Keith Dysart wrote:
It happens without effort because the output impedance
of the Norton/Thevenin equivalent circuit is the same
as the circuit it replaces, otherwise it is not an
equivalent.

False. There can be a large difference in the output
impedance of an amplifier designed to drive a 50 ohm
load and a 50 ohm Thevenin equivalent circuit.
--
73, Cecil http://www.w5dxp.com