Do it yourself operational transconductance amp?
The OTA is what the name inducates: A TRANCONDUCTANCE amplifier which means: High-resistive voltage differential input and a high-resistive current output (output/input=I/V=conductance gm). And - yes - commercial OTAs have a control node for injecting a control current Iabc (amplifie bias current).
Therefore, the transconductance gm can be externally controlled. 

Of course, no ideal OTA is possible - which means: It is up to you to decide if a well-designed differential amplifier can meet your requirements or not (input and output resistances). I think, a darlinton pair at the input can provide a sufficient input resistance - and an active load (current mirror) provides a good output resistance.