Operational transconductance amplifier

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Schematic symbol for the OTA. 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, I_abc and I_bias, explained in Basic operation and Subsequent improvements, below.

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 the 1960s (before being acquired by General Electric), in the form of the CA3080, and they have been improved since that time. Although most units are constructed with bipolar transistors, field effect transistor units are also produced. The OTA is not very useful by itself in the vast majority of standard op-amp functions because its output is a current. Its principal use is in implementing electronically controlled applications such as variable frequency oscillators and filters and variable gain amplifier stages.

1 Principal differences from standard operational amplifiers
2 Basic operation
3 Non-ideal characteristics
4 Subsequent improvements
5 See also
6 External links

[edit] Principal differences from standard operational amplifiers

First, its output of a current contrasts to that of standard operational amplifier whose output is a voltage.

Second, except for its input stage (which is a simple two transistor differential amplifier), its internal circuitry is completely different. The OTA is constructed completely of transistors and diodes; it uses no resistors or capacitors.

Third, 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.

[edit] Basic operation

In the ideal OTA, the output current is a linear function of the differential input voltage, calculated as follows:

$I_\mathrm{out} = (V_\mathrm{in+} - V_\mathrm{in-}) \cdot g_\mathrm{m}$

where V_in+ is the voltage at the non-inverting input, V_in− is the voltage at the voltage at the inverting input and g_m is the transconductance of the amplifier.

The amplifier's output voltage is the product of its output current and its load resistance:

$V_\mathrm{out} = I_\mathrm{out} \cdot R_\mathrm{load}$

The voltage gain is then the output voltage divided by the differential input voltage:

$G_\mathrm{voltage} = {V_\mathrm{out} \over (V_\mathrm{in+} - V_\mathrm{in-})} = R_\mathrm{load} \cdot g_\mathrm{m}$

The transconductance of the amplifier is usually controlled by an input current, denoted I_abc ("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.

[edit] 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

Temperature sensitivity of transconductance

Variation of input and output impedance, input bias current and input offset voltage with the transconductance control current I_abc.

[edit] Subsequent improvements

Earlier versions of the OTA had neither the I_bias terminal shown in the diagram nor the diodes shown adjacent to it. They were all added in later versions. As depicted in the diagram, the anodes of the diodes are attached together and the cathode of one is attached to the non inverting input (Vin+) and the cathode of the other to the inverting input (Vin−). The diodes are biased at the anodes by a current (I_bias) that is injected into the I_bias terminal. These additions make two substantial improvements to the OTA. First, when used with input resistors, the diodes distort the differential input voltage to offset a significant amount of input stage non linearity at higher differential input voltages. Second, the action of the biased diodes offsets much of the temperature sensitivity of the OTA's transconductance.

A second improvement is the integration of an optional-use output buffer amplifier to the chip on which the OTA resides. This is actually a convenience to a circuit designer rather than an improvement to the OTA itself; dispensing with the need to employ a separate buffer. It also allows the OTA to be used as a traditional op-amp, if desired, by converting its output current to a voltage.

An example of a chip combining both of these features is the LM13600 and its successor, the LM13700.