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Operational transconductance amplifier
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Operational transconductance amplifier
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. 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 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. 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. But the OTA differs in that:
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.
In the ideal OTA, the output current is a linear function of the differential input voltage, calculated as follows:
where Vin+ is the voltage at the non-inverting input, Vin− is the voltage at the inverting input and gm is the transconductance of the amplifier.
If the load is just a resistance of to ground, the OTA's output voltage is the product of its output current and its load resistance:
The voltage gain is then the output voltage divided by the differential input voltage:
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Operational transconductance amplifier
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. 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 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. 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. But the OTA differs in that:
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.
In the ideal OTA, the output current is a linear function of the differential input voltage, calculated as follows:
where Vin+ is the voltage at the non-inverting input, Vin− is the voltage at the inverting input and gm is the transconductance of the amplifier.
If the load is just a resistance of to ground, the OTA's output voltage is the product of its output current and its load resistance:
The voltage gain is then the output voltage divided by the differential input voltage:
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