Introduction to Operational Transconductance Amplifier (OTA) — The Engineering Knowledge

5 min readAug 1, 2020

Hello students I hope you all are doing great. In today’s post, we will have a detailed look at Introduction to Operational Transconductance Amplifier (OTA). In a normal operational amplifier, the value of output voltage is the multiple of the input voltage with the gain. While the OTA is voltage to a current amplifier that has output current multiple of input voltage which are multiple of gain.

In this post, we will discuss its operation working circuit and some related factors. So let’s get started with Introduction to Operational Transconductance Amplifier (OTA).

Operational Transconductance Amplifier (OTA)

Similar to normal operational amplifiers the OTA comprises of 2 differential input terminals a large input impedance and large value of CMRR.
Contrary to the conventional operational amplifier there is bias current input connection exists in the OTA large value impedance at an output and there is no fixed open-loop voltage gain.

What is Transconductance

The transconductance of the electronic components is the ratio between the output current and input voltage.
In the case of OTA voltage is input and current is output so the ratio between the output current and the input voltage is called gain.
Resultantly the voltage to a current gain of OTA is called transconductance and denoted as gm.

Gm=Iout/Vin

For OTA transconductance is depend on the constant K multiple with the bias current.
The value of the constant depends on the inner circuit structure.

Gm=KIBIAS

Iout = gmVin = KIBIASVin

The relation of transconductance and the bias current in OTA is significant for characteristics.
The graphical representation in the below figure indicates this significant relation.

Note that the transconductance rises in linear behavior with the bias current.
The proportionality constant K is the line slope.
In this condition, K is almost sixteen microseconds per microampere.

Basic OTA Circuits

The below figure indicates the OTA circuit representation that is shown as an inverting amplifier having a fixed value of voltage gain.
The value of voltage gain is adjusted by the transconductance and the load resistance value is given as.

Vout = IoutRL

Vout/Vin = (Iout/Vin) RL

Av = gmRL

The most important factor of OTA is that the voltage gain can be regulated through the quantity of bias current.
It can be created through the manual way as shown in the below figure with the use of variable resistance in series combination with the circuitry shown in the above figure.

Through varying the resistance we can generate a variation in which there is a change in transconductance.
The change in the transconductance varies the voltage gain.
The voltage gain can be regulated through the outerly given changeable voltage as shown in figure denoted as b.
The change in the given bias voltage produces a variation in the bias current.

Certain type of OTA

The LM13700 is a certain category of OTA and operates as a representative component.
The LM13700 is dual-module casing comprises of 2 OTAs and buffer circuitries.
The below figure indicates the pin arrangements with the use of a single OTA in the casing.
The extreme dc voltage value is plus-minus eighteen volts and its transconductance parameters occur to the similar as shown in the above graph.
In the case of LM13700, the bias current id find by this below formula.

IBIAS = (+VBIAS — (-V) — 1.4 V)/ RBIAS

The 1.4 volts in this formula is due to inner circuitry where base-emitter junction and diode linked the outer Rbias through the negative voltage source.
The positive bias voltage can be get through a positive power source.

Two OTA Applications

Amplitude Modulator

When the constant value of amplitude input signal is given the amplitude of the output signal will change with respect to the modulation voltage at the bias input.
The gain is dependent on the bias current and bias current is corespondent to the modulation voltage according to the given expression.

IBIAS = (VMOD — (-V) — 1.4 V)/ RBIAS

Schmitt Trigger

The below figure indicates OTA in Schmitt trigger arrangments.
The Schmitt trigger is comparator having hysteresis where the input voltage has such value to run the module into its saturated condition.
When the input voltage is larger than the specific threshold value or called trigger point the component changes state to one of its saturated output conditions.
When the value of the input is less than the threshold value the component changes its state to saturated condition.
For OTA Schmitt trigger the threshold level and is adjusted by the current with the use of resistance.
The extreme output current in OTA is equal to the bias current.
So in saturated output condition, the extreme positive voltage is IoutR1 and this voltage is a positive threshold value or upper trigger point.
When the input voltage larger than this value the output moves to its extreme negative voltage.
That is -IoutR1 As Iout= IBIAS, the trigger points can be regulated through the bias current.
The below figure defines this function.

That is a detailed post about Introduction to Operational Transconductance Amplifier (OTA) if you have any further query ask in comments. Thanks for reading. Have a good day.

Author: Henry

http://www.theengineeringknowledge.com

I am a professional engineer and graduate from a reputed engineering university also have experience of working as an engineer in different famous industries. I am also a technical content writer my hobby is to explore new things and share with the world. Through this platform, I am also sharing my professional and technical knowledge to engineering students.

Originally published at https://www.theengineeringknowledge.com on August 1, 2020.