On 16/05/2014 20:44, gareth wrote:
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?

One is a static value the other is dynamic, you seem to be wondering why
a real calculation should be different to a complex one.
Once the signal passes through any phasing element the vector will shift
off the real axis and the V=IR. That's why it is measured in impedance
and not resistance.

As for calculation - for simple stuff it is a matter of number crunching
with complex numbers - for more complicated circuits then you need to
look at some network analysis over the whole frequency range you will be
using.

Andy