A Song for You

You can imagine - as a rough approach - that an OTA is an opamp without the last stage.
1.) More correct: An OTA (operational transconductance amp) is an amplifying unit with a current output (high output resistance) in contrast to the classical opamp having a low output resistance (voltage source).
2.) Therefore, the transfer characteristic of the OTA is "Transconductance gm" (current divided by input voltage).
3.) Important feature: Commercial OTA´s have a separate pin to control the bias current for the internal diff. amplifier resp. the transconductance gm.
4.) An output voltage is created only if there is a load impedance (but often an additional buffer is needed)
5.) OTA allow negative feedback although normally it is used without feedback.

1.) OPAMP: Differential high impedance input and very low impedance output (voltage source) and a very high gain.

2.) OTA: Differential high impedance input and rather high impedance output (current source).
Thus, the relation between output and input is current/voltage=transconductance gm which has a final value. As an important feature the value of gm in commercial OTAs can be varied by a current into a separate pin.

Application:

3.) Opamps always are used with external feedback in order to create amplifier, filter, adding circuits, integrators,...... (Of course, they can drive also capacitive loads up to a certain limit)

4.) OTAs are used normally without external feedback (although it is possible and allowed). They have better high frequnecy capability than opamps. The output voltage is simply output current times load impedance. Since gm can be varied the gain can be controlled by an external control voltage.