Operational Amplifier as Voltage summation or adder and subtractor.

Voltage summation or adder

A voltage adder or voltage summing circuit is an electronic circuit that combines two or more input voltages into a single output voltage. There are different types of voltage adders, but one common circuit configuration is the op-amp summing amplifier.

The op-amp summing amplifier has multiple input terminals and a single output terminal. The input voltages are applied to the input terminals, and the output voltage is the sum of the weighted input voltages. The weights are determined by the feedback resistors in the circuit.

Figure shows an inverting amplifier circuit that is used to sum three inputs signals. Input signals V1, V2, V3 are applied through resistors R1, R2 and R3 to an inverting terminal. Rf is feedback resistance between input and output of the amplifier. As the point 'S' being virtually grounded then we can write

Therefore, output voltage is sum of input voltages. The negative sign in the formula indicates that the output voltage is inverted with respect to the input voltages. However, the magnitude of the output voltage can be adjusted by choosing the appropriate values for the resistors.

Voltage adders are commonly used in analog signal processing and control systems, where multiple signals need to be combined and processed.

Voltage Subtraction

Voltage subtraction is the process of subtracting one voltage from another. In electronic circuits, this can be accomplished using an operational amplifier (op-amp) configured as a differential amplifier.

A differential amplifier amplifies the difference between two input voltages, while rejecting any common-mode voltage that is present at both inputs. By choosing the appropriate resistor values, the differential amplifier can be configured to subtract one voltage from another.

Suppose we want to produce an output voltage that equals the mathematical difference between two input signals. The operation can be performed by using amplifier in a differential mode, where signals are connected through appropriate resistor network to the inverting and non inverting terminals.

First assume V2 is shorted to ground then we can write

Again assume V1 is shorted to ground we have

Therefore, both signals present, the output voltage is given as

which implies output voltage is subtraction of input voltages.

To perform voltage subtraction using an op-amp, the two input voltages are applied to the non-inverting and inverting inputs of the op-amp, respectively. The output voltage is taken from the output terminal of the op-amp.

In some cases, the output of the differential amplifier may need to be inverted. This can be accomplished by adding an additional inverting stage to the circuit, using another op-amp configured as an inverter.

Voltage subtraction can be useful in many applications, such as in instrumentation and control systems, where it may be necessary to measure the difference between two signals or to cancel out a common-mode voltage.

This note is a part of the Physics Repository.

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