Introduction to Operational Transconductance Amplifier (OTA)

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).

Introduction to operational Transconductance Amplifier (OTA)

• In the below figure you can see the symbolic representation of OTA. The double circle symbolic representation at the output end denotes the output current source which depends on the bias current.

• 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

• The output current is regulated through the input voltage and biases current as shown by the given formula.

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

• If we divide both sides with Vin then we have.

Vout/Vin = (Iout/Vin) RL

• As Vout/Vin is the voltage gain and Iout/Iin is equal to gm.

Av = gmRL

• The transconductance of the amplifier is shown in below figure is found through the bias current that is set with the dc power source and the bias resistance R_Bias

 

• 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.

 Types of OTA

• The LM13700 is a certain category of OTA and operates as a representative component.
• The LM13700 is a dual-module casing comprised 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 the base-emitter junction and diode link the outer Rbias through the negative voltage source.
• The positive bias voltage can be get through a positive power source.

 OTA Applications

Amplitude Modulator

• The figure below shown define the OTA linked as an amplitude modulator. The voltage gain changes with the application of modulation voltage to bias input.

• When the constant value of the 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 the bias current is correspondent to the modulation voltage according to the given expression.

IBIAS = (VMOD – (-V) – 1.4 V)/ RBIAS

• This modulation process is shown in the above figure for large frequency sine wave input voltage and less frequency sin modulating voltage.

Schmitt Trigger

• The below figure indicates OTA in Schmitt trigger arrangments.
• The Schmitt trigger is a 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 triggers 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 is 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.

Is the transconductance of an OTA dependent on a bias current?

The transconductance of the amplifier is regulated with input current I_abc (“amplifier bias current”). The amplifier’s transconductance is directly proportional relation with this current.

Read also:

• LM324 – Single Supply Quad Operational Amplifiers worksheet
• Difference Between An Operational Amplifier and Comparator
• Amplifier circuit using LA4440 IC
• Audio Amplifier Circuit
• Difference Between Inverting and Non-Inverting Amplifier
• Introduction to 2N5089 NPN Amplifier Transistor
• Introduction to Summing Amplifiers

FAQS

Is the OTA is a voltage-to-current amplifier?

The OTA is amplifiers that output current proportional to the input voltage. it is voltage voltage-controlled current source.

What are the different types of operational transconductance amplifiers?

(1) five-transistor OTAs, (2) telescopic cascode OTAs, (3) folded cascode OTAs, and (4) current mirror OTAs.

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