Which statement about summing amplifiers is true?

Which statement about summing amplifiers is true?

• A. They can sum multiple inputs with appropriate gains and inversion.
• B. They can never sum more than two inputs.
• C. They always preserve phase with respect to each input.
• D. They do not use feedback.

Answer: (A)

Summing signals with weighted gains relies on using the op-amp’s inverting input as a virtual ground and feeding multiple signals through separate resistors. Each input voltage V[i] produces a current I[i] = V[i]/R[i] that flows into the summing node. The feedback resistor Rf converts the total current (sum of I[i]) into an output voltage: Vout = -Rf × (V1/R1 + V2/R2 + ...). This shows you can sum any number of inputs and set their relative contributions (gains) by choosing R[i], with the overall scale set by Rf, and the output being inverted relative to the inputs. This also explains why the other statements aren’t correct. You’re not limited to two inputs—any number can be added via additional input resistors. The output is not guaranteed to preserve the same phase as each input because the standard summing configuration inverts the signal. And negative feedback is essential to establish the virtual ground at the inverting input and to achieve a stable, predictable sum; without it, the amplifier wouldn’t produce a controlled weighted sum.

Summing signals with weighted gains relies on using the op-amp’s inverting input as a virtual ground and feeding multiple signals through separate resistors. Each input voltage V[i] produces a current I[i] = V[i]/R[i] that flows into the summing node. The feedback resistor Rf converts the total current (sum of I[i]) into an output voltage: Vout = -Rf × (V1/R1 + V2/R2 + ...). This shows you can sum any number of inputs and set their relative contributions (gains) by choosing R[i], with the overall scale set by Rf, and the output being inverted relative to the inputs.

This also explains why the other statements aren’t correct. You’re not limited to two inputs—any number can be added via additional input resistors. The output is not guaranteed to preserve the same phase as each input because the standard summing configuration inverts the signal. And negative feedback is essential to establish the virtual ground at the inverting input and to achieve a stable, predictable sum; without it, the amplifier wouldn’t produce a controlled weighted sum.