When the output signal of an amplifier is 180° out-of-phase with the input signal, it is called a(n) __________.

When the output signal of an amplifier is 180° out-of-phase with the input signal, it indicates that the amplifier inverts the input signal. This behavior is characteristic of an inverter, which takes an input voltage and produces an output voltage that is equal in magnitude but opposite in polarity. For example, if the input is a positive voltage, the output will be a negative voltage of the same magnitude.

In the context of electronic circuits, this inversion signifies that when the input goes high, the output goes low, and vice versa. This phase relationship is fundamental to the functionality of inverters in both analog and digital circuits, such as in logic gates where they perform the NOT function.

The other options represent different types of amplifiers or functions that do not describe this specific behavior of phase inversion. A bipolar amplifier refers to a type of amplifier using bipolar junction transistors, a buffer is designed to isolate and not invert signals, and a quadrant amplifier does not pertain to the 180° phase relationship found in basic signal inversion. Each of these terms has its own distinct role in electronic applications but does not specifically represent the inversion characteristic described in the question.