Definition
An electronic component in an autopilot or auto-flight system that takes a small input signal indicating how much and in which direction a flight control needs to move, boosts that signal to a usable power level, and sends it to a servo motor that physically moves the control surface.
Plain English
A small device that takes a weak command signal and turns it into a strong enough electrical output to drive the motor that moves a flight control.
Context Anchor
Seen in aircraft maintenance discussions of autopilot, automatic flight control, trim, and other systems that move something in response to an electrical command.
Derivation
Servo comes from the Latin servus, meaning servant. A servo is a device that does work on command. Amplifier comes from the Latin amplificare, meaning to enlarge. Together: a device that enlarges a command signal so a servant motor can carry it out.
Why Pilots Care
If the servo amplifier fails or drifts, the autopilot may move controls incorrectly, weakly, or not at all. Knowing where it sits in the signal chain helps narrow down whether a malfunction is in the sensing, the amplification, or the motor itself.
Analogy
Like the amplifier in a stereo: the signal coming out of a phone is too weak to drive big speakers, so the amplifier boosts it. Here, the autopilot's command signal is too weak to drive a control-moving motor, so the servo amplifier boosts it.
Intuition Check
Do not think of this as an audio amplifier for sound. In this context, “amplifier” means it strengthens a control signal so a servo can move.
Example Sentence 1
When the autopilot would not hold heading, the technician traced the fault to a failed servo amplifier in the roll channel.
Example Sentence 2
A failing servo amplifier produced sluggish aileron response when the autopilot was engaged in heading mode.