Lindy Mayfield wrote:
> D3 is a zener diode with a reverse breakdown voltage of 5.6 volts, sort
> of like a voltage sensitive switch.
>
> So R2 and D3 in this configuration functions as a stable voltage
> regulator to the base of Q1.

Right.  The point of R2 and D3 is to produce a 5.6V source that is
reasonably independent of the actual voltage on the 8.2V rail.  Do you see
how the top (cathode) of D3 will be close to 5.6V even if the "8.2V" rail
were 7V or 10V, for example?  I think you do, but this is important to
understand before proceeding.

Now figure out what Q1 is doing and how it works in that circuit.  You've
already figured out that R2 and D3 are producing 5.6V, so now just assume
the base of Q1 will be driven with 5.6V and that the collector will be at
somewhere around 8.2V.  Explain what happens as other parts of the circuit
try to draw current coming out the emitter of Q1.  What function is Q1
performing?


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