While quickly prototyping some of diy-synth circuits I realized that I don't have OTA at hand, but I wanted to continue fiddling with the circuit. Is there a way to make OTA out of op-amps and transistors at hand? Quick googling couldn't find me any solutions. If I understand correctly, OTA is OP-amp with current-controlled gain. Well, a differential amp with controlled current source should do the trick, right? Or there is something more to it?
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.