I am teaching students about the differential pair (see circuit below). I addressed many issues like the difference between differential and common mode gain, taking arbitrary vb1 and vb2 signals and expressing them with common mode + differential components, and finally using current mirrors for single-ended conversion. 
One thing I have not addressed is that the differential pair is easy to saturate. Very seldom does the differential pair operate as a linear amplifier, but rather the outputs usually are pushed to their limit (to their rails). In fact, the range of differential inputs that produce linear outputs is very small (perhaps +/- two thermal voltages as seen below). What I would like to know from you is if there are any applications of the differential pair that depend on using the linear region only? Telecom antenna preamps? Wheatstone bridges?
Thanks for your help – (this is my first stackexchange post)
Electronic – small-signal differential amplifiers used
amplifierbjtdifferentiallinear
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Best Answer
You'll see this circuit used in voltage controlled amplifiers. Here an audio signal is feed into one of the inputs and the current through the emitters controls the gain of the amplifier.
A google search for "discrete synth vca" will give you several example circuits. Here is a simple one: