Electronics · Free Calculator
Op Amp Summing Amplifier Calculator
Find the output of an inverting summing amplifier with two or three inputs. Leave V3 and R3 empty for two inputs.
Op amp adderWeighted sumUp to 3 inputsAudio mixer
V
V
V
V
Output voltage−1.7 V
Weight of V1−1
Weight of V2−1
Weight of V3−1
How this result was calculated
- Vout = −Rf × (V1/R1 + V2/R2 + V3/R3) = −1.7 V
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Summing Amplifier Formula
Vout = −Rf × (V1/R1 + V2/R2 + V3/R3)
Equal resistors: Vout = −(V1 + V2 + V3)
Equal resistors: Vout = −(V1 + V2 + V3)
Worked example
V1 = 1 V, V2 = 0.5 V, V3 = 0.2 V with all resistors 10 kΩ: Vout = −(1 + 0.5 + 0.2) = −1.7 V.
Key insight: the inverting input is a virtual ground, so the inputs do not interact; each source sees only its own resistor, which is why this is the classic audio mixer and DAC circuit.
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Frequently Asked Questions
What is a summing amplifier?
An inverting op amp with several input resistors. The output is the negative weighted sum of the inputs.
How do I make a non inverted sum?
Follow the summer with a unity gain inverting stage.
Can the weights differ?
Yes. Each input gain is −Rf/Rn, so choose Rn to weight each input.
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