Summing amplifier principle: Which property of the op-amp input node enables the standard inverting summing amplifier to add multiple input voltages linearly?

Introduction / Context:
The inverting summing amplifier is a workhorse op-amp configuration that produces an output proportional to a weighted sum of several input voltages. The key enabler is the behavior of the inverting input node under negative feedback, often described by the “virtual ground” concept. Grasping this concept clarifies why currents from multiple inputs add algebraically at a single node without interaction.

Given Data / Assumptions:

• Ideal op-amp with infinite open-loop gain, infinite input impedance, and zero output impedance.
• Negative feedback via a feedback resistor from output to the inverting input.
• Noninverting input tied to ground (reference 0 V).

Concept / Approach:
With large open-loop gain and negative feedback, the op-amp drives its output so that the differential input voltage approaches zero. If the noninverting input is at 0 V, the inverting input node is held at approximately 0 V as well, even though it is not physically connected to ground. This is the “virtual ground.” Because the inverting input draws negligible current, currents from each source resistor flow into the summing node and then through the feedback resistor, adding linearly by Kirchhoff’s Current Law.

Step-by-Step Solution:

Verification / Alternative check:
SPICE simulations show the inverting node clamped near 0 V while the output adjusts to satisfy KCL. Practical op-amps with finite gain still maintain the node within microvolts to millivolts of ground in linear operation.

Why Other Options Are Wrong:
Virtual ground at the noninverting input does not create the summing node. Low/high-voltage ratings are irrelevant to the summing mechanism. Positive feedback would break linear summation and risk oscillation.

Common Pitfalls:
Assuming significant current enters the op-amp input; forgetting that resistor values set weights; ignoring output swing limits that can cause clipping.

Final Answer:
Virtual ground at the inverting input