Define common-mode input range and explain why it matters for single-supply operation.

Define common-mode input range and explain why it matters for single-supply operation.

• A. The range of output voltages where the amplifier operates.
• B. The range of input voltages over which both inputs can operate without large output error; with single-supply op-amps, the common-mode range typically does not include both rails.
• C. The voltage difference between inputs at which the output saturates.
• D. The range of supply voltages required for operation.

Answer: (B)

Common-mode input range is the range of voltages that can be applied to both inputs at the same time while the amplifier still operates in its linear region and without large errors in the output. In single-supply operation (0 to Vcc), the internal transistor stages need some headroom away from the supply rails, so the inputs cannot swing all the way to either rail. As a result, the common-mode range typically does not include both rails, meaning you can’t safely drive the inputs all the way down to ground or all the way up to Vcc. This matters because if the input signal moves outside this range, the amplifier’s input stage can saturate or cut off, causing distortion, gain loss, or incorrect output. To handle signals on a single supply, you usually bias the inputs around mid-supply (or use a rail-to-rail input op-amp) so the entire signal stays within the allowable common-mode window.

Common-mode input range is the range of voltages that can be applied to both inputs at the same time while the amplifier still operates in its linear region and without large errors in the output. In single-supply operation (0 to Vcc), the internal transistor stages need some headroom away from the supply rails, so the inputs cannot swing all the way to either rail. As a result, the common-mode range typically does not include both rails, meaning you can’t safely drive the inputs all the way down to ground or all the way up to Vcc. This matters because if the input signal moves outside this range, the amplifier’s input stage can saturate or cut off, causing distortion, gain loss, or incorrect output. To handle signals on a single supply, you usually bias the inputs around mid-supply (or use a rail-to-rail input op-amp) so the entire signal stays within the allowable common-mode window.