The operational transconductance amplifier (OTA) is a type of an operational amplifier which differential input voltage produces an output current.
The operational transconductance amplifier (OTA) is a special type of operational amplifier, which works as a voltage controlled current source (VCCS).
The OTA have high impedance voltage inputs as a standard op-amp, a current output and two additional Difference of voltage between non-inverting \$V_{in+}\$ and inverting \$V_{in-}\$ inputs produces output current \$I_{out}\$, as follows:
\$ I_{out} = g_m \times (V_{in+} - V_{in-}) \$
where \$g_m\$ is a transconductance of an amplifier. The value of transconductance depends on currents of additional "biasing" inputs of OTA: \$I_{bias}\$ and/or \$I_{abc}\$ - amplifier bias current.
Further reading:
Operational transconductance amplifier, Wikipedia
A Short Discussion of the Operational Transconductance Amplifier (OTA) , Synths'R'Us
