Operational amplifier configurations common mistakes

Common mistakes

• Input connected to non‑inverting input

  Students often think the input of an inverting amplifier is connected to the non‑inverting (+) input of the op‑amp.

  Fix itIn an inverting amplifier the input signal is applied to the inverting (−) input through the input resistor, the non‑inverting (+) input is grounded, and a feedback resistor connects the output back to the inverting input.

• Incorrect sign of voltage gain

  Students often calculate the voltage gain of an inverting amplifier as +Rf/Rin, ignoring the negative sign that indicates phase inversion.

  Fix itUse the formula Av = -Rf/Rin. Substitution: For Rf = 10 kΩ and Rin = 2 kΩ, Av = -(10 kΩ)/(2 kΩ) = -5. The negative sign shows the output is inverted. The magnitude of the gain is 5, so the output voltage is 5 × Vin but inverted.

• Misunderstanding of phase inversion in inverting amplifier

  Students think the output is simply the negative of the input voltage, ignoring the feedback resistor ratio and the virtual ground concept.

  Fix itExplain that the inverting amplifier forces the inverting input to a virtual ground; the output is Vout = - (Rf/Rin) × Vin, so the negative sign indicates phase inversion and the magnitude depends on the resistor ratio.

• Input applied to negative terminal

  Connecting the input signal to the op‑amp’s negative (inverting) input instead of the positive input

  Fix itIn a non‑inverting amplifier the signal must be applied to the positive (non‑inverting) input; the negative input is used for the feedback network.

• Missing the +1 term in non‑inverting gain formula

  Students often calculate the gain as Rf/Rin, ignoring the +1 that comes from the op‑amp’s input configuration.

  Fix itFormula: Vout/Vin = 1 + (Rf/Rin) → Substitution: Rf = 10 kΩ, Rin = 1 kΩ → Working: 1 + (10 kΩ / 1 kΩ) = 1 + 10 = 11 → Answer: 11 (dimensionless) → Conclusion: The correct voltage gain is 11, not 10.

• Phase inversion misconception

  Students assume the output of a non‑inverting amplifier is inverted because op‑amps invert signals in general.

  Fix itExplain that in a non‑inverting configuration the input is applied to the + terminal, so the output follows the input with the same phase; the op‑amp only inverts signals when the input is applied to the – terminal.

• Incorrect assumption of direct voltage addition

  Students often think that the output voltage of a summing amplifier is simply the sum of the input voltages, ignoring the resistor ratios and the negative sign.

  Fix itFormula/rule: Vout = - (Rf/R1 * V1 + Rf/R2 * V2 + ...). Substitution: Rf = 10kΩ, R1 = 5kΩ, V1 = 1V, R2 = 5kΩ, V2 = 2V. Working: Vout = - (10k/5k * 1V + 10k/5k * 2V) = - (2 * 1V + 2 * 2V) = - (2V + 4V) = -6V. Answer: -6V. Units/conclusion: The output is the negative weighted sum of the inputs, not the simple sum.

• Neglecting the inversion sign in summing amplifier output

  Students often write Vout = (Rf/R1)V1 + (Rf/R2)V2 for a summing amplifier, ignoring the negative sign that arises from the inverting input.

  Fix itTrace reasoning: Initial state: Rf = 10 kΩ, R1 = 5 kΩ, R2 = 5 kΩ, V1 = 2 V, V2 = 3 V. Step 1: Calculate the individual voltage gains: Rf/R1 = 10 kΩ/5 kΩ = 2, Rf/R2 = 10 kΩ/5 kΩ = 2. Step 2: Multiply each input by its gain: 2 V × 2 = 4 V, 3 V × 2 = 6 V. Step 3: Sum the contributions: 4 V + 6 V = 10 V. Step 4: Apply the inversion sign because the op‑amp input is inverting: Vout = –10 V. Final state: Vout = –10 V. Conclusion: Always include the negative sign when using the formula Vout = –(Rf/R1)V1 – (Rf/R2)V2 for a summing amplifier; omitting it leads to an incorrect positive output.

• Input Resistor Role Misconception

  Students think input resistors only set the gain, not the signal summing.

  Fix itFormula: V_out = -R_f (V1/R1 + V2/R2 + ...). Substitution: For two inputs, V_out = -R_f (V1/R1 + V2/R2). Working: If R1 = R2, each input contributes equally; if R1 ≠ R2, the input with the smaller resistor contributes more. Answer: Input resistors determine the weighting of each input and the input impedance. Units: Resistances in Ω, voltages in V. Conclusion: They set the contribution of each input to the output, not just the overall gain.

• Incorrect assumption about output voltage of summing amplifier

  Students often think the output voltage of a summing amplifier is simply the arithmetic sum of the input voltages, ignoring the resistor ratios and the negative sign from the inverting configuration.

  Fix itRemember that for an ideal op‑amp in a summing configuration, Vout = –(Rf/R1)V1 – (Rf/R2)V2 …; the negative sign indicates phase inversion and the resistor ratios set the weighting of each input. Choose Rf and Ri to obtain the desired signal weighting and maintain high input impedance.