Which statement about a difference amplifier is false?

Which statement about a difference amplifier is false?

• A. A difference amplifier does not saturate.
• B. It subtracts two input voltages.
• C. It can reject common-mode signals.
• D. It requires matched resistor ratios for proper operation.

Answer: (A)

The key idea is that a difference amplifier is designed to take two input voltages and produce an output proportional to their difference, while canceling signals that are common to both inputs when the resistor ratios are matched. This means it can reject common-mode signals, which is why it’s used to extract the true difference in the presence of noise or interference that appears on both lines. But saturation can occur. If the op-amp’s output would need to swing beyond its supply rails, or if the input common-mode range is violated, the output will clip at the supply limits. So the statement that it does not saturate is not generally true; saturation depends on the inputs, the gain set by the resistor ratios, and the power supply. The other statements describe the intended behavior accurately: it subtracts two input voltages, and it can reject common-mode signals when resistor ratios are matched, which is also why matching those ratios is required for proper differential operation.

The key idea is that a difference amplifier is designed to take two input voltages and produce an output proportional to their difference, while canceling signals that are common to both inputs when the resistor ratios are matched. This means it can reject common-mode signals, which is why it’s used to extract the true difference in the presence of noise or interference that appears on both lines.

But saturation can occur. If the op-amp’s output would need to swing beyond its supply rails, or if the input common-mode range is violated, the output will clip at the supply limits. So the statement that it does not saturate is not generally true; saturation depends on the inputs, the gain set by the resistor ratios, and the power supply.

The other statements describe the intended behavior accurately: it subtracts two input voltages, and it can reject common-mode signals when resistor ratios are matched, which is also why matching those ratios is required for proper differential operation.