The operational transconductance amplifier (OTA) is an amplifier whose differential input voltage produces an output current. Thus, it is a voltage controlled current source (VCCS). There is usually an additional input for a current to control the amplifier's transconductance. The OTA is similar to a standard operational amplifier in that it has a high impedance differential input stage and that it may be used with negative feedback. The first commercially available integrated circuit units were produced by RCA in 1969 (before being acquired by General Electric) in the form of the CA3080. Although most units are constructed with bipolar transistors, field effect transistor units are also produced. The OTA is not as useful by itself in the vast majority of standard op-amp functions as the ordinary op-amp because its output is a current. One of its principal uses is in implementing electronically controlled applications such as variable frequency oscillators and filters and variable gain amplifier stages which are more difficult to implement with standard op-amps. Principal differences from standard operational amplifiers Its output of a current contrasts to that of standard operational amplifier whose output is a voltage. It is usually used "open-loop"; without negative feedback in linear applications. This is possible because the magnitude of the resistance attached to its output controls its output voltage. Therefore, a resistance can be chosen that keeps the output from going into saturation, even with high differential input voltages. Basic operation In the ideal OTA, the output current is a linear function of the differential input voltage, calculated as follows: where Vin+ is the voltage at the non-inverting input, Vin− is the voltage at the inverting input and gm is the transconductance of the amplifier. The amplifier's output voltage is the product of its output current and its load resistance: The voltage gain is then the output voltage divided by the differential input voltage: The transconductance of the amplifier is usually controlled by an input current, denoted Iabc ("amplifier bias current"). The amplifier's transconductance is directly proportional to this current. This is the feature that makes it useful for electronic control of amplifier gain, etc. Non-ideal characteristics As with the standard op-amp, practical OTA's have some non-ideal characteristics. These include: Input stage non-linearity at higher differential input voltages due to the characteristics of the input stage transistors. In the early devices, such as the CA3080, the input stage consisted of two bipolar transistors