In an inverting summing amplifier, what happens to the polarity of the summed voltages?

In an inverting summing amplifier, what happens to the polarity of the summed voltages?

• A. The polarity is unchanged.
• B. The polarity is inverted.
• C. The polarity is random.
• D. Polarity depends on the resistor values.

Answer: (B)

In an inverting summing amplifier, the polarity of the summed voltages is inverted. This happens because the inputs are connected to the inverting input, while the noninverting input is at ground. The op-amp forces the inverting node to act like a virtual ground (nearly 0 V) by adjusting its output through the feedback resistor. Currents from each input voltage flow through their respective input resistors into this summing node, and all of those currents must flow through the feedback resistor to the output. This creates an output current that is proportional to the sum of the input currents, but with the sign reversed, so the output voltage is Vout = -Rf*(V1/R1 + V2/R2 + ...). Therefore, positive input voltages produce a negative output (and vice versa), confirming that the summed polarity is inverted.

In an inverting summing amplifier, the polarity of the summed voltages is inverted. This happens because the inputs are connected to the inverting input, while the noninverting input is at ground. The op-amp forces the inverting node to act like a virtual ground (nearly 0 V) by adjusting its output through the feedback resistor. Currents from each input voltage flow through their respective input resistors into this summing node, and all of those currents must flow through the feedback resistor to the output. This creates an output current that is proportional to the sum of the input currents, but with the sign reversed, so the output voltage is Vout = -Rf*(V1/R1 + V2/R2 + ...). Therefore, positive input voltages produce a negative output (and vice versa), confirming that the summed polarity is inverted.