Which circuit illustrates a difference amplifier?

Which circuit illustrates a difference amplifier?

• A. Circuit A: A four-resistor differential amplification arrangement
• B. Circuit B: Classic four-resistor differential amplifier
• C. Circuit C: Non-inverting amplifier configuration
• D. Circuit D: Inverting amplifier configuration

Answer: (B)

Difference amplifiers are built to take two input voltages and produce an output proportional to their difference. The key is the symmetric four-resistor network around the op-amp: each input is fed through its own resistor, and the feedback and output resistors are paired so that the gains on both inputs match. When the resistor ratios are matched (R1 to R3 equals R2 to R4), the output becomes proportional to the difference of the two inputs, specifically Vout ≈ (R2/R1)·(Vplus − Vminus). This arrangement is what distinguishes a difference amplifier from other common configurations. The other options don’t subtract two signals. A non-inverting amplifier takes a single input at the plus terminal and uses a feedback network to set gain, with the minus input influencing the output but not combining a second input signal. An inverting amplifier also uses a single input fed through a resistor into the minus input, with the plus input grounded, producing an inverted, amplified version of that single signal. Only the four-resistor differential arrangement implements the subtraction of two inputs, yielding a differential output.

Difference amplifiers are built to take two input voltages and produce an output proportional to their difference. The key is the symmetric four-resistor network around the op-amp: each input is fed through its own resistor, and the feedback and output resistors are paired so that the gains on both inputs match. When the resistor ratios are matched (R1 to R3 equals R2 to R4), the output becomes proportional to the difference of the two inputs, specifically Vout ≈ (R2/R1)·(Vplus − Vminus). This arrangement is what distinguishes a difference amplifier from other common configurations.

The other options don’t subtract two signals. A non-inverting amplifier takes a single input at the plus terminal and uses a feedback network to set gain, with the minus input influencing the output but not combining a second input signal. An inverting amplifier also uses a single input fed through a resistor into the minus input, with the plus input grounded, producing an inverted, amplified version of that single signal. Only the four-resistor differential arrangement implements the subtraction of two inputs, yielding a differential output.