Which statement is false regarding the circuit gain factor?

Which statement is false regarding the circuit gain factor?

• A. R3 forms an essential element of the circuit gain factor
• B. R2 sets the input impedance
• C. The gain equals the ratio of the feedback resistor to the input resistor
• D. The summing node is at virtual ground

Answer: (A)

The gain of a standard inverting op-amp is set by the ratio of the feedback resistor to the input resistor, and the summing node sits at a virtual ground because the op-amp drives its output to keep the inverting input at the same potential as the non-inverting input (usually ground). That means the input current flows through the input resistor into the feedback network, producing output A = -Rf/Rin (magnitude Rin is the resistor in the input path, and Rf is the feedback resistor). R3 does not determine this ratio unless it lies in the same signal path as the input or feedback resistors. So calling R3 an essential element of the circuit gain factor is not correct; it would only affect gain if it actually changes Rin or Rf, or affects biasing, not the fundamental gain relationship. The statements about the gain being set by the resistor ratio and the summing node being at virtual ground are the correct descriptions.

The gain of a standard inverting op-amp is set by the ratio of the feedback resistor to the input resistor, and the summing node sits at a virtual ground because the op-amp drives its output to keep the inverting input at the same potential as the non-inverting input (usually ground). That means the input current flows through the input resistor into the feedback network, producing output A = -Rf/Rin (magnitude Rin is the resistor in the input path, and Rf is the feedback resistor). R3 does not determine this ratio unless it lies in the same signal path as the input or feedback resistors. So calling R3 an essential element of the circuit gain factor is not correct; it would only affect gain if it actually changes Rin or Rf, or affects biasing, not the fundamental gain relationship. The statements about the gain being set by the resistor ratio and the summing node being at virtual ground are the correct descriptions.