Questions: Operational Amplifier Fundamentals

Question 1

An inverting amplifier is built with R_f = 100 kΩ and R_in = 10 kΩ. What is the voltage gain?

• A. +10
• B. -10
• C. +11
• D. -11

Answer: (B)

The inverting amplifier gain is -R_f/R_in = -100kΩ/10kΩ = -10. The negative sign indicates phase inversion. A gain of -11 (or +11) would be the non-inverting configuration gain formula (1 + R_f/R_in), which does not apply here. The gain depends only on the resistor ratio, not on the op-amp's own internal characteristics, which is the power of negative feedback.

Question 2

The virtual short rule means that the inverting (-) and non-inverting (+) input terminals of an ideal op-amp are electrically shorted together.

• T. True
• F. False

Answer: False

Virtual short means the two inputs are at the same potential — they have equal voltage — not that they are physically connected. In fact, the ideal op-amp has infinite input impedance, so no current flows between or into the input terminals. The equal-voltage condition arises because negative feedback drives the output until the differential input (V+ - V-) approaches zero. Treating them as a physical short would incorrectly allow current to flow between them.

Question 3

Why does the non-inverting amplifier have a gain of (1 + R_f/R_in) rather than simply R_f/R_in?

Model answer: In the non-inverting configuration, the input signal connects directly to the non-inverting terminal, so the virtual short sets the inverting terminal at the input voltage V_in. KCL at the inverting node gives V_in/R_in = (V_out - V_in)/R_f, which solves to V_out = V_in(1 + R_f/R_in). The extra '1' comes from the fact that even with R_f = 0 (a wire), the gain would be 1, because the input is passed through directly.

Compare with the inverting amplifier, where the non-inverting terminal is grounded and the signal enters through R_in. There, the virtual short sets the inverting terminal at 0 V, and KCL gives V_out/R_f = -V_in/R_in, yielding gain -R_f/R_in with no '1'. The non-inverting configuration always has gain ≥ 1, while the inverting configuration can have gain less than 1 in magnitude.