differential pair

Works well for amplifying small signal(under 100mv)

100mv peak input difference.

1.9v peak output difference( input amplified 19 times).

Trying to  apmplify further more.

outputs hit the limit( binded).

peak output difference 12.5v( 125 times amplification!).

By adding resistor at the emitter, we have more control of the gain.
AC gain = 2*Rc/[2*(RE + Re)] = Rc/(RE + Re). (RE is within hybrid pi model of transistor, Re is the resistor we just added). In this case Re >> RE, gain ~ Rc/Re = 50.

peak output difference 5v( 50 times input).
We need to adjust VCC, current source, and Rc to keep transistor in operation mode.