Difference Amplifier or OP Amp Subtractor

Difference Amplifier or OP Amp Subtractor

1. Presented by
Dr. R. RAJA, M.E., Ph.D.,
Assistant Professor, Department of EEE,
Muthayammal Engineering College, (Autonomous)
Namakkal (Dt), Rasipuram – 637408
MUTHAYAMMAL ENGINEERING COLLEGE
(An Autonomous Institution)
(Approved by AICTE, New Delhi, Accredited by NAAC, NBA & Affiliated to Anna University),
Rasipuram - 637 408, Namakkal Dist., Tamil Nadu.
Difference Amplifier or OP Amp Subtractor

2. Difference Amplifier or Op Amp Subtractor
A difference amplifier or op amp subtractor is a specially designed op amp
based amplifier circuit, which amplifies the difference between two input
signals and rejects any signals common to both inputs.

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3. Contd..
Let us consider the above op amp circuit. Now, by applying Kirchhoff Current Law
at node 1, we get,



We have written this equation by assuming that there is no current entering in the
inverting terminal of the op amp.
Now, by simplifying the above equation, we get,

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4. Contd..
Now, by applying Kirchhoff Current Law, at node 2, we get,







We know that, in ideal op amp, voltage at inverting input is same as the voltage at
non inverting input. Hence,



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5. Contd..
So, now from equation (i) and (ii), we get,







The difference amplifier must reject any signal common to both inputs. That means,
if polarity and magnitude of both input signals are same, the output must be zero.


This condition must be satisfied only when,
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6. Contd..
In that case, equation (iii) becomes



Again, if we make, R
1 = R
2, then equation (iv) becomes,


So, if R
1 = R
2 and also R
3 = R
4 then the difference amplifier becomes a perfect
subtractor, which subtracts directly the input signals.
Finally, the circuit of op amp substractor becomes,


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7. Thank You
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