The Summing Amplifier

The Summing Amplifier is a very flexible circuit based upon the standard Inverting Operational Amplifier configuration. As its name suggests, the “summing amplifier” can be used for combining the voltage present on multiple inputs into a single output voltage.

We saw previously in the Inverting Operational Amplifier that 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.

Summing Amplifier Circuit

summing amplifier


The output voltage, ( Vout ) now becomes proportional to the sum of the input voltages, V1, V2, V3 etc. Then we can modify the original equation for the inverting amplifier to take account of these new inputs thus:

summing amplifier formula


However, if all the input impedances, ( Rin ) are equal in value, we can simplify the above equation to give an output voltage of:

Summing Amplifier Equation

summing amplifier equation

We now have an operational amplifier circuit that will amplify each individual input voltage and produce an output voltage signal that is proportional to the algebraic “SUM” of the three individual input voltages V1, V2 and V3. We can also add more inputs if required as each individual input “see’s” their respective resistance, Rin as the only input impedance.

This is because the input signals are effectively isolated from each other by the “virtual earth” node at the inverting input of the op-amp. A direct voltage addition can also be obtained when all the resistances are of equal value and is equal to Rin.

A Scaling Summing Amplifier can be made if the individual input resistors are “NOT” equal. Then the equation would have to be modified to:

scaling summing amplifier equation


To make the math’s a little easier, we can rearrange the above formula to make the feedback resistor RF the subject of the equation giving the output voltage as:

summing amplifier feedback equation


This allows the output voltage to be easily calculated if more input resistors are connected to the amplifiers inverting input terminal. The input impedance of each individual channel is the value of their respective input resistors, ie, R1, R2, R3 … etc.

Sometimes we need a summing circuit to just add together two or more voltage signals without any amplification. By putting all of the resistances of the circuit above to the same value R, the op-amp will have a voltage gain of unity and an output voltage equal to the direct sum of all the input voltages as shown:

unity gain summing amplifier


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

Summing Amplifier Example No1

Find the output voltage of the following Summing Amplifier circuit.

Summing Amplifier

summing op-amp circuit


Using the previously found formula for the gain of the circuit

inverting op-amp gain


We can now substitute the values of the resistors in the circuit as follows,

summing amplifier input gain


We know that the output voltage is the sum of the two amplified input signals and is calculated as:

summing amplifier output voltage


Then the output voltage of the Summing Amplifier circuit above is given as -45 mV and is negative as its an inverting amplifier.

Summing Amplifier Applications

So what can we use summing amplifiers for?. If the input resistances of a summing amplifier are connected to potentiometers the individual input signals can be mixed together by varying amounts. For example, measuring temperature, you could add a negative offset voltage to make the output voltage or display read “0” at the freezing point or produce an audio mixer for adding or mixing together individual waveforms (sounds) from different source channels (vocals, instruments, etc) before sending them combined to an audio amplifier.

Summing Amplifier Audio Mixer

summing amplifier audio mixer circuit


Another useful application of a Summing Amplifier is as a weighted sum digital-to-analogue converter. If the input resistors, Rin of the summing amplifier double in value for each input, for example, 1kΩ, 2kΩ, 4kΩ, 8kΩ, 16kΩ, etc, then a digital logical voltage, either a logic level “0” or a logic level “1” on these inputs will produce an output which is the weighted sum of the digital inputs. Consider the circuit below.

Digital to Analogue Converter

digital to analogue converter


Of course this is a simple example. In this DAC summing amplifier circuit, the number of individual bits that make up the input data word, and in this example 4-bits, will ultimately determine the output step voltage as a percentage of the full-scale analogue output voltage.

Also, the accuracy of this full-scale analogue output depends on voltage levels of the input bits being consistently 0V for “0” and consistently 5V for “1” as well as the accuracy of the resistance values used for the input resistors, Rin.

Fortunately to overcome these errors, at least on our part, commercially available Digital-to Analogue and Analogue-to Digital devices are readily available with highly accurate resistor ladder networks already built-in.

In the next tutorial about Operational Amplifiers, we will examine the effect of the output voltage, Vout when a signal voltage is connected to the inverting input and the non-inverting input at the same time to produce another common type of operational amplifier circuit called a Differential Amplifier which can be used to “subtract” the voltages present on its inputs.

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28 Responses to “The Summing Amplifier”

  1. Denis Kirchoff

    Now in the case u have R1,R2,R3,,,,,V1,V2 and Rf only…how can i get the voltage output of that summing amp?

  2. Godfrey

    Wayne @ur reason, I don’t think it would work exactly so. I kind of read that since the summing amplifier is generally an inverting amplifier.

    • johnpaul benedict

      i thnk you need to up the game. do some practicals on proteus or pspice. since it is anolog in nature some small variations are prone to happen.try this

  3. perveen

    actually I tried to solve an equation of opamp using two inverting terminals of opamp … so that the output equal to Vo =-V1+V2+V3-V4 , I am not getting exact result can u tell y??

    • Wayne Storr

      Hello Perveen, Summing amplifiers will add together the values of two or more voltages on its inputs. If the values of all resistors are the same, then the output is a direct sum. Then for example, if V1 = -4v, V2 = 8v, V3 = 2v and V4 = -1v, the output will be (-4)+8+2+(-1) = -5v as its and inverting amplifier. To convert to a positve voltage output (+5v) pass through a unity gain non-inverting amplifier. Are you using dual or single supply op-amps.

  4. Ian Newcombe

    With regard to the 741 Op-Amp supply voltages quoted by Wayne, it must be stated that these are maximum values. Typical values used are +/- 15 volts, especially if the person experimenting with Op-Amps is not very familiar with the integrated circuits.
    It is easy to damage these devices by applying greater voltages than the maximum.

  5. VIDYA

    I have made summing amplifier but when I am applying only one voltage the output I am getting as the applied voltage. But when the second voltage source is connected it’s output is not agreeing the summing formula. What would be the reason?

  6. Malik Awais

    can anyone tell me how much Vcc should apply to the 7th pin of 741 IC for operating the summing amplifier…. i am making this circuit but still not working it rightly,,,,,,, :(


    why in summing amplifier the number of inputs in op-amp is taken as voltage gain(Av)

  8. Rod

    Hi, in the audio mixer example above, is there a particular reason why you chose the inverting input for the audio input rather than having the mixed audio going into the non-inverting input and setting the gain as you would a regular non-inverting amplifier?


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