OPERATIONAL AMPLIFIER APPLICATIONS

OPERATIONAL AMPLIFIER APPLICATIONS

    A high-gain, the direct-coupled amplifier is what an operational amplifier is. It has two inputs and an output, but generally, it is operated in single-ended input - single-ended output mode. Op-amp can perform several arithmetic operations like addition, subtraction, differentiation, integration, comparison, analog to digital conversion, etc. Op-amps are used in analog computers.

   IC 741 is a widely used OP-Amp. In this IC when the input is zero, the output can be adjusted to zero by varying the 10kΩ potentiometer between ‘offset null’ terminals. 

 

Applications of OP-Amp: There are many applications of Op-amp. It has a high open-loop gain, high input impedance, and low output impedance. It has a common-mode rejection ratio. Due to these favorable characteristics, it is used for different applications. Important Linear applications are- buffer, inverter, adder, and subtractor. Important non-linear applications of Op-amp are- Integrator, Differentiator, and Logarithmic amplifier.          

Linear Applications

• BUFFER-Unity gain voltage follower

·        A buffer amplifier is a stage that separates the previous and subsequent stages.  As the gain of this amp. Is 1, it is called ‘unity gain voltage follower.

·        It is a non-inverting amplifier. Output is feedback directly to the inverting input terminal. So, Rf = 0.

Therefore, Vo=Vi  i.e. Output signal is in phase with the input signal.

·        USE- Buffer is used to providing high impedance and very low output impedance. Therefore, the preceding stage is highly loaded the and buffer can drive heavy loads (small load resistance).

• INVERTER or Sign Changer

·        Input is applied to inverting input terminal in an inverter. In inverting op-amp closed-loop gain is:

In inverter Rf=Ri=R,

Closed-loop gain= -1

Output voltage= Vo= -Vi

Thus, gain of this op-amp is -1 and it can be used to change the sign of the input signal.

• ADDER – Inverting Summing Amplifier

·        Adder is an inverting amplifier. Voltages V1 and V2 are applied at the input terminal through R1 and R2 respectively. 

·        The non-inverting terminal is grounded and the inverting input terminal is at virtual ground.

At inverting input terminal,

V1R1 + V2R2 - VoRf= 0

Vo= -RfV1R1+V2R2

OP-Amp is said to be INVERTING SUMMING AMPLIFIER

Here, say Rf=R1=R2=R

Then, Vo= -[V1+V2]

·        The output is the sum of the input voltages but with a negative sign which indicates 180 degree phase shift of the output with the input.

• SUBTRACTOR – Differential Amplifier

·        As input impedance of op-amp is very high, current absorbed by the amplifier is zero. 

·        The differential mode input is equal to p.d. between A and B.

Gain=A=VoVab

Vab =VoA

As A→ ∞, Vab≈0

Va=Vb=v

V1-vR1=v-VoRf

V1R1=v1R1+1Rf-VoRf        …(1)

Also,V2R1=v1R1+1Rf  ….(2)

Solving eq1and eq2

V1R1=V2R1-VoRf

Vo=RfR1[V2-V1]

If R1=Rf, Vo=V2-V1

·        Thus, the output voltage is difference between two input voltages.

Non-linear Applications- The circuits in which the shape of the output signal is different from that of the input, are called as ‘non-linear circuit’.

• INTEGRATOR

·        The output voltage of an integrator is proportional to the integral of the input voltage. 

·        This is an inverting op-amp, where feedback is provided through the capacitor C. Now, voltage across the capacitor is

Vc=qC.  But q= idt

Vc= idtC

·        As Vout is the potential of output terminal with respect to the ground.

Vout= -idtC

As X is at virtual ground, i=VinR1

Vout= -VindtR1C= -1R1CVin dt

Thus, the output is proportional to the time integral of the input voltage.

·        As a result, low-frequency signals travel through the system more easily than high-frequency signals. As a result, this integrator is known as a 'low pass filter.'

• DIFFERENTIATOR

·        A differentiator produces an output voltage proportional to the slope of the input voltage. The input voltage is applied to the inverting input terminal through a capacitor.

·        The difference between integrator and differentiator circuit is that resistor and capacitor are interchanged.

·        All applied input voltage would applied across the capacitor, as point X is at virtual ground.

Vc = Instantaneous voltage across the capacitor, then

Vc=qC. Hence   q=CVc

Instantaneous current across capacitor would be

Iin=dqdt=C.dVindt

Due to high input impedance of an Op-amp,

Iin=If

But If=0-VoutR= -VoutR

As Iin=If,  C.dVidt= -VoR

Vo= -RCdVidt

·        As frequency increases, output voltage increases. Therefore, differentiator is called ‘high pass filter’. 

• LOGARITHMIC AMPLIFIER

·        An op-amp based logarithmic amplifier produces a voltage at the output, which is proportional to the logarithm of the voltage applied to the resistor connected to its inverting terminal.

·        The voltage at an inverting input terminal will be identical to the voltage at its non-inverting input terminal, according to the virtual short idea. As a result, the inverting input terminal voltage will be zero volts.

0-ViR1+If=0

If=ViR1   ….(1)

·        When the diode is forward biased, equation of current flowing through it-

If=IseVfnVt …..(2)

where,

Is:  the saturation current of the diode,

Vf : the voltage drop across diode, when it is in forward bias,

Vt:  the diode’s thermal equivalent voltage.

·        KVL across feedback loop, 0-Vo-Vf=0,      Vo=Vf

From eq(2), IfIs=e-VonVt          …….(3)

Applying natural logarithm on both side,

InViR1Is= -VonVt

Vo= -nVtlnViR1Is

As Vt and Is are constant, output voltage Vo proportional to the natural logarithm of the input voltage Vi and fixed value of resistance R1. The negative sign indicates 180degree phase shift of the output voltage

CONCLUSION- We had seen many applications how by using inverting and non-inverting terminals of Op Amp it can act as various devices and can be used at a number of places. It is used extensively in signal conditioning, filtering or to perform mathematical operations such as add, subtract, integration and differentiation. A number of functions can be performed by a simple Op Amp just by changing components attached to it, or simply editing the design or just altering the voltage level according to the requirements. In modern electronics, the use of this multitasking device is increasing day by day and it is available in advanced design and in compatible form.