Definition
An electronic amplifier circuit biased so that the output device (transistor or vacuum tube) conducts current during the entire input signal cycle. Because the device never switches off, it reproduces the input waveform with very low distortion, but it also draws current continuously, which makes it inefficient and causes it to run warm even with no signal applied.
Plain English
An amplifier that stays switched on the whole time the signal is passing through it. It gives a very clean, accurate output, but it wastes power as heat because it is always working.
Context Anchor
Seen in aircraft radio, audio, and avionics circuit discussions, especially during maintenance or troubleshooting.
Derivation
The 'Class' system (A, B, AB, C) was introduced by early radio engineers to label amplifier circuits by how much of the input cycle the output device conducts. Class-A was named first because it was the simplest and earliest design, where the device conducts for 100% of the cycle.
Why Pilots Care
Class-A stages are common in the audio paths of cockpit communication gear. Knowing the type helps when troubleshooting issues like persistent warmth, higher current draw, or clean-but-quiet audio output from a radio or intercom.
Analogy
It is like a person holding a door partly open all the time so they can respond instantly and smoothly. That works well, but it takes effort even when nobody is passing through.
Intuition Check
“Class A” does not mean “top quality” here. It means the amplifier is kept on through the whole signal so the output stays smooth.
Example Sentence 1
The intercom's audio output uses a Class-A amplifier, which is why the unit feels warm to the touch even when no one is speaking.
Example Sentence 2
During the preflight check the technician confirmed the Class-A Amplifier was operating within limits.