Operational Transconductance Amplifiers (OTA) Information
Operational transconductance amplifiers (OTA) are devices that convert an input voltage to an output current. They are primarily voltage-to-current amplifiers. Unlike traditional operational amplifiers, or op-amps, OTAs represent a voltage-controlled current source (VCCS). Their transconductance parameter is controlled by an external, amplifier-bias current and expressed as a function of the applied voltage. Because an OTA’s output impedance is high, some operational transconductance amplifiers have on-chip controlled impedance buffers for driving resistive loads. The dependence of open-loop bandwidth, closed-loop bandwidth, and frequency responses are similar to those of conventional operational amplifiers, however. For OTA circuits that use negative feedback, there is a very close relationship between the closed-loop bandwidth, the amplifier bias current, and the closed-loop gain.
Like voltage-controlled current sources (VCCS), OTAs represent an ideal transistor. Because the variation of current with temperature holds the transconductance, however, OTAs remain relatively constant with temperature. Like a transistor, an OTA has three terminals:
- a high-impedance input or base
- a low-impedance input/output or emitter
- the current output or collector
Unlike a VCCS, an OTA is both self-biased and bipolar. Typically, an external resistor is used to adjust the transconductance and optimize bandwidth, quiescent current, and gain trade-offs. Absolute maximum ratings for OTAs include:
- power supply
- internal power dissipation
- differential input voltage
- input common-mode voltage range
- storage temperature range
- junction temperature
- electrostatic discharge (ESD) rating
Electrical specifications include bandwidth, slew rate, rise time and rise fall, harmonic distortion, and base input voltage noise.
OTAs are used in a variety of radio frequency (RF), intermediate frequency (IR), and high-performance video applications. When used as electronic building blocks, OTAs can be used in products such as automatic gain control (AGC) amplifiers, fast control loop amplifiers, and control amplifiers for capacitive sensors and active filters. OTAs can also be used in light emitting diode (LED) driver circuits for fiber optic transmissions, integrators for fast pulses, and as current-controlled oscillators. Additional applications for OTAs include timers, multiplexers, sample-and-hold circuits, video and broadcast equipment, high-speed data acquisition devices, and communications equipment.