For what portion of a signal cycle does a Class A amplifier operate?

A Class A amplifier is biased so that its active electronic device conducts current during the entire 360 degrees of the input signal cycle. This ensures the output is an exact, continuously amplified replica of the input, resulting in high linearity and very low distortion. Because it conducts continuously—even when there is no input signal applied—a Class A amplifier is highly inefficient and converts a significant amount of power into heat. For comparison, other amplifier classes operate for a smaller portion of the cycle to improve efficiency: Class B conducts for exactly 180 degrees, Class AB conducts for between 180 and 360 degrees, and Class C conducts for less than 180 degrees.