Applications of OP-AMP

Introduction • Operational amplifier using IC's is inexpensive, versatile and easy to use. For this

reason they are used not only for negative feedback amplifiers but also used for

wave shaping, filtering and mathematical operations. Some commonly used

applications are

– Inverting Amplifier

– Summing Amplifier

– Non-Inverting Amplifier

– Voltage Follower

– Comparator

– Zero Cross Detector

– Integrator

– Differentiators

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An Inverting Amplifier Using Op-Amp

• The non Inverting input is grounded.

• A “feedback network” composed of

resistors R1 and R2 is connected

between the inverting input, signal

source and amplifier output node,

respectively.

This amplifier uses voltage shunt negative feed back for controlling the gain

It looses high input resistance property because of shunt mixing at i/p

There exist a virtual short circuit between the 2-inputs of Op-Amp.

The inverting i/p is at virtual groundwww.ustudy.in

Inverting Amplifier : Voltage Gain Av

• The negative sign of voltage gain implies that there is a 1800 phase shift between input and output signals.

• The gain magnitude can be greater than 1 if R2 > R1

• The gain magnitude can be less than 1 if R1 > R2

• The inverting input of the Op Amp is at ground potential (although it is not connected directly to ground) and is said to

be at virtual ground.

0ov22

i1

isv RRs

1

svsi R

But is= i2 and v- = 0 (since vid= v+ - v-= 0)

and

1

2

svov

R

RvA

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Inverting Amplifier: Input and Output Resistance

01

since 1

sisv

vRinR

Rout is found by applying a test current

(or voltage) source to the amplifier

output and determining the voltage (or

current) after turning off all

independent sources. Hence, vs = 0

11i

22iov RR

But i1=i2

)12

(1iv RRo

Since v- = 0, i1=0. Therefore vo = 0 irrespective of the value of io .

0 outR

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Summing (Scaling) Amplifier

Because the op-amp Inv-Input is at virtual ground the multiple inputs are treated as separate inputs.

3

3

f2

2

f1

1

fV

R

RV

R

RV

R

RVo

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The Non-inverting Amplifier

• The input signal is applied to the non-inverting input.

• A part of the output signal is fed back to the inverting input.• There exist the virtual short circuit between two input terminals of Op

Amp, v1 is same as vs

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Voltage follower

• The voltage follower, also called a buffer, provides a high

input impedance, a low output impedance, and unity gain.

As the input voltage changes, the output and inverting input

will change by an equal amount. The voltage gain of this

configuration is unity or 1.

• This circuit is useful though it does not have any voltage

gain. Since it has a high input impedance, it makes an

excellent isolation amplifier.

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• The inverting amplifier exhibits a much lower input

impedance due to the virtual ground created by the negative

feedback.

• A high impedance signal source will suffer loading effects

when connected to such amplifier and therefore it is

sometimes necessary to isolate the signal source by connecting

it to a voltage follower to the next stage.

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Comparator

• Compares two voltages and switches its output to indicate which voltage is larger.

• (where Vs is the supply voltage and the opamp is powered by + Vs and − Vs.)

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The inverting input is at virtual ground. Hence,

Op-Amp Integrator

dt

dvCi o

2

iR

vi in

21

Applying KCL at the inverting input

i1+i2 = 0

0R

v

dt

dvC ino

)initial(vdtvRC

1v oino

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The Difference Amplifier

• This circuit is also called a differential amplifier, since it amplifies the difference between the input signals.

• Rin2 is series combination of R1 and R2 because i+ is zero.

• For v2=0, Rin1= R1, as the circuit reduces to an inverting amplifier.

• For general case, i1 is a function of both v1 and v2.

1v

1

2-v

1

21)-v1

v(

1

2-v

21i-v

22i-vov

R

R

R

RR

R

R

RR

vR

2R1R

2

v2

Also,

Since v-= v+)

2v

1(v

1

2v R

Ro

For R2= R1)

2v

1(vv o

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