Operational transconductance amplifier

Schematic symbol for an OTA with differential input. Like the standard operational amplifier, it has both inverting (−) and noninverting (+) inputs; power supply lines (V+ and V−); and a single output. Unlike the traditional op-amp, it has two additional biasing inputs, Iabc and Ibias.

The operational transconductance amplifier (OTA) is an amplifier that outputs a current proportional to its input voltage. Thus, it is a voltage controlled current source (VCCS). Three types of OTAs are single-input single-output, differential-input single-output, and differential-input differential-output (a.k.a. fully differential),[1] however this article focuses on differential-input single-output. There may be an additional input for a current to control the amplifier's transconductance.

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.[2] Although most units are constructed with bipolar transistors, field effect transistor units are also produced.

Like a standard operational amplifier, the OTA also has a high impedance differential input stage and may be used with negative feedback.[3] But the OTA differs in that:

  • The OTA outputs a current while a standard operational amplifier outputs a voltage.
  • The OTA 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.

These differences mean the vast majority of standard operational amplifier applications aren't directly implementable with OTAs. However, OTAs can implement voltage-controlled filters, voltage-controlled oscillators (e.g. variable frequency oscillators), voltage-controlled resistors, and voltage-controlled variable gain amplifiers.[4]

  1. ^ https://www.ijsr.net/archive/v2i3/IJSRON2013566.pdf [bare URL PDF]
  2. ^ CA3080
  3. ^ Jung, W.G., IC Op-Amp Cookbook (Howard W. Sams -Bobbs-Merrill First Ed. 1974) p. 440 et seq.
  4. ^ Cite error: The named reference :0 was invoked but never defined (see the help page).