Distortion in Amplifiers

DISTORTION IN AMPLIFIERS

If the output of an amplifier is not a complete sine wave, then it is known as distortion using Fourier analysis, any distorted periodic waveform can be broken into different frequency components. These components are known as harmonics of the fundamental frequency. Harmonics are integer multiples of a fundamental frequency. For example. 1^st harmonic is 1 x F KHz.

Amplitude (or) Non Linear Distortion

Due to non-linear characteristics of the transistor, the output is different from the input. This kind of distortion is known as amplitude or harmonic or non-linear distortion. If fundamental frequency has amplitude A₁ and nth frequency, component has an amplitude of A_n.

Harmonic distortion % D

Second harmonic distortion occurs in class B and class AB amplifiers.

Frequency Distortion

When different frequency components of the input signal are amplified differently, frequency amplification takes place. This occurs due to the internal capacitance effect of the transistors.

Delay or Phase Shift Distortion

If the phase shift introduced by the amplifier is not proportional to the frequency, then phase distortion takes place.

Cross Over Distortion

In Push Pull operation of Class B amplifiers, the output waveform is not a perfect sinusoidal waveform. The output waveform will be as shown in Figure 5.23.

The drive signal applied to class B transistor must reach a certain minimum level before its collector current is properly in active region.

The Fig.5.23 initial rise of collector current in class B transistor lags the initial rise of input voltage. Also, the collector current prematurely drops to 0, when the input voltage approaches zero. Therefore, distortion occurs during each half-cycle due to combination of Class-B transistor. This distortion is known as cross over distortion, because it occurs when the composite waveform crosses the zero voltage axes.

COMPARISON OF POWER AMPLIFIERS