Op-Amp Series – Part 4: The Summing Amplifier

The Summing Amplifier

In this part of the op-amp series, we’re going to build and test summing amplifiers.

A summing amplifier combines multiple input signals into one output, making it one of the most useful and flexible op-amp building blocks.

In this post we will:

• Build two types of summing amplifier
• Measure real voltages on the bench
• Match the math directly to what we measure
• Highlight practical limitations with single-supply op-amps

What Is a Summing Amplifier?

A summing amplifier is an op-amp circuit that adds multiple input signals together and produces a single output voltage.

Key idea (very important):
👉 Summing amplifiers add currents, not voltages.

👉 The op-amp output converts the summed current back into a voltage.

Each input voltage is converted into a current by a resistor.
Those currents are summed at the op-amp input and converted back into a voltage at the output.

There are two basic types of summing amplifiers: -

1. Inverting
2. Non-Inverting

Type 1 — Inverting Summing Amplifier

This is the most common summing amplifier and the best place to start.

Inverting Summing Amplifier

• Each input voltage (V1, V2, V3…) connects to the inverting (–) input
• Each input has its own resistor (R2, R3, R4…)
• A feedback resistor (Rf) connects output to the inverting input
• The non-inverting (+) input is connected to ground

Just like the basic inverting amplifier, the inverting input is held at virtual ground.

How the Inverting Summer Works

Because of negative feedback and the ideal op-amp rules:

• The inverting input stays at ~0 V
• No current flows into the op-amp input
• All input currents must flow through the feedback resistor

Each input creates a current:

I2 = V1 / R2
I3 = V2 / R3
I4 = V3 / R4

These currents add together:

I_total = I2 + I3 + I4

That total current flows through Rf, creating the output voltage.

Output Voltage Equation (Inverting Summer)

Vout = - Rf ( V1/R2 + V2/R3 + V3/R4 )

If all resistors are equal:
R2 = R3 = R4 = R
Rf = R
Then the equation simplifies to:
Vout = - ( V₁ + V₂ + V₃ )
This is a true voltage summer (with inversion).
This configuration is excellent for:
Demonstrations
Learning
Simple mixers where all inputs are equal

What the Equation Tells Us

• The minus sign means the output is inverted
• Each input can be weighted by choosing resistor values
• Smaller resistor → larger influence on output
• Larger resistor → smaller influence

This is why summing amplifiers are used as mixers.

Practical Build — Inverting Summing Amplifier

Lets build a basic inverting summing amplifier where all resistors are equal at 10kΩ and with two voltage inputs.

For each of the voltage inputs we will use a voltage divider to obtain a different voltage input from the original supply.

With the inputs: -

• V1 = 1 V
• V2 = 2 V

The expected output should be: -

Vout = - (1 + 2) = -3 V

Circuit Diagram

Inverting Summing Amplifier Practical Build Circuit Diagram

Parts Required

• LM358 operational amplifier
• 5 × 10kΩ resistors
• 1 × 40kΩ resistors
• Breadboard
• 6V DC power supply
• Input voltage source (potentiometer, signal generator, or fixed voltage source)
• Multimeter or oscilloscope

Step-by-Step Wiring

Place the LM358

• Put the LM358 across the breadboard centre gap.
• Identify pin 1 (dot/notch on the chip).

Power the op-amp

• Connect pin 8 to +6 V
• Connect pin 4 to 0 V (ground)

Connect Power Rails to Op-Amp

Choose which op-amp inside the LM358
I will use op-amp A:

• Pin 1 = Output
• Pin 2 = Inverting (–)
• Pin 3 = Non-inverting (+)

Ground the non-inverting input

• Connect pin 3 to ground

Pin 3 to Ground

Add the feedback resistor

• Connect Rf (10kΩ) from pin 1 (output) to pin 2 (inverting input)

Rf between Pin 1 and Pin 2

Add input resistor for V1

• Connect the 6V rail to R5 (40kΩ) 
• Connect R5 to R6 (10kΩ) then the midpoint is V1 source to pin 2 though R1 (10kΩ)
• Connect the other end of R6 to ground.

Add input resistor for V2

• Connect 6V rail to R7 (10kΩ) 
• Connect R7 to R8 (10kΩ) then the midpoint is V2 source to pin 2 though R2 (10kΩ)
• Connect the other end of R8 to ground.

Supply Voltages

Measured Result

Measure V1 and V2 directly (make sure they’re really 1 V and 2 V).
Measure Vout at pin 1 relative to ground.
You should see Vout sit at (or near) 0 V because it’s trying to go negative.
Optional: vary V1 / V2 and watch Vout stay pinned at ground.

V1 Supply

V2 Supply

Vout

On a single-supply LM358, the output cannot go negative, so the output clips at ground.

