On the inverting amplifier, the summing junction is at

On the inverting amplifier, the summing junction is at

• A. Real ground
• B. Virtual ground
• C. Floating node
• D. Non-inverting input

Answer: (B)

In an inverting amplifier, the node where the input resistor and the feedback resistor meet is kept at nearly zero volts by negative feedback. The non-inverting input is tied to real ground, so the op-amp adjusts its output to force the inverting input to match that ground potential. Because the op-amp inputs draw negligible current, this summing junction is not physically connected to earth; it simply behaves like ground for voltage while currents flow through the resistors to the output. This is the virtual ground: it has zero (or near-zero) voltage, but it is not a true ground. With an ideal op-amp, V- equals V+ = 0, so the input current is V_in/R_in and the current through the feedback resistor is V_out/R_f, and Kirchhoff’s current law at the summing node gives V_out ≈ - (R_f/R_in) V_in.

In an inverting amplifier, the node where the input resistor and the feedback resistor meet is kept at nearly zero volts by negative feedback. The non-inverting input is tied to real ground, so the op-amp adjusts its output to force the inverting input to match that ground potential. Because the op-amp inputs draw negligible current, this summing junction is not physically connected to earth; it simply behaves like ground for voltage while currents flow through the resistors to the output. This is the virtual ground: it has zero (or near-zero) voltage, but it is not a true ground.

With an ideal op-amp, V- equals V+ = 0, so the input current is V_in/R_in and the current through the feedback resistor is V_out/R_f, and Kirchhoff’s current law at the summing node gives V_out ≈ - (R_f/R_in) V_in.