DISTORTION IN OUTPUT SIGNAL

DISTORTION

The change m amplified output signal waveform with respect to the input signal is called distortion.

Under ideal conditions, the amplified output signal must have exactly the same waveform as the input signal. But, in a practical amplifier, the output signal waveform is changed from the Input waveform signal, such a change is called the distortion. The useful information which is carried by the signal may be changed. 

TYPES OF DISTORTION

The four basic types of distortions are :

(i) Amplitude distortion or Non-linear distortion.

(ii) Harmonic distortion

(iii) Frequency distortion

(iv) Phase distortion

(i) Amplitude Distortion: The dynamic characteristics of a transistor is non-linear in nature.
Due to the nonlinearity of the' transconductance curve, the output waveform (Ic) of an amplifier will be slightly different from the ac input waveform. That is the resulting collector current Ic is not truely sinusoidal. This type of distortion is known as amplitude distortion or Non-linear distortion. Since this output current flows through a load resistance, the output voltage will also have a non-linear distortion. Such a distortion produces new frequency components in the output which are not present in the input i. signal. These frequencies are usually harmonics of the input frequency.

(ii) Harmonic Distortion: The transfer characteristics of an amplifier is nonlinear. Due to the nonlinear characteristics, one half of the input signal is amplified more than the other half of the input signal. Then the output signal contains a dc component along with fundamental components and harmonics. Frequencies that are integral multiples of input fundamental frequency signal are called harmonics. Due to the presence of harmonics in the output signal, the output voltage waveform will be distorted. This is called harmonic distortion.
Usually, the type of distortion is analyzed using the Fourier Series. The relationship between ac collector current ic and the input base current ib is shown by Fourier Series,
Since the base current ib = 1b cost,
where ICQ is a dc component of collector current 10 is an extra dc component due to rectification Il is the amplitude of fundamental component 12 is the amplitude of the second harmonic component.
The frequency component which has the same frequency of the input is known as the fundamental frequency component. The other frequency component in the output which are integer multiples of fundamental frequency component is known as Harmonics.

(iii) Frequency Distortion: Frequency distortion is caused by an amplifier when signal components of different frequencies are amplified differently. Frequency distortion may be caused either by the electrode capacitances of the amplifying device or by the reactive elements associated with the circuit. In the case of frequency distortion, the amplifier gain is changed with a change in the frequency of input ac signal. Generally, the low frequency and high-frequency input signals are attenuated. The amplifier gain remains constant in the mid-frequency range. The quality of speech or music signals is affected by such distortion. Fig. 1.8 shows the frequency distortion signal. The input signal of the amplifier contains much equal amplitude sinusoidal component, but the output signal contains a different amplitude of the sinusoidal component.

(iv)Phase Distortion: Phase shift distortion takes place due to the unequal phase shift of the input signal at different frequencies. The phase distortions are not detectable by the human ears as they are insensitive to the phase changes. If the phase shift varies with frequency, the different frequency components of the signal are delayed by a different amount of time.