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For an ideal transistor amplifier, the output must be an exact replica of input except that the required characteristics(Voltage, Power) is increased according to the gain of amplifier.

In the otherhand For a practical amplifier the ideal conditions may not be achieved. Some of the changes take place in the output wave form other than the amplitude increases. Such a change can be termed as DISTORTION.
DISTORTION is undesirable because it may affect the information carried by the signal.

The amplifier distortion can be broadly classified into two :-



The classification is based on the region of the characteristic used by the transistor, the related circuit and the reactance of the devices used.

Non-linear distortion occurs when the transistor operates in the Non-linear region of its output.

1. When the signal is viewed under time domain it is known as Amplitude Distortion or waveform distortion.

2. When the signal is considered under Frequency domain, it is known as Harmonic distortion

Non-linear distortion occurs in the case of large signal inputs when the active device is driven into the Non-linear region of its output characteristics.

Linear Distortion occurs when the active device operates on the Linear part of its output with small signal inputs. It is mainly due to Frequency dependent reactances associated with the active device itself and occurs if the input signal is composite (speech- contains too many Frequency)


For a small swing of input signal, only a small part of the transconductance curve, this small part is almost linear and operation over this curve is Linear Operation because the changes in the output Current is proportional to the changes in the input Voltage. In such a case, the shape of the input waveform is almost similar to output wave and therefore no Distortion occurs.
But for large input signal, the Operation became Non-linear.

Because of the non-linearity of transconductance curve, the resulting collector Current Ic is no longer truly sinusoidal. The upper half of the wave is elongated. Since the output Current flows through the load resistance, and the output Voltage will also have a non-linear Distortion.


The Frequency domain of Non-linear Distortion (HARMONIC Distortion) is shown in the Figure below:-

The input is a single Frequency(f1) signal and the output consist of dc component and different harmonics. These harmonics are the integral multiples of the output signal Frequency. The result is Distorted output. The magnitude depends on the the strength and number of These harmonics.

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