Operational transconductance amplifier (OTA): An overview
Article By : Jean-Jaques (JJ) DeLisle
OTA, commonly used in analog circuits, is a direct-coupled differential voltage-controlled current source (DVCCS) with high output impedance.
Operational transconductance amplifiers (OTAs) have become essential building blocks of many modern analog and mixed signal circuits. OTAs are used as a key element in a wide variety of circuits that benefit from voltage control. In essence, an OTA is a direct-coupled differential voltage-controlled current source (DVCCS) with high output impedance.
Hence, a differential input voltage determines the output current as a function of the differential input voltage and the transconductance of the OTA. The transconductance of an OTA can also often be controlled with a setup current. This is why an OTA is typically used as a high impedance differential input stage. Also, OTAs are used for precision control of amplifier gain or filter frequency with a relatively wide range.
Figure 1 The operational transconductance amplifier (OTA) here is used in the bandgap voltage reference. Source: ResearchGate
The devices that often come with OTA elements include:
Analog-to-digital converters (ADC)
Digital-to-analog converters (DAC)
Oscillators
Mixers
Automatic gain control amplifiers (AGCA)
Voltage-controlled amplifier (VCA)
Current-feedback amplifier (CFB)
Operational amplifiers (as a core amplifier)
Active filters/voltage-controlled filters (VCF)
Memcapacitors and meminductors
LED driver circuits
Fast-pulse integrators
Capacitive sensor control loops
OTAs differ from operational amplifiers as the key output parameter for OTAs is current while it’s voltage for an operational amplifier. OTAs are also more commonly used in open-loop configurations without negative feedback in linear applications. This is a result of the high resistance at the output that controls the output voltage and can specifically be selected to prevent the OTA from going into saturation, even with relatively high differential input voltages. With the addition of an output buffer amplifier to an OTA, the hybrid device can be used as an operational amplifier by converting the OTA’s current output to a voltage.
An ideal OTA presents an output current that is a product of the transconductance of the OTA multiplied by the differential input voltage. The output voltage of an ideal OTA is a product of the output current and the load resistance. Hence, the voltage gain on the output is a product of the load resistance and the transconductance of the OTA. The transconductance is directly proportional to an amplifier bias current (I_abc), which is used to control the transconductance.
In real OTA circuits, there is a practical limit to the differential input voltage that yields a linear response and is generally a result of the input stage transistor characteristics. OTAs also tend to exhibit transconductance behavior that is sensitive to temperature variations. The input and output impedance of an OTA, as well as the input bias current and input, offset voltage as a function of the transconductance control current.
Figure 2 LM13700, a dual transconductance amplifier, includes simple emitter follower buffers for each OTA because OTAs are modeled as output current sources. Source: Texas Instruments
There are three main OTA configurations that result in different input-output functions: the single input single output (SISO) OTA (one input one output), the differential input and single output (DISO) OTA, and the differential input differential output (DIDO) OTA (fully differential).
The SISO OTA is as described in the ideal OTA discussion presented earlier with the exception that the output current is only a function of the transconductance multiplied by the input voltage referenced to ground. The DISO OTA configuration is as described in the ideal OTA discussion. Lastly, the DIDO configuration presents a differential output current that is a function of the transconductance multiplied by a differential input voltage.
This article was originally published on Planet Analog.
Jean-Jaques (JJ) DeLisle, an electrical engineering graduate (MS) from Rochester Institute of Technology, has a diverse background in analog and RF R&D, as well as technical writing/editing for design engineering publications. He writes about analog and RF for Planet Analog.