Generally, comparators are classified into various types such as electrical comparators,

electronic comparators, mechanical comparators, optical comparators, sigma comparators,

pneumatic comparators, digital comparators, and so on. These comparator circuits are typically

used in designing electrical and electronics projects. In this article, we discuss how to use an op

amp as comparator circuit and working operation of op amp as comparator circuit. But, primarily

we must know what is an operational amplifier and comparator circuit.

Operational Amplifier

Operational Amplifier

The DC coupled electronic voltage amplifier with high gain, that consists of two input terminals is

shown in the figure. The differential input is fed to the two input terminals (inverting input terminal

and non-inverting input terminal) of the op amp and it produces a single output potential at Vout

terminal. Thus, potential difference fed to its two input terminals is amplified for producing an

amplified output. This amplified output is equal to hundreds of thousands of times of difference

between the input signals.

The output of the amplifier can be given as

Vout = AOL (V+ – V-)

Where,

o AOL is an open loop gain of the amplifier

o V+ is the non-inverting input of the amplifier

o V- is the inverting input of the amplifier

Even though there are various types of operational amplifiers, 741 op amps are frequently used

as a comparator circuit in several electronics circuits.

Comparator Circuit

The device that consists of two input terminals, in which reference input signal is fed to one

terminal and the actual value of the signal is fed to another terminal. Then, an output signal is

generated at the output terminal based on the difference between the two input signals fed to the

two input terminals. This generated output signal is either 0 (low) or 1 (high).

In electrical and electronics terminology, the device used for comparing two voltage signals or

current signals that are fed to the two analog input terminals, thereby producing one binary digital

output signal to indicate the larger input signal is called as a comparator circuit.

Comparator Circuit

The two analog input terminals are represented as V+ (Vin) & V- (Vref) in the above comparator

circuit. The digital output is generated at the output terminal V0 (Vout). The output signal of

comparator circuit is given by

If V+ > V- (Vin is greater than Vref), then V0 = 1 and

If V+ < V- (Vin is less than Vref), then V0 = 0

Generally, comparators are used in the devices such as relaxation oscillators, analog to digital

converters (ADCs), and also in the devices that are used to measure analog signals. The

comparators consist of high-gain differential amplifiers and we can use an op amp as comparator

circuit.

Op Amp as Comparator

741 operational amplifiers are basic operational amplifiers that can be used as a comparator

circuit in many electronics circuits. For example, if we consider a temperature controlled switch;

then switching operation is performed based on the temperature. If the actual temperature value

exceeds the preset reference temperature value, then an output voltage (low or high) is

produced by temperature sensor accordingly.

If we consider the basic comparator arrangement, then there will be high frequency voltage

variations caused due to noise. This problem is needed to be considered in case of operational

amplifiers that are particularly designed as comparator circuits. This noise is produced whenever

the input voltage signal and reference voltage signal are close to each other.

Op Amp as Comparator Circuit

The high frequency voltage variations are caused due to the random nature of noise, due to this,

in rapid successions, the input signal voltage becomes greater than or less than the reference

voltage. Thus, the output signal will oscillate between its maximum voltage level and minimum

voltage level. This problem can be reduced by applying hysteresis. We can adjust the hysteresis

gap in the Schmitt trigger circuit arrangement by applying hysteresis to an op amp comparator

circuit using positive feedback. The figure shows op amp as comparator circuit with hysteresis.

Op Amp as Comparator Circuit Working Operation

In general, the output of an Op amp fluctuates positive and negative to extreme voltage that is

approximately equal to the supply potentials. If a 741 op amp is connected to a +/-18V, then the

maximum output voltage is given as +/-15V. This is due to the extreme high open loop gain of

the op amp (10,000 to 1 million). Thus, if +/-150 microvolts of voltage difference is created by any

input, then it will be amplified approximately by one million times and output is driven into

saturation. Thus, the output remains at its maximum or minimum value.

Op Amp as Comparator Circuit Diagram Working Operation

While using op amp as a comparator in instrumentation, the open loop can be used to compare

the two voltages. Therefore, depending on the difference between the input voltage value and

the reference voltage value, the output Vout will be equal to the maximum high value or minimum

low value (input voltage value will be greater than or less than the reference voltage value few by

micro volts).

The reference voltage is fed to non-inverting input terminal of the op amp and variable voltage is

fed to inverting input terminal of the op amp. Consider the op amp comparator circuit diagram

shown in the figure, if the voltage fed to pin 2 is greater than the reference voltage fed to pin 3,

then the output voltage becomes low and it is marginally greater than –Vs. If the voltage fed to

pin 2 is less than the reference voltage fed to pin 3, then the output voltage becomes high and it

is marginally less than +Vs.

There are many op amps dedicated for comparators operation, these op amp comparator circuits

are used for high speed comparisons. The output state of these op amp comparator circuits

changes in less than 1 microsecond. But, these high speed comparing op amp comparator

circuits consume more power, depending on the speed of comparison. Based on the speed of

comparisons and the amount of power consumption, these comparators are classified into

different types. The specific op amp comparator can be used for a particular application

depending on the required speed and/or power consumption.

Application of Op Amp as Comparator in Practical Electroncis Circuits

The temperature humidity monitoring system of soil based on wireless sensor networks

using Arduino project is designed for developing an automatic irrigation system that controls the

switching operation (on and off) pump motor by sensing the soil moisture content.

The temperature humidity monitoring system of soil based on wireless sensor

networks using Arduino by Edgefxkits.com

The sensing arrangement senses the moisture of soil and appropriate signal is given to the

Arduino board. This is achieved using an op amp as comparator circuit acting as interface

between sensing arrangement and microcontroller. Based on the signal received from the

sensing arrangement, the water pump is operated. The LCD display is used for displaying the

status of soil moisture content and water pump.

Furthermore, technical assistance can be provided based on your queries posted in the

comments section below. You can download our free eBook to design electronics projects on

your own.

Do you know any embedded systems applications in which op-amp is used as comparator

circuit?