Olin Lathrop said:

> 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.

Olin,

Think about it from a current sense as well, please. I think you are not
seeing the forest for the trees. Collector and base current in this
transistor are less than 1% different. The speaker is putting current into
the emitter. If the speaker is a voltage source in series with a
resistor(say it's the same as the external one, 8.2 ohms), then the
voltage gain is 5600/4.1 assuming an impossible zero resistance
transistor.

In reality the resistance in the transistor drops the gain a lot, but
overall it is a linear circuit, and it is very close to the way some low
impedence microphone preamps I've seen do it. They were not audiophile
grade, but were in good quality public address systems.

Also note that Jack Smith's Spice result of .57% THD shows we're talking
about a circuit that is respectable for its low parts count.

In answer to your latest challenge, ripple on the power supply will get
through, but it won't be amplified. Normally as long as it doesn't get
amplified it's accepted in a small circuit block like this.

When always using common-emitter connections it's all too easy to forget
the many different ways to use a transistor. This is one of them. Enjoy!

Cheerful regards,

Bob

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