Difference amplifier and subtractor
- 1. A Presentation on Difference Amplifier And Subtractor For Analog circuit Design course of Bachelor of Engineering Presented by :- Bhavesh M. Chaudhari
- 3. Introduction :- A difference amplifier is a special purpose amplifier designed to measure differential signals, otherwise known as a subtractor. A key feature of a difference amplifier is its ability to remove unwanted common mode signals, known as common mode rejection (CMR). Unlike most types of amplifiers, difference amplifiers are typically able to measure voltages beyond the supply rails, and are used in applications where large dc or ac common-mode voltages are present. They are ideal for current and voltage monitoring. Analog Devices offers a range of difference amplifiers, optimized for low distortion, low power, or high voltage performance.
- 4. Expression for Output Voltage :- 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,
- 5. 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,
- 6. • 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.
- 7. This condition must be satisfied only when, In that case, equation (iii) becomes, Again, if we make, R1 = R2, then equation (iv) becomes,
- 8. Conclusion :- So, if R1 = R2 and also R3 = R4 then the difference amplifier becomes a perfect substractor, which subtracts directly the input signals. Finally, the circuit of op amp substractor becomes