Class A Amplifier

Class A Amplifier

• To achieve high linearity and gain, the output stage of a class A amplifier is biased “ON” (conducting) all the time.
• Then for an amplifier to be classified as “Class A” the zero signal idle current in the output stage must be equal to or greater than the maximum load current (usually a loudspeaker) required to produce the largest output signal.
• As a class A amplifier operates in the linear portion of its characteristic curves, the single output device conducts through a full 360 degrees of the output waveform.
• Then the class A amplifier is equivalent to a current source.
• Since a class A amplifier operates in the linear region, the transistors base (or gate) DC biasing voltage should by chosen properly to ensure correct operation and low distortion. However, as the output device is “ON” at all times, it is constantly carrying current, which represents a continuous loss of power in the amplifier.
• Due to this continuous loss of power class A amplifiers create tremendous amounts of heat adding to their very low efficiency at around 30%, making them impractical for high-power amplifications.
• Also due to the high idling current of the amplifier, the power supply must be sized accordingly and be well filtered to avoid any amplifier hum and noise. 
• Therefore, due to the low efficiency and over heating problems of Class A amplifiers, more efficient amplifier classes have been developed.