In a non-inverting summing amplifier, which statement about the inverting input is true?

In a non-inverting summing amplifier, which statement about the inverting input is true?

• A. It must be at virtual ground
• B. It is always at ground
• C. It is always at the positive supply
• D. It may not be at virtual ground

Answer: (D)

The key idea is that an op-amp with negative feedback forces its two inputs to be at the same voltage (a virtual short). But that common voltage isn’t fixed at zero; it follows whatever voltage is applied to the non-inverting input. In a non-inverting summing amplifier, the non-inverting input is fed by a resistor network that combines the input signals into a certain reference voltage. Because of that, the inverting input will be driven to the same reference voltage, not necessarily ground. It will only be at virtual ground if that reference voltage happens to be zero. So the inverting input may not be at virtual ground in general.

The key idea is that an op-amp with negative feedback forces its two inputs to be at the same voltage (a virtual short). But that common voltage isn’t fixed at zero; it follows whatever voltage is applied to the non-inverting input. In a non-inverting summing amplifier, the non-inverting input is fed by a resistor network that combines the input signals into a certain reference voltage. Because of that, the inverting input will be driven to the same reference voltage, not necessarily ground. It will only be at virtual ground if that reference voltage happens to be zero. So the inverting input may not be at virtual ground in general.