Transconductance in bjt diff amps

[Andetroy]

I've been readn about bjt differential amplifiers and stumbled across the term "transconductance". Being new to electronics, it's quite baffling and i'd like to know what it means and what its significance is in diff amps. Is there a way of calculating it in a bjt diff amp with emitter degeneration resistors?

 

[HMS1021]

transconductance is (change of drain current)/(change of gate voltage).

BJTs, FETs (and vacuum tubes) are basically voltage controlled current sources so "transconductance" is a measure of performance and is the amount of output current change versus the amount of input voltage change. It can not be called gain. Gain is Vout/Vin, Power out/Power in, or Iout/Iin and does not make sense with Iout/Vin. Once a circuit with a BJT is designed it's O/I may fit one of the definitions of gain.

From wiki: Transconductance is a contraction of transfer conductance. The old unit of conductance, the mho (ohm spelled backwards), was replaced by the SI unit, the siemens, with the symbol S (1 siemens = 1 ampere per volt).