Master the Basics: Combine Signals

Operational amplifiers (op-amps) are powerful tools in electronics, capable of performing a range of mathematical operations. You might already be familiar with op-amps used for amplification — essentially multiplying or dividing voltages. But op-amps can also add signals together. In this post, we’ll look at how to build a summing amplifier, an op-amp circuit that combines multiple voltages into a single output.

What Is a Summing Amplifier?

A summing amplifier is built on the foundation of the inverting amplifier configuration. By connecting additional input resistors to the inverting terminal of the op-amp, you can apply multiple input voltages (e.g., VA and VB) and combine them into one output.

• Each input voltage passes through its own resistor (RA, RB, etc.)

• The non-inverting (+) terminal is grounded

• The op-amp adjusts its output to keep the inverting (-) terminal at 0V

• All input currents are summed through a single feedback resistor (RF)

Output Voltage Equation

The total output voltage of the summing amplifier is calculated using Ohm’s Law:

Vout = -RF × [(VA / RA) + (VB / RB) + ...]

This negative sign means the output is inverted compared to the sum of the inputs.

Real-World Example

Let’s use:

• RA = RB = RF = 1kΩ

• VA = 1V, VB = 2V

Then:

• Current from VA: 1V / 1kΩ = 1mA

• Current from VB: 2V / 1kΩ = 2mA

• Total current through RF = 3mA

• Vout = - (1kΩ × 3mA) = -3V

Customizing the Mix

You can control how much each input affects the output by adjusting the input resistors:

• Doubling RB to 2kΩ reduces VB’s current to 1mA

• Now total current = 1mA (from VA) + 1mA (from VB) = 2mA

• New Vout = - (1kΩ × 2mA) = -2V

Want to adjust the overall gain? Change RF:

• Set RF = 3kΩ → Output scales by ×3

• Same input current now yields Vout = -6V

 Add More Inputs

The summing amplifier isn’t limited to two inputs. Add more resistors and input voltages, and the op-amp will still sum them — weighted by the resistor values you choose.

 Why Use a Summing Amplifier?

• Mix audio signals

• Create weighted sensor inputs

• Perform analog computing

• Customize signal levels before ADC (analog-to-digital conversion)

Key Takeways

Summing amplifiers are a great way to add and scale multiple voltages using a single op-amp. With flexible resistor values and a simple layout, this circuit becomes an essential tool in analog signal processing. Whether you're blending audio or combining sensor data, a summing amplifier is your go-to building block.