AC Linked Differential Pair

The AC Linked Differential Pair has the 2 emitters coupled together for AC via a capacitor.

This simplifies biasing especially where the transistors are not matched.

The linking capacitor needs to have a very low impedance at the operating frequency ( a few ohms at most.)

It can reduce low frequency noise (such as 1/f noise), preventing it from being transmitted from 1 transistor to another. 

The capacitor could be replaced by a crystal for linking at the series resonant frequency and any harmonic frequencies the crystal has, though the gain would be quite limited as crystals have an impedance of about 50 ohms at their specified frequency.