The inverting input voltage is approximately 0 V, referred to as virtual ground.

The inverting input voltage is approximately 0 V, referred to as virtual ground.

• A. Absolute zero
• B. Virtual ground
• C. Vin
• D. Vout

Answer: (B)

Virtual ground is the idea that the inverting input of an op-amp in negative feedback sits at a voltage very close to ground, even though it isn’t literally connected to ground. The non-inverting input is at ground, and the op-amp’s huge gain drives its outputs so that the two inputs are nearly equal. As a result, the inverting input voltage is about 0 V, which is why we treat it as ground for signal analysis. This doesn’t mean the node is a real ground conductor—the input current into the op-amp is ideally zero, so the current from Vin through the input resistor must flow through the feedback resistor to the output. That relationship gives the familiar Vout = −(Rf/Rin)·Vin for an ideal inverting amplifier. Other options don’t fit because absolute zero is a temperature, while Vin and Vout are the actual input and output voltages, not the potential at the inverting input. The inverting input is the “virtual ground” region that helps simplify analysis.

Virtual ground is the idea that the inverting input of an op-amp in negative feedback sits at a voltage very close to ground, even though it isn’t literally connected to ground. The non-inverting input is at ground, and the op-amp’s huge gain drives its outputs so that the two inputs are nearly equal. As a result, the inverting input voltage is about 0 V, which is why we treat it as ground for signal analysis.

This doesn’t mean the node is a real ground conductor—the input current into the op-amp is ideally zero, so the current from Vin through the input resistor must flow through the feedback resistor to the output. That relationship gives the familiar Vout = −(Rf/Rin)·Vin for an ideal inverting amplifier.

Other options don’t fit because absolute zero is a temperature, while Vin and Vout are the actual input and output voltages, not the potential at the inverting input. The inverting input is the “virtual ground” region that helps simplify analysis.