Summing Amplifier using Op-Amp

Summing Amplifier using Op-Amp

1. Aim

To build an inverting summing amplifier using OP07 and verify that output is proportional to the algebraic sum of input voltages.

2. Apparatus / Components Required

  • SEELab3 unit
  • OP07 op-amp
  • $R_1 = 1\text{ kOhm}$, $R_2 = 1\text{ kOhm}$, $R_f = 1\text{ kOhm}$
  • Dual supply for op-amp (about $\pm 6\text{ V}$)
  • Breadboard and connecting wires

3. Theory & Principle

For an inverting summing amplifier: \[V_{out} = -R_f\left(\frac{V_1}{R_1} + \frac{V_2}{R_2}\right)\]

With $R_1 = R_2 = R_f$: \[V_{out} = -(V_1 + V_2)\]

So for $V_1 = 1\text{ V}$ and $V_2 = 2\text{ V}$, expected output is: \[V_{out} = -3\text{ V}\]

The same circuit can be tested with AC signals to observe real-time waveform addition and inversion.

4. Circuit Diagram / Setup

  1. Power OP07 with dual supply.
  2. Ground the non-inverting input (+).
  3. Apply input $V_1$ (PV1) through $R_1$ to inverting node.
  4. Apply input $V_2$ (PV2) through $R_2$ to the same inverting node.
  5. Connect $R_f$ from output to inverting node.
  6. Measure output using A1/scope.

5. Procedure

  1. Set PV1 = 1 V and PV2 = 2 V.
  2. Record output voltage and compare with theoretical -3 V.
  3. Repeat for multiple input pairs.
  4. Replace one or both DC inputs with AC signals and observe summed output.
  5. Increase amplitude and note clipping near supply rails.

6. Observation Table

Trial$V_1$ (V)$V_2$ (V)Expected $V_{out}$ (V)Measured $V_{out}$ (V)Error (%)
11.02.0-3.0  
2     
3     
4     

7. Results and Discussion

  • The circuit performed weighted addition with inversion.
  • For equal resistors, measured output followed $V_{out} \approx -(V_1 + V_2)$.
  • At larger input combinations, output clipping appeared near op-amp rail limits.

8. Precautions

  1. Ensure common ground for all input sources and SEELab3.
  2. Verify resistor values before power-on.
  3. Start with low input levels to avoid clipping.
  4. Check OP07 supply polarity and pinout.

9. Troubleshooting

SymptomPossible CauseCorrective Action
Output incorrect signWrong input connected to non-inverting pathRecheck inverting summing topology
Output not equal to sumWrong resistor valuesConfirm $R_1$, $R_2$, $R_f$ values
Output clippedInput sum too largeReduce input amplitude/DC level

10. Viva-Voce Questions

Q1. Why is summing amplifier output negative for positive inputs?

Ans: Because input signals are applied to the inverting terminal with negative feedback.

Q2. How does unequal resistor choice change behavior?

Ans: Each input gets a different weight: $V_{out} = -R_f(V_1/R_1 + V_2/R_2)$.