difference between op amp and other amplifiers
OTA is a TRANSCONDUCTANCE amplifier which means that it's GAIN is given by Io/Vin=A (For a certain Voltage input you will have an output current Io which will be A times Vin). This means that the output current is independent of your load value (in theory).
an OPAMP is a Voltage to Voltage amplifier. Its GAIN is given by Vo/Vin=A, which means that the output voltage it's independent of the load attached to the circuit.
hope this helps,
diemilio