Amplifier

An amplifier is anything that receives an input signal and puts out an output signal that is stronger than it was when it came in.

The incoming signal can be just about anything, but frequently the incoming signal represents sound.

Many electronic devices have amplifiers to boost a signal that represents sound. TVs, radios, stereos, tape players and CD players, for example, have amplifiers. The goal of those amplifiers is to boost but otherwise remain faithful to the volume at all frequencies within the range that people can hear.

Hearing aids are audio amplifiers, also, but they have a completely different goal. Their goal is to add varying amounts of "gain" depending on the frequency and the amount of hearing loss the wearer has at those frequencies.

Hearing aid amplifiers also can be set to either stop adding gain to incoming signals above a certain volume (clipping) or to add less gain as the incoming volume increases (compression).

As hearing aids have gotten more sophisticated. Early aids added a varying amount of "gain" based on frequency, but the "curve" of the gain was largely fixed by the electronics of the aid. Adjustments could increase the overall amount of gain and could determine how much affect compression or clipping somewhat, but there wasn't much else you could do to adjust the shape of the response curve.

Later aids added the ability to control high and low frequencies of the same amplifier differently.

Modern digital aids may have many bands that the amount of gain (and other characteristics) can be independently set to best offset the hearing loss of the wearer.

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