# 50 ohm output impedance

I saw some circuits that uses a emitter follower as a buffer for output. The emitter resistance is 470$\Omega$. The author says that, since the ac emitter resistance is very small, he put a 50$\Omega$ resistor in series with the output terminal, so that the circuit would look like 50$\Omega$ from the output.

But what I don't understand is, if you now connect the output to a 50$\Omega$ load, the emitter resistance then becomes (470$\Omega$ || 50+50$\Omega$) = 82$\Omega$! This could affect a lot of things, for example the collector current would suddenly change 5-fold, the signal source would see a 80% reduction of load, everything could go wrong. What am I missing here?

• Got a schematic? And are there any AC coupling caps? – alex.forencich Dec 20 '14 at 8:22