"gareth" wrote in message
...
One thing that puzzled me for years was that the output impedance of
an amplifying stage could be much higher than the voltage drop
across the active element divided by the current through it.

Now, clearly as current sources, this would be true, and it could be
measured.,
by changing the loads.

But, how could you calculate such a value up front?

Consider for a moment the equivalent circuit of a current generator
with a shunt impedance feeding a load.

The voltage across the load is the same as that across the current
generator,
and measuring the current coming out of the generator at any one time will
tell you
about the load impedance, but not the shunt impedance.

However, by changing the load impedance and again measuring the voltage
and the current, then you will ahve sufficient information to calcualte the
source impedance.