A 2-input gate that can be used to pass a digital waveform unchanged at certain times and inverted at other times is ___________

Some logic gates can act as a controllable switch, letting one input signal decide whether a second, changing signal passes through unchanged or gets flipped. Identifying which gate behaves this way means checking its output for both settings of the controlling input.

An XOR gate outputs 1 whenever its two inputs differ and outputs 0 whenever they are the same. If one input is held at 0 while the other input carries a changing digital waveform, the output simply follows that waveform unchanged, because a signal paired with 0 keeps its own value under exclusive-OR. If the control input is instead held at 1, the output becomes the exact opposite of the waveform at every instant, because a signal paired with 1 gets flipped under exclusive-OR. This makes the XOR gate a controllable inverter: unchanged at one control setting, inverted at the other.

The AND and OR gates cannot reproduce this behaviour, since holding one input fixed at either 0 or 1 forces their output to either always follow or always block the other signal, never to invert it. A NAND gate can invert a signal but does not offer this same unchanged-versus-inverted switching behaviour based on a control input in the way XOR does.

Exam takeaway: recognise the exclusive-OR gate as the standard controlled inverter, passing a waveform through untouched with one control level and flipped with the other.