This perfectly demonstrates why biasing is required for real single-supply designs.

Type 2 — Non-Inverting Summing Amplifier

Non-Inverting Summing Amplifier Circuit

A non-inverting summing amplifier works differently:

• Inputs are combined using a resistor network into the non-inverting (+) input
• The op-amp is configured as a non-inverting amplifier
• Output polarity matches the input signals (no inversion)

This type is useful when you want a summed output that stays positive and non-inverted on a single supply.

How It Works

Step 1: the resistor network creates a combined voltage

For equal resistors feeding the + input, the combined voltage is the average:

Vsum = (V1 + V2 + V3) / 3

​With two inputs:

$${\mathrm{Vsum\; =\; (V1\; +\; V2)\; /\; 2}}_{}$$

Step 2: the op-amp amplifies that voltage

Non-inverting gain is:

Av = 1 + (Rf / Rg)

So:

Vout = Vsum × (1 + (Rf / Rg))

Practical Build 2 — Non-Inverting Summing Amplifier 

Lets build a basic non-inverting summing amplifier where all resistors are equal at 10kΩ and with two voltage inputs.

For each of the voltage inputs we will use a voltage divider to obtain a different voltage input from the original supply.

With the inputs: -

• V1 = 1 V
• V2 = 2 V

Parts Required

• LM358 op-amp
• 2 × 10kΩ resistors (input resistors to the + node)
• 1 × 10kΩ resistor (Rg)
• 1 × 10kΩ resistor (Rf)
• 2 × 10kΩ resistor (Voltage divider)
• 1 × 40kΩ resistor (Voltage divider)
• Breadboard + jumper wires
• 6V DC supply
• Multimeter (oscilloscope optional)

Circuit Diagram

Non-Inverting Summing Amplifier Practical Build Circuit Diagram

Example Values

Inputs:

• V1 = 1.0 V
• V2 = 2.0 V

Input network:

• Two equal resistors (10kΩ each) feeding + input

Gain network:

• Rg = 10kΩ
• Rf = 10kΩ

Gain = 1 + (10k/10k) = 2

Expected Output

First, compute the summed voltage:

Vsum = (V1 + V2) / 2

Vsum = (1.0 + 2.0) / 2

Vsum = 3.0 / 2

Vsum = 1.5 V

Then apply gain:

Vout = Vsum * Av

Vout = 1.5 * 2

Vout = 3.0 V

Step-by-step wiring guide

Place and power the LM358

• Pin 8 to +6 V
• Pin 4 to ground

Use op-amp A again

• Pin 1 = output
• Pin 2 = inverting (–)
• Pin 3 = non-inverting (+)

Build the non-inverting gain resistors

• Connect Rg (10kΩ) from pin 2 to ground
• Connect Rf (10kΩ) from pin 1 (output) to pin 2

Create the summing node for the + input

• Make a single breadboard row as your “Vsum node”
• Connect pin 3 to that node

Create V1 into the Vsum node

• Connect +6V rail to a 40kΩ resistor the end of this is the V1 node
• Connect 10kΩ from V1 to the Vsum node
• Connect a 10kΩ from the V1 node to ground

Create V2 into the Vsum node

• Connect +6V rail to a 10kΩ resistor the end of this is the V2 node
• Connect 10kΩ from V2 to the Vsum node
• Connect a 10kΩ from the V2 node to ground

Vsum node

• Connect the Vsum node to Pin 3 of the op-amp

Non-Inverting Op-Amp Full Build

Step-by-step measurement guide

Measure V₁ and V₂ directly.
Measure Vsum node (pin 3). You should read about 1.5 V.
Measure Vout at pin 1. You should read about 3.0 V.
Vary V1/V2 and confirm:

• Vsum follows the average
• Vout follows Vsum × gain

V1 Supply

V2 Supply

Sum of the voltages going in is 3.6V approx. I had to use different resistors as I didn't have the exact values.

Vout

As can see though the output is still correct and equals the sum of the inputted voltages.

YouTube Video

Common Uses of Summing Amplifiers

• Audio mixers
• Combining sensor signals
• Adding DC offsets (biasing)
• Analogue computation
• Feedback and control systems

Key Takeaways

• Summing amplifiers add multiple signals into one
• Inputs are summed as currents
• Inverting summing amplifiers are the simplest and most common
• Non-inverting summing amplifiers preserve signal polarity
• Resistor values directly control signal contribution
• Bench measurements match the maths exactly (when limits are respected)

What’s Next?

Next up, we’ll move to the differential amplifier, where instead of adding signals, we subtract them, opening the door to noise rejection and instrumentation amplifiers.

👉 Op-Amp Series – Part 5: The Differential Amplifier

Hope you enjoyed this post.
Thanks for reading
Matty