Operational transconductance amplifier

A transconductance amplifier ( OPV or VC- VC- OP), also called the Operational Transconductance Amplifier (OTA), is a special operational amplifier, which converts the differential voltage at the two inputs into a proportional output current. Typically, it is, like all operational amplifier implemented as an integrated circuit.


In contrast to traditional operational amplifier with low impedance voltage output of the transconductance amplifier has a high-impedance current output.

Other differences to the operational amplifier with voltage output are:

A resultant characteristic is that allows an OTA build analog circuits such as analog filters without ohmic resistors. Ohmic resistors with corresponding accuracy depend on manufacturing processes are difficult to achieve in integrated circuits. Therefore OTAs play as a basic element in programmable analog circuits such as Field Programmable Analog Array ( FPAA ) play a role.

The first transconductance amplifier CA3080 was produced in 1969 by the company RCA. OTAs are available today with improved data by different manufacturers. Examples are the LM13700 from National Semiconductor or the LT1228 from Linear Technology. As a stand-alone circuit of the OTA, but does not have the same meaning as the operational amplifier output voltage.


The basic function is given by:

To the output current Ia input differential voltage UD and the transconductance and transconductance gm with the dimensions of a conductance.

An output voltage Vout can be obtained by an externally load resistance Ra:

The voltage gain G is thus given as:

The conductance gm of the amplifier can be adjusted typically about three to four decades. For this, a, is provided in the circuit symbols usually not shown, own control input can be adjusted at the IABC, the operating point with a control current. The commonly used data sheets and technical literature Index ABC stands for Amplifier Bias Current and has the transconductance following relationship:

The temperature thereby occurring voltage UT has a value of approximately 25 mV at room temperature.