OP-AMP Based Circuits

Summing Amplifier


To study the characteristic of a summing Amplifier


An operational amplifier ("op-amp") is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. An op-amp produces an output voltage that is typically hundreds of thousands times larger than the voltage difference between its input terminals.

The circuit symbol for an op-amp:

The Summing Amplifier

One of the most common applications for an op amp is to algebraically add two (or more) signals or voltages to form the sum of those signals. Such a circuit is known as a summing amplifier, or just as a summer. The point of using an op amp to add multiple input signals is to avoid interaction between them, so that any change in one input voltage will not have any effect on the other input.

The Summing Amplifier circuit is based upon the standard Inverting Operational Amplifier configuration that can be used for combining multiple inputs. The inverting amplifier has a single input voltage, (Vin) applied to the inverting input terminal. If we add more input resistors to the input, each equal in value to the original input resistor, Rin we end up with another operational amplifier circuit called a Summing Amplifier, "summing inverter" or even a "voltage adder" circuit as shown below.

Fig 1. Summing Amplifier Circuit

The output voltage, (Vout) now becomes proportional to the sum of the input voltages, V1, V2, V3 etc. Then we can write:



From eq. 1 and 2


If all the input impedances, (Rin) are equal in value the final equation for the output voltage is given as:

Summing Amplifier Equation

A direct voltage addition can also be obtained when all the resistances are of equal value and Rf is equal to Rin.

A Scaling Summing Amplifier can be made if the individual input resistors are "NOT" equal. Then the equation would can be written as:

The input impedance of each individual channel is the value of their respective input resistors, i.e., R1, R2, R3 ... etc.

The Summing Amplifier is a very flexible circuit indeed, enabling us to effectively "Add" or "Sum" together several individual input signals. If the inputs resistors, R1, R2, R3 etc, are all equal a unity gain inverting adder can be made. However, if the input resistors are of different values a "scaling summing amplifier" is produced which gives a weighted sum of the input signals.