your amplifier. Feed a small signal voltage at say 1 kHz into the input
and measure the signal voltage between the base of Tr1 and ground, and the
signal current entering the base of Tr1. From the current and voltage you
can calculate the input resistance of the amplifier at the base of Tr1,
exclusive of the resistors, R1, R2, & R3. Next, take a spare 10m
capacitor and connect it across resistor R7 in the emitter circuit of Tr2,
and again measure the signal voltage between the base of Tr1 and ground,
and the signal current entering the base of Tr1. Again calculate the
input resistance of the amplifier at the base of Tr1, using the new
voltage and current values. Now explain the substantial difference
observed in the two input resistance values? The presence of resistor R7
in the emitter circuit of Tr2 effects not only Tr2, but also has a major
impact on the resistance looking into the base of Tr1. The only way I can
see to explain this effect is that the feedback loop created by R7
encompasses both Tr1 & Tr2, do you have an alternate explanation for this
effect?

While the effect you describe is theoretically correct,

a) I wouldn't describe this as 'feedback', since it's simply a load
variation on the output of Tr1.

b) The presence of R4 reduces this effect to negligible proportions.

c) If it bothers you that much, remove Tr1, rebias Tr2, and drive from
a low-impedance source.
--

Stewart Pinkerton | Music is Art - Audio is Engineering