Amplifier Classes: Most mobile amplifiers use complementary transistor pairs to drive the speakers. In this configuration there is a transistor (or group of transistors) which conducts current from the positive power supply voltage for the positive half of the audio waveform and a different transistor (or group of transistors) which conducts current from the negative power supply voltage for the negative half of the waveform. There are some amplifiers which use the same transistor(s) to drive both the positive and the negative halves of the waveform. NOTE:Amplifiers in classes A, B, and AB operate their output transistors in a 'linear' mode. Class 'D' amplifiers operate their outputs in 'switch' mode. Mode examples: Linear mode: Imagine that you are the amplifier's output device(s) and you must support a 10 pound iron weight (the speaker load). The most difficult method (linear mode) would be to hold the weight straight out in front of you. This would very roughly simulate the linear mode architecture. Your muscles would start to ache in a short amount of time. Think of this pain as the power dissipation in output transistors. Switch mode: In this example, you can support the weight in one of two positions. In the first position, you can hold the iron weight directly over your head with your elbows locked so that your're not really using very much effort to support the weight. In the second position, you would let the weight hang down by your side. This would also use very little effort from your muscles. If you held it directly over your head half of the time and by your side for the other half of the time, it's position would 'average' out to be the same as if you held it out straight in front of you like in the previous (linear mode) example. This would roughly simulate the switch mode which we will discuss later in this page. You can see that with this method (switch mode), there would also be little pain (power dissipation) involved in supporting the weight. --------------------------------------------------------------------------------
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. You should remember from an earlier page, that 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 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'
As we said earlier, a class 'A' amplifier is very inefficient. This is not good for a car audio amplifier. We also said that a class 'B' amplifier will 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.
CLASS 'D'
We said that class 'A' amplifiers were VERY inefficient. Class 'AB' amplifiers are also inefficient but are more more efficient than class 'A' amplifiers. Class 'AB' mobile amplifiers are generally 60% efficient when driving a 4 ohm load at maximum power (just before clipping). The reason that these amplifier configurations are inefficient is because there is a difference of potential (voltage) across the output transistors and current flowing through the output transistors. When you have voltage across the device and current flow through the device, there will be power dissipation in the form of heat. The power needed to produce this heat is wasted power. When there is (virtually) no voltage drop across a device (such as a large piece of wire or a transistor), there can be a significant amount of CURRENT flow through the device with (virtually) no power dissipation. This means that there is virtually no heat given off (highly efficient). The inverse is also true. If you have a significant amount of VOLTAGE across the device (transistor, wire...) but no current flow through the device, again, there will be no wasted power. OK, now to the point. A class 'D' amplifier, which may also be known as a switching amplifier or a digital amplifier, utilizes output transistors which are either completely turned on or completely turned off (they're operating in switch mode). This means that when the transistors are conducting (switched on) there is virtually no voltage across the transistor and when there is a significant voltage across the transistor (switched off), there is no current flowing through the transistor. This is very similar to the operation of a switching power supply which is very efficient.
