Here are some low frequency examples:

Rc is the resistor at the collector - Re is
the resistor at the emitter

This model assumes that you are getting your RF from a tap between the collector and
collector resistor.

http://en.wikipedia.org/wiki/Common_emitter

I am still working on trying to figure out this impedance thing. N0FP added another
bit of complexity... what happens if you have a tank circuit vs. no tank circuit.
... Argghhh.

I need to pull out SSDRA and do some more reading or re-reading as the case might
be.