Bjt differential amps

[Andetroy]

I've bn reading on bjt differential amplifiers and i stumbled on the term "transconductance" i believe it's the ratio of the amperage to the voltage. My question is: what exactly does it mean in bjt diff amps? How is it calculated for an active loaded bjt diff amp with emitter degeneration resistors?

 

[(*steve*)]

It is the ratio of change in output current to change in input voltage.

See here.