Published:2011/8/2 0:56:00 Author:Li xiao na From:SeekIC
By Harry Baggen
Class A
Here we begin with the simplest configuration, the Class A final amplifier, which is also one of the best configurations for high-quality audio reproduction. In its basic form, this configuration can be implemented using a standard emitter follower (Figure 1). The quiescent current through the transistor is equal to the peak AC output current, which means that the transistor is biased in the middle of its working range and simply conducts more or less current when driven by an alternating voltage. The efficiency is very low: 25 % at maximum output amplitude, and even less at low signal levels. The efficiency can be improved by using a symmetrical design with two transistors, but even then the highest efficiency that can be achieved is 50 %.
Class B
The Class B configuration employs two transistors, each of which conducts for exactly half of the signal cycle (Figure 2). In the quiescent state, no current at all flows through the transistors. The efficiency of a Class B output stage is around 78 %, but the primarily disadvantage of this configuration is the ’transfer distortion’ that occurs each time the load must be transferred from one transistor to the other. Due to the sharp bend at the bottom end of the transfer characteristic. the two halves of the signal waveform do not properly align with each other. This leads to the notorious problem of crossover distortion, which is a quite audible degradation of the signal waveform.
To solve this problem. Class A and Class B were combined to produce Class AB. This is a Class B configuration in which a small quiescent current flows, causing the output stage to actually work in Class A at low power levels. This approach is presently used in various forms in most final amplifiers. The efficiency remains approximately the same as for Class B.
G and H
Hey. just a minute! Haven’t we skipped a few classes"? We have indeed, but we did so on purpose. Classes C. E and F also exist, but they are actually only suitable for high-frequency applications, which means they more or less fall outside the scope of what we’re talking about here. And the design of Class D amplifiers is so different from Class A and Class B that it we decided to deal with it separately. So. let’s first look at classes G and H. which have an important feature in common. This is that in both of these classes, the supply voltage is adjusted according to the magnitude of the output signal. In Class G (Figure 3). the supply voltage is continuously adjusted to match the desired amplitude of the output signal. Such a "tracking’ supply voltage can be implemented relatively easily using modem switching power supplies. although it is of course important to have a good regulator circuit to allow the supply voltage to respond sufficiently quickly to changes in the amplitude of the signal generated by the output stage.
In Class H (Figure 4). what happens is essentially the same as in Class G. except that here the supply voltage is switched between several distinct levels (usually two) instead of being continuously varied. This allows the dissipation in the output stage to be considerably reduced, especially when large amounts of power are involved.
Class D
With the Class D amplifier configuration, the letter D’ doesn’t have anything to do with ’digital’ (that’s just a coincidence). It refers to a switching amplifier that uses pulse-width modulation (Figure 5). The input signal is compared with a triangular waveform, and the signal from the comparator switches the output stage to the positive or negative supply voltage. This is done using a very high switching frequency, which is usually ten times or more higher than the audio bandwidth (which means 200 kHz or above).
With this form of modulation, the pulse width depends on the level of the input signal. If a low-pass filter is placed after the output stage, the pulse-width signal is integrated and what is left is an analogue signal with the same form as the input signal. but of course amplified.
As the output stage only has to switch, its efficiency is very high. However, there are also a number of drawbacks to this approach. It is rather difficult to keep the signal waveform free of distortion, a hefty output filter is required, and drastic measures must be taken to limit radiated interference. For low-distortion amplification, it is always necessary to use negative feedback (analogue or digital).
Reprinted Url Of This Article: http://www.seekic.com/blog/project_solutions/2011/08/02/THAT'S_CLASS__Audio_Amplifiers_from_A_to_T_(2).html
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