A common mode input voltage is described as which of the following?

A common mode input voltage is described as which of the following?

• A. Is simultaneously present at both inputs of a difference amplifier
• B. Is only present at one input
• C. Is converted to a differential signal
• D. Is never present in a real circuit

Answer: (A)

The concept being tested is that a common-mode input voltage is a signal that appears on both inputs of a differential amplifier or op-amp with the same magnitude and phase. Because the amplifier’s output is driven by the difference between the two inputs, equal voltages on both inputs should cancel and produce no output in an ideal case. That’s why the defining description is that the common-mode voltage is present simultaneously at both inputs. In real devices, nothing is perfect, and finite common-mode rejection means a small residual can show up at the output, but the key idea remains: the voltage is shared by both inputs at the same time. The other statements don’t fit: if it were only on one input, it wouldn’t be common-mode; if it were converted to a differential signal, that describes imperfect rejection rather than the basic definition; and common-mode voltages do occur in real circuits, so they are not never present.

The concept being tested is that a common-mode input voltage is a signal that appears on both inputs of a differential amplifier or op-amp with the same magnitude and phase. Because the amplifier’s output is driven by the difference between the two inputs, equal voltages on both inputs should cancel and produce no output in an ideal case. That’s why the defining description is that the common-mode voltage is present simultaneously at both inputs.

In real devices, nothing is perfect, and finite common-mode rejection means a small residual can show up at the output, but the key idea remains: the voltage is shared by both inputs at the same time. The other statements don’t fit: if it were only on one input, it wouldn’t be common-mode; if it were converted to a differential signal, that describes imperfect rejection rather than the basic definition; and common-mode voltages do occur in real circuits, so they are not never present.