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> I agree with Mike here, you need to study the circuit a little more.
> Common-base amps are pretty linear.

They can be, but this one isn't.

> The negative feedback is
> degenerative, but it results in low distortion.

Yes, that's usually the intent of properly done negative feedback.
However this circuit has no *AC* negative feedback.

For sake of discussion, lets forget about the second stage.  It is an
emitter follower that has roughly unity voltage gain, and is there to
provide a lower output impedence (power gain).  I don't have a problem
with that.

So, let's look at the first stage in detail.  I've attached the schematic
GIF to this message so we can all see the same thing (it's only
4.4Kbytes).

The 5.6Kohm resistor produces a voltage accross it proporional to the
current thru the transistor.  This means any supply voltage variations
show up in the output, by the way.

The 2.2Mohm resistor provides base bias to the transistor so that its
quiescent point is in the "on" region.  This resistor does provide some
negative feedback, but due to the capacitor this feedback only effects the
DC operating point, not the AC signal.  Note that the time constant
between the base bias resistor and the capacitor is about 8 minutes.  This
will clearly filter out any audio frequencies, in addition to providing an
annoyingly long startup delay.  So, from an AC point of view the base is
being held at a fixed voltage (which makes this a common base circuit).

The 8.2ohm emitter resistor is so small compared to the 5.6Kohm resistor
that it will have very little voltage accross it.  It therefore has little
effect on the DC operating point.  It's only purpose apparently is to load
the speaker with a fixed impedence.

Now let's look at AC characteristics.  The speaker, series capacitor, and
8.2ohm resistor together act as a low impedence source producing small
audio voltages when the speaker receives sounds.  This low impedence AC
voltage source is directly connected to the emitter.

So, for AC analisys, we've got a fixed voltage on the base and the input
signal is a low impedence voltage on the emitter.  In other words, the
input signal is presented directly accross the transistor's B-E junction.
This is essentially a diode, and the voltage from current response of a
diode is very non-linear.  The transistor has the added effect of
multiplying this diode current by the transistor gain and making that the
collector current.  The output voltage is therefore a scaled up signal
proportional to the current thru a diode resulting from the input voltage.

Yes, this has high gain, but the output signal will have significant
distortion, and any AC on the power supply will be added to it directly.
Much of these problems could be corrected with negative feedback, but this
circuit has no *AC* feedback path.

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