Olin Lathrop <olin_piclist@EMBEDINC.COM> wrote:
> 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.  The small voltage signals there cause
> large (but non-linear) changes in collector current, which is where all
> the voltage gain comes from.

I don't think you're applying the correct analysis to this circuit.

The speaker is a very low-impedance source, and its AC current output is
much more significant than its voltage output.

The common-base arrangement of the transistor acts to impose the AC
current of the speaker across the much larger load resistor connected
to the collector, providing voltage gain equal to the ratio of the
impedances.

The AC voltage across the B-E junction is on the order of 1 mV or less,
and the nonlinearity of this junction has little effect on circuit
performance.

The biggest problem with this circuit is the fact that the impedance of
the speaker varies quite a bit with frequency, which makes the voltage
gain of the circuit vary as well. The low-value emitter resistor helps
to limit this effect.

In general, the common-base amplifier is useful whenever you need to
match a low-impedance source to a high-impedance load.

-- Dave Tweed

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