If the transistors biasing point is correct, the output waveform should have the same shape as that of the input waveform only bigger, (amplified). Amplitude Distortion due to Incorrect Biasing This non-linearity of the output waveform is shown below. Amplifier Distortion Due To Amplitude DistortionĪmplitude distortion occurs when the peak values of the frequency waveform are attenuated causing distortion due to a shift in the Q-point and amplification may not take place over the whole signal cycle. Careful choice of the transistor and biasing components can help minimise the effect of amplifier distortion. Then amplifier distortion occurs because the amplifier is not linear and a type of amplifier distortion called Amplitude Distortion will result. However, this Beta value will vary from transistors of the same type, in other words, the Q-point for one transistor is not necessarily the same as the Q-point for another transistor of the same type due to the inherent manufacturing tolerances. Consider the common emitter amplifier circuit below.Ĭommon emitter or even common source type transistor circuits work fine for small AC input signals but suffer from one major disadvantage, the calculated position of the bias Q-point of a bipolar amplifier depends on the same Beta value for all transistors.
In other words the amplifier will suffer from what is commonly called Amplifier Distortion. However, if we incorrectly design our amplifier circuit and set the biasing Q-point at the wrong position on the load line or apply too large an input signal to the amplifier, the resultant output signal may not be an exact reproduction of the original input signal waveform. The Power, Voltage or Current Gain, (amplification) provided by the amplifier is the ratio of the peak output value to its peak input value (Output ÷ Input). The bias Q-point setting will give us a “Class-A” type amplification configuration with the most common arrangement being the “Common Emitter” for Bipolar transistors or the “Common Source” configuration for unipolar FET transistors. A DC bias is required so that the amplifier can amplify the input signal over its entire cycle with the bias “Q-point” set as near to the middle of the load line as possible. For a signal amplifier to operate correctly without any amplifier distortion of the output signal, it requires some form of DC Bias on its Base or Gate terminal.
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