Common Transmitter Amplifier Circuit
Distortion of the output signal waveform can occur for the following reasons:
- Due to incorrect polarity levels, amplification may not occur throughout the entire signal cycle.
- The input signal can be very large, which can cause amplifier transistors to be limited to the feed voltage.
- Amplification may not be a linear signal along the entire frequency range of inputs.
- This means that during the upgrade process of the signal waveform, some form of Amplifier Distortion occurs.
Amplifiers are mainly designed to amplify small voltage input signals to much larger output signals, which means that the output signal is constantly changed by a factor or value called gain, which is multiplied by the input signal for all input frequencies. We have seen before that this multiplication factor is called the beta, β value of the transistor. Common emitter or even common weld type transistor circuits work well for small AC input signals but suffer from a major disadvantage, the pre-polarization of a bipolar amplifier, the calculated position of the Q-point depends on the same Beta value for all transistors. However, this Beta value will differ from transistors of the same type, that is, the Q point of a transistor is not necessarily the same as the Q point for another transistor of the same type due to their natural production tolerance. Then, amplifier corruption occurs because the amplifier is not linear. Careful selection of transistor and pre-polarization components can help minimize the impact of amplifier distortion.
Amplitude distortion occurs when the peak values of the frequency waveform are weakened, causing distortion due to a shift in point Q, and amplification may not occur throughout the entire signal cycle. This nonlinearity of the output waveform is shown below.
Amplitude Deterioration Caused by Incorrect Polarization
If the polarity point of the transistors is correct, the output waveform must have the same shape as the input waveform. If there is insufficient deviation and the Q point is located in the lower half of the load line, the output waveform will look like the one on the right with the negative half of the output waveform "truncated" or cropped. Similarly, if there is a lot of deviation and the Q point is located in the upper half of the load line, the output waveform will look like the one on the left, which is positively semi-"truncated" or cropped. Also, when the pre-voltage is set too small, the transistor does not transmit fully during the negative half of the cycle, so the output is adjusted by the supply voltage. When the polarism is too large, the positive half of the cycle saturates the transistor, and the output drops to almost zero. Even if the correct polarization voltage level is set, it is still possible to distort the output waveform due to a large input signal powered by circuit gain. The output voltage signal is trimmed in both the positive and negative parts of the waveform, and even if the deviation is correct, it is no longer like a sinus wave. This type of amplitude deterioration is called Cropping and is the result of "overloading" the input of the amplifier. When the input amplitude is too large, cropping becomes important and forces the output waveform signal to exceed the power supply voltage rails, and the top (+and half) and bottom (-and-and-half) parts of the waveform signal are flattened. To prevent this, the maximum value of the input signal should be limited to a level that will prevent this clipping effect, as shown above.
Amplitude Disruption Due to Cropping
Amplitude Distortion greatly reduces the efficiency of an amplifier circuit. These "flat peaks" of the distorted output waveform due to incorrect polarization or excessive driving of the input do not contribute to the power of the output signal at the desired frequency. Having said all this, some well-known guitarists and rock bands prefer that their distinctive sounds are quite distorted or "overloaded", trimming the output waveform both + and – and intensely on the power supply rails. In addition, increasing the amounts of clippings on a sinusoid will produce so many amplifier distortions that it will eventually produce an output waveform similar to the "square wave" shape that can be used in electronic or digital synthesizer circuits. We found that with a DC signal, the gain level of the amplifier can vary with the signal amplitude, but ac signals in amplifier circuits such as Frequency Distortion and Phase Distortion and other types of amplifier distortion may also occur.
Frequency Distortion is another type of amplifier distortion that occurs in a transistor amplifier when the level of amplification changes with frequency. Most of the input signals that a practical amplifier will amplify consist of the necessary signal waveform called "Basic Frequency" and a series of different frequencies called "Harmonics" superimposed on it. Normally, the amplitude of these harmonics is part of the basic amplitude, and therefore has little or no effect on the output waveform. However, if the amplitude of these harmonic frequencies increases according to the basic frequency, the output waveform may be disturbed. For example, consider the following waveform:
Harmonic Frequency Distortion
In the example above, the input waveform consists of one basic frequency plus a second harmonic signal. The resulting output waveform is shown on the right side. Frequency distortion occurs when the basic frequency is combined with the second harmonic to disrupt the output signal. Harmonics are therefore multiples of the basic frequency and a second harmonic is used in our simple example.
Therefore, the frequency of the harmonic is twice that of the base, 2*ε or 2ε. Then a third harmonic becomes 3ε, the fourth, 4ε, etc. Frequency distortion caused by harmonics is always a possibility in amplifier circuits containing reactive elements such as capacitance or inductancy.
Phase Distortion or Delay Distortion is a type of amplifier failure that occurs in a nonlinear 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 input and output is zero at the basic frequency, the resulting phase angle delay will be the difference between harmonic and basic. This time delay will depend on the structure of the amplifier and will gradually increase with the frequency within the amplifier's bandwidth. For example, consider the following waveform:
Phase Distortion Due to Delay
Apart from high-end audio amplifiers, most practical amplifiers will have a kind of Amplifier Distortion, a combination of both "Frequency Distortion" and "Phase Distortion", together with amplitud distortion. In most applications, such as audio amplifiers or power amplifiers, amplifier distortion usually does not affect the operation or output sound of the amplifier, unless it is excessive or severe.
In the next tutorial on 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 sound power amplifiers.