In Fig. 9-9, very little signal current flows
in RE. so it does not appear in the voltage gain
equation. Q1 is driven at its base by the signal
source. Its emitter signal current must flow
through its 47 Q of ac emitter resistance. This
emitter signal also drives the emitter of Q2 and
must overcome its 47 Q of ac emitter resistance.
Q2 acts as a common-base amplifier in this cir-
cuit and it is driven at its emitter by the emitter
of Q1. This is why the denominator of the gain
equation contains 2x rr (the two 47-(Z resistances are acting in series for signal current).
RE is much larger than the ac emitter resistances
and its effect is small enough to be ignored. In
circuits of this type, the signal current in RE is
only about 1 percent of the signal current in the
transistors.