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power amplifier
Amplifier circuits are classified as A, B, AB and C for analog designs, and class D and E for switching designs. For the analog classes, each class defines what proportion of the input signal cycle (called the angle of flow) is used to actually switch on the amplifying device:
1. Class A -100% of the input signal is used (conduction angle a = 360° or 2π)
2. Class AB - more than 50% but less than 100% is used. (181° to 359°)
3. Class B - 50% of the input signal is used (a = 180° or π)
In an analog amplifier, the signal is applied to the input terminal of the device (base, gate or grid), and this causes a proportional output drive current to flow out of the output terminal. The output drive current is obtained from the power supply. The voltage signal shown is thus a larger version of the input, but has been changed in sign (inverted) by the amplification. Other arrangements of amplifying device are possible, but that given (common emitter, common source or common cathode) is the easiest to understand and employ in practice. Non-linearity is the origin of distortion within an amplifier. The class of amplifier (A, B and AB) depends on how the amplifying device is biased — in the diagrams the bias circuits are omitted for clarity.
CLASS 'A'
Many class A amplifiers use the same transistor(s) for both halves of the audio waveform. In this configuration, the output transistor(s) always has current flowing through it, even if it has no audio signal (the output transistors never 'turn off'). The current flowing through it is D.C. A pure class 'A' amplifier is very inefficient and generally runs very hot even when there is no audio output. The current flowing through the output transistor(s) (with no audio signal) may be as much as the current which will be driven through the speaker load at FULL audio output power. Many people believe class 'A' amps to sound better than other configurations (and this may have been true at some point in time) but a well designed amplifier won't have any 'sound' and even the most critical 'ear' would be hard-pressed to tell one design from another.
NOTE: Some class A amplifiers use complimentary (separate transistors for positive and negative halves of the waveform) transistors for their output stage.
CLASS 'B'
A class 'B' amplifier uses complimentary transistors for each half of the waveform. A true class 'B' amplifier is NOT generally used for audio. In a class 'B' amplifier, there is a small part of the waveform which will be distorted. It takes approximately 6 volts (measured from base to emitter) to get a bipolar transistor to start conducting. In a pure class 'B' amplifier, the output transistors are not "biased" to an 'on' state of operation. This means that the part of the waveform which falls within this .6 volt window will not be reproduced accurately. The output transistors for each half of the waveform (positive and negative) will each have a 6 volt area in which they will not be conducting. The distorted part of the waveform is called 'crossover' or 'notch' distortion. Remember that distortion is any unwanted variation in a signal (compared to the original signal). The diagram below shows what crossover distortion looks like.
CLASS 'AB'
A class 'A' amplifier is very inefficient. This is not good for a car audio amplifier. A class 'B' amplifier will also cause a signal to be distorted, which is not good in any audio amplifier. A class 'AB' amplifier is the best compromise. A class 'AB' amplifier is a class 'B' amplifier which has a small amount of "bias" current flowing through the output transistors at all times. This eliminates virtually all of the crossover distortion. The bias current is flowing because the output transistors are always conducting current (even without an audio signal). This differs from a pure class 'A' amplifier in the amount of current flow. A pure class 'A' amplifier has an enormous amount of current flowing through its output transistors with NO audio signal. A pure class 'B' amplifier has NO current flowing through its outputs with no input signal. A class 'AB' amplifier is much more efficient than the class 'A' but without the distortion of the class 'B'. MANY of the car audio amplifiers which claim to be a class 'A' amplifier are just a high bias class 'AB' design. These amplifiers are only class 'A' at very low power output levels. At higher power levels, one of the output transistors will switch off while the other output transistor is conducting. I don't want you to think that I am telling you that there are no class 'A' amplifiers. There are a few high quality mobile amplifiers which are a true class 'A' design.
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