In a properly designed difference amplifier, common-mode signals are which of the following?

In a properly designed difference amplifier, common-mode signals are which of the following?

• A. Greatly attenuated
• B. Greatly amplified
• C. Unchanged
• D. Dominant in output

Answer: (A)

The key idea is that a properly designed difference amplifier aims to pass only the difference between its two inputs while rejecting voltages that are the same on both inputs (common-mode). Because the two input paths are matched, any voltage that appears identically on both inputs contributes equally to the output in opposite ways and cancels out. If both inputs carry the same voltage (a common-mode signal), the difference between them is zero, so the ideal output is driven to zero. In practice, the circuit is designed so that common-mode signals are greatly attenuated at the output; the residual effect depends on how well the resistor values are matched and the amplifier's own common-mode rejection capability. So, common-mode signals are greatly attenuated, rather than amplified or left unchanged, and they are not dominant in the output.

The key idea is that a properly designed difference amplifier aims to pass only the difference between its two inputs while rejecting voltages that are the same on both inputs (common-mode). Because the two input paths are matched, any voltage that appears identically on both inputs contributes equally to the output in opposite ways and cancels out.

If both inputs carry the same voltage (a common-mode signal), the difference between them is zero, so the ideal output is driven to zero. In practice, the circuit is designed so that common-mode signals are greatly attenuated at the output; the residual effect depends on how well the resistor values are matched and the amplifier's own common-mode rejection capability.

So, common-mode signals are greatly attenuated, rather than amplified or left unchanged, and they are not dominant in the output.