Operational transconductance amplifier

The operational transconductance amplifier (OTA) is an <a href="/facts/Amplifier/pnRgxncC">amplifier</a> that outputs a <a href="/facts/Electric_current/Dc1xdnUX">current</a> proportional to its input voltage. Thus, it is a voltage controlled <a href="/facts/Current_source/1UkYH0aG">current source</a>. Three types of OTAs are single-input single-output, differential-input single-output, and differential-input differential-output (a.k.a. fully differential), however this article focuses on differential-input single-output. There may be an additional input for a current to control the amplifier's <a href="/facts/Transconductance/lwh8krQa">transconductance</a>. 
The first commercially available integrated circuit units were produced by <a href="/facts/RCA/NWyjNN6R">RCA</a> in 1969 (before being acquired by <a href="/facts/General_Electric/bBm5CzVY">General Electric</a>) in the form of the CA3080. Although most units are constructed with bipolar transistors, field effect transistor units are also produced.
Like a standard <a href="/facts/Operational_amplifier/hunKdYb9">operational amplifier</a>, the OTA also has a high <a href="/facts/Electrical_impedance/I06NSXtN">impedance</a> differential input stage and may be used with <a href="/facts/Negative_feedback/G9byVgpN">negative feedback</a>. But the OTA differs in that:

<ul><li>The OTA outputs a current while a standard operational amplifier outputs a voltage.</li>
<li>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 <a href="/facts/Operational_amplifier/hunKdYb9">saturation</a>, even with high differential input voltages.</li></ul>
These differences mean the vast majority of standard <a href="/facts/Operational_amplifier_applications/m7VjdVDl">operational amplifier applications</a> aren't directly implementable with OTAs. However, OTAs can implement <a href="/facts/Voltage-controlled_filter/R7o8sYsM">voltage-controlled filters</a>, <a href="/facts/Voltage-controlled_oscillator/85sdYF0m">voltage-controlled oscillators</a> (e.g. <a href="/facts/Variable_frequency_oscillator/l6e0IT4r">variable frequency oscillators</a>), <a href="/facts/Voltage-controlled_resistor/GzHQqKUu">voltage-controlled resistors</a>, and voltage-controlled <a href="/facts/Variable_gain_amplifier/FSUqSByl">variable gain amplifiers</a>.

Operational transconductance amplifier open-in-new

Operational transconductance amplifier