This DC bias is needed 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. This bias q-point setting then gave 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.
We also learnt that 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). 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. In other words the amplifier will suffer from what is commonly called Amplifier Distortion. Consider the common emitter amplifier circuit below.
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Common Emitter Amplifier
Distortion of the output signal waveform may occur because:
- Amplification may not be taking place over the whole signal cycle due to incorrect biasing levels.
- The input signal may be too large, causing the amplifiers transistors to be limited by the supply voltage.
- The amplification may not be a linear signal over the entire frequency range of inputs.
This means then that during the amplification process of the signal waveform, some form of Amplifier Distortion has occurred.
Amplifiers are basically designed to amplify small voltage input signals into much larger output signals and this means that the output signal is constantly changing by some factor or value, called gain, multiplied by the input signal for all input frequencies. We saw previously that this multiplication factor is called the Beta, β value of the transistor.
Common 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. 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.
Then amplifier distortion occurs because the amplifier is not linear and a type of amplifier distortion called Amplitude Distortion will result. Careful choice of the transistor and biasing components can help minimise the effect of amplifier distortion.
Amplitude 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. This non-linearity of the output waveform is shown below.
Amplitude Distortion due to Incorrect Biasing
If the transistors biasing point is correct, the output waveform should have the same shape as that of the input waveform only bigger, (amplified). If there is insufficient bias and the Q-point lies in the lower half of the load line, then the output waveform will look like the one on the right with the negative half of the output waveform “cut-off” or clipped. Likewise, if there is too much bias and the Q-point lies in the upper half of the load line, then the output waveform will look like the one on the left with the positive half “cut-off” or clipped.
Also, when the bias voltage is set too small, during the negative half of the cycle the transistor does not fully conduct so the output is set by the supply voltage. When the bias is too great the positive half of the cycle saturates the transistor and the output drops almost to zero.
Even with the correct biasing voltage level set, it is still possible for the output waveform to become distorted due to a large input signal being amplified by the circuits gain. The output voltage signal becomes clipped in both the positive and negative parts of the waveform an no longer resembles a sine wave, even when the bias is correct. This type of amplitude distortion is called Clipping and is the result of “over-driving” the input of the amplifier.
When the input amplitude becomes too large, the clipping becomes substantial and forces the output waveform signal to exceed the power supply voltage rails with the peak (+ve half) and the trough (-ve half) parts of the waveform signal becoming flattened or “Clipped-off”. To avoid this the maximum value of the input signal must be limited to a level that will prevent this clipping effect as shown above.
Amplitude Distortion due to Clipping
Amplitude Distortion greatly reduces the efficiency of an amplifier circuit. These “flat tops” of the distorted output waveform either due to incorrect biasing or over driving the input do not contribute anything to the strength of the output signal at the desired frequency.
Having said all that, some well known guitarist and rock bands actually prefer that their distinctive sound is highly distorted or “overdriven” by heavily clipping the output waveform to both the +ve and -ve power supply rails. Also, increasing the amounts of clipping on a sinusoid will produce so much amplifier distortion that it will eventually produce an output waveform which resembles that of a “square wave” shape which can then be used in electronic or digital synthesizer circuits.
We have seen that with a DC signal the level of gain of the amplifier can vary with signal amplitude, but as well as Amplitude Distortion, other types of amplifier distortion can occur with AC signals in amplifier circuits, such as Frequency Distortion and Phase Distortion.
Frequency Distortion is another type of amplifier distortion which occurs in a transistor amplifier when the level of amplification varies with frequency. Many of the input signals that a practical amplifier will amplify consist of the required signal waveform called the “Fundamental Frequency” plus a number of different frequencies called “Harmonics” superimposed onto it.
Normally, the amplitude of these harmonics are a fraction of the fundamental amplitude and therefore have very little or no effect on the output waveform. However, the output waveform can become distorted if these harmonic frequencies increase in amplitude with regards to the fundamental frequency. For example, consider the waveform below:
Frequency Distortion due to Harmonics
In the example above, the input waveform consists a the fundamental frequency plus a second harmonic signal. The resultant output waveform is shown on the right hand side. The frequency distortion occurs when the fundamental frequency combines with the second harmonic to distort the output signal. Harmonics are therefore multiples of the fundamental frequency and in our simple example a second harmonic was used.
Therefore, the frequency of the harmonic is twice the fundamental, 2 x ƒ or 2ƒ. Then a third harmonic would be 3ƒ, a fourth, 4ƒ, and so on. Frequency distortion due to harmonics is always a possibility in amplifier circuits containing reactive elements such as capacitance or inductance.
Phase Distortion or Delay Distortion is a type of amplifier distortion which occurs in a non-linear transistor amplifier when there is a time delay between the input signal and its appearance at the output.
If we say that the phase change between the input and the output is zero at the fundamental frequency, the resultant phase angle delay will be the difference between the harmonic and the fundamental. This time delay will depend on the construction of the amplifier and will increase progressively with frequency within the bandwidth of the amplifier. For example, consider the waveform below:
Phase Distortion due to Delay
Other than high end audio amplifiers, most practical amplifiers will have some form of Amplifier Distortion being a combination of both “Frequency Distortion” and “Phase Distortion”, together with amplitude distortion. In most applications such as in audio amplifiers or power amplifiers, unless the amplifiers distortion is excessive or severe it will not generally affect the operation or output sound of the amplifier.
In the next tutorial about Amplifiers we will look at the Class A Amplifier. Class A amplifiers are the most common type of amplifier output stage making them ideal for use in audio power amplifiers.