Unwanted characteristics of amplifier output, apart from the desired output, are collectively termed as distortion. Distortion refers to any alteration or modification of the original signal waveform that occurs during the amplification process. It can introduce additional frequencies or alter the amplitude and phase of the original signal, thus degrading its quality.

Distortion can occur in various forms, and each type has its own impact on the output signal. Some common types of distortion include:

1. Harmonic Distortion: This type of distortion occurs when the amplifier introduces harmonics or multiples of the original signal frequency. Harmonic distortion is usually caused by non-linearities in the amplifier circuitry, which result in the amplification of frequencies that were not present in the original signal. Harmonic distortion can lead to a loss of clarity and fidelity in the output signal.

2. Intermodulation Distortion: Intermodulation distortion occurs when the amplifier produces sum and difference frequencies between two or more input signals. It is caused by non-linearities in the amplifier's transfer function, which result in the mixing of multiple input signals. Intermodulation distortion can cause unwanted frequencies to be generated, leading to a distorted output signal.

3. Phase Distortion: Phase distortion refers to the alteration of the phase relationship between different frequency components of the signal. It can occur due to frequency-dependent phase shifts introduced by the amplifier circuitry. Phase distortion can result in a loss of sharpness and accuracy in the output signal, particularly in applications where phase coherence is critical, such as audio reproduction.

4. Amplitude Distortion: Amplitude distortion refers to any alteration in the amplitude or dynamic range of the output signal. It can be caused by non-linearities in the amplifier's gain response, resulting in signal compression or expansion. Amplitude distortion can lead to a loss of detail and dynamics in the output signal.

Distortion in amplifier output is generally undesirable as it degrades the quality and fidelity of the reproduced signal. Therefore, amplifier design and selection should focus on minimizing distortion to ensure accurate and faithful reproduction of the input signal. Various techniques, such as feedback compensation and careful component selection, can be employed to reduce distortion and improve overall amplifier performance.