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以下一段摘自http://sound.westhost.com/efficiency.htm Power Amplifier Efficiency Explained（功放效率的解释）
Class-A
Class-A amps come in many 'flavours', but can be loosely separated into two separate groups. The first (and simplest) is as shown in Figure 2, where a transistor (although it can easily be a MOSFET or valve) is simply loaded with a constant current source. Although a resistor can be used instead of the current source, this reduces performance. An inductor or transformer is usually used for valve Class-A amplifiers.
Figure 2 - A Basic Class-A Amplifier
The bias voltage of the amp in Figure 2 must be high enough to ensure that the transistor's quiescent current is about equal to (or slightly greater than) the peak speaker current. The efficiency of this amp can be easily calculated by the same method as described above. A Class-A amp conducts at all times and the DC is at least equal to the peak output current. A 20V peak signal will still create a 2.5A peak current in the load, so the DC quiescent current must be equal to (or greater than) this figure. Note that the supply is not bipolar - a single +40V supply is the same as a ±20V supply for the purposes of calculation. The output to the load will still be 25W, but the DC input power is now 100W (2.5A * 40V). Efficiency now (from equation 4) is ...
Eff = 25 / 100 * 100 = 25%
This is again the best that can be expected, and there are some Class-A topologies that are considerably worse than this. Without going into extreme detail, efficiency of this type of Class-A stage can range from about 12.5% up to about 22% (allowing an additional amount of current to ensure that the transistors never reach a point where no current flows). The 'modulated current source' (as used the original Linsley Hood amp), can reach efficiencies up to about 30 to 35%. With a Class-A amp, the efficiency falls with reduced power, until at zero output power, efficiency is 0%. At an output power of 1W, efficiency is 1%, and so on (with the voltages and currents as described).
Another type of Class-A amplifier uses the same circuit as Figure 1, but the transistors are biased to around ½ the peak speaker current. With signal, the transistors can draw up to double the quiescent current at waveform peaks, or zero at the opposite peak (one transistor will be double and the other zero as the waveform alternates). Dynamic analysis of this arrangement is harder than either of the other topologies described, but if we examine the quiescent state and the maximum output, a reasonable estimate is possible. Using the same values as before, we have 20V across each transistor, at a quiescent current of 1.25A. Each transistor will therefore dissipate 25W with no signal, and maximum power is also 25W. Efficiency is therefore ...
Eff = 25 / 50 * 100 = 50%
Again, this is the absolute maximum possible efficiency, and in general it will be lower. How much lower depends on many factors, but one can reasonably expect the actual figure to be perhaps 40% or so at best. The most common application of this technique is with amplifiers that claim to be Class-A over part of their maximum power, reverting to Class-AB for higher powers. This allows the amp to operate in Class-A up to perhaps 10W or more, and the majority of listening will probably be within this range unless the loudspeakers are very inefficient. |
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楼主
发表于 2010-7-2 21:57
钻牛角尖了，我无话可说。