Olin Lathrop wrote:
> > "The first transistor operates in common base mode."
>
> Yes, I know.
>
> > The feedback at variable voltage achieved due to collector
> > current changes make voltage changes on emitter resistor.
>
> That is only DC feedback to set the bias point of the
> first transistor.  I agree this works.  I should have
> been more clear in that I was talking about AC (the
> sound) feedback.  This circuit doesn't have any.


Can't agree fully with you, Olin.

( http://www.mitedu.freeserve.co.uk/Circuits/Misc/Sp-mic.htm )

"DC feedback for bias purposes" is getting by collector and
base resistors.
"AC (the sound) feedback" you were talking about is getting
by emitter resistor.


> I stand by my comment that this is a "crappy"
> circuit.
...
> Yes Mike, I understand how common base works.  Actually this circuit
> has about the same response characteristics as common emitter would
> with the emitter tied to ground (assuming a suitable DC bias
> arrangement).  In either case, small AC input signal is presented
> directly accross the B-E junction of the first transistor.

In a "common emitter mode", if we assume this DC bias arrangement
suitable (collector and base resistors and capacitor), then, anyway, we
need emmiter resistor for the linearity.

And to feed the base of the transistor with an AC input signal
we need extra resistor from the base to capacitor and some
capacitor between AC input signal and the base. The base
resistor will spoil stability, extra capacitor is extra part.

In "common base mode" even emitter resistor could be substituted
with loudspeaker's coil resistance.


> > Great circuit, by the way.

> Not!  See above.


Mike.

P.S. Hmm, Peter's got better score :-)

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