From the previous tutorials we learnt that for a signal amplifier to work correctly it requires some
form of DC Bias on its Base or Gate terminal so that it amplifies 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 then gave
us a Class "A" type amplification with the most common configuration being Common Emitter
Bipolar transistors and Common Source for unipolar transistors. We also saw that the Power,
Voltage or Current Gain, (amplification) provided by the amplifier is the ratio of the peak input
value to its peak output value. 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, the
resultant output signal may not be an exact reproduction of the original input signal waveform.
Consider the common emitter amplifier circuit below.
Common Emitter Amplifier
Distortion of the signal waveform may take place because:
Amplification may not be taking place over the whole signal cycle due to incorrect biasing.
The input may be too large, causing the amplifier to limit.
The amplification may not be linear 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 times
the input signal at all input frequencies. We saw previously that this multiplication factor is called
the Beta, ß value of the transistor. Common Emitter or even common Source type circuits work
fine for small AC input signals but suffer from one major disadvantage, the bias Q-point of a
bipolar amplifier depends on the same Beta value which may vary from transistors of the same
type, ie. 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. If this occurs the
amplifier may not be linear and Amplitude Distortion will result but careful
choice of the transistor can minimise this effect.
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
Amplitude Distortion greatly reduces the efficiency of an amplifier circuit. These "flat tops"
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
power supply rails. Also, excessive amounts of clipping can also produce an output which
resembles a "square wave" shape which can then be used in electronic or digital 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 distortion can occur with AC
signals in amplifier circuits, such as Frequency Distortion and Phase Distortion.
Frequency Distortion 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
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 2
times the fundamental, 2 x f or 2f. Then a third harmonic would be 3f, a fourth, 4f, 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 occurs in a non-linear transistor amplifier (or Op-Amp)
when there is a time delay between the input signal and its appearance at the output. If we call
the phase change between the input and the output 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
Any practical amplifier will have a combination of both "Frequency" and "Phase"
together with amplitude distortion but in most applications such as in audio amplifiers or power
amplifiers, unless the distortion is excessive or severe it will not generally affect the operation