Published:2011/8/2 0:40:00 Author:Li xiao na From:SeekIC
By Harry Baggen
The final amplifier is the power source of every audio installation. Its job is to convert a small alternating voltage into a powerful signal suitable for driving loudspeakers, with as little distortion as possible. During the years since the invention of electronic audio systems, designers have come up with various approaches to this problem. It all started with Class A...
A little more noise, a lot more power
For many people, the amount of power an amplifier can produce is an important factor in judging its characteristics (So your amplifier delivers 2 x 40 watts? Mine does 2 x 701). But in practice, power plays only a minor role.
You can already make a lot of noise with just a few watts. If you use a set of loudspeakers that can produce a sound pressure level of 86 dB at 1 W (which is a value commonly stated by manufacturers in speaker specifications), you can manage 90 dB with just 2.5 watts. With 25 watts, you have enough for 100 dB. That’s already rather loud (and harmful to your ears as well!).
Our ears perceive each 6-dB increase in the sound pressure level as a doubling of the volume level, but this requires increasing the power by a factor of four. This means that if you really want to have a bigger final amplifier with more power than what you presently have, you will need an amplifier with at least four times as much power to actually notice any difference.
Delivering a large amount of power is not a simple task for an amplifier. Voltage amplification and current amplification are both necessary in order to provide sufficient power to the speakers connected to the amplifier. This is because loudspeakers have an efficiency of only a few percent, which means that several watts are certainly necessary to generate an adequate sound pressure level in a living room. In the case of concerts or outdoor events, quite a bit more is required, and the necessary power can easily amount to several kilowatts. To produce power amplification in a final amplifier, various concepts have been developed for using transistors or FETs to generate highquaiity output signals and/or improve the efficiency of the output stage. (Here we leave valve amplifiers out of the picture.)
When devising an output stage, the designer must take into account the specific properties of the semiconductor devices being used. If we could work with "ideal’ transistors or FETs. it would be much easier to build good amplifiers. Unfortunately, all semiconductor devices suffer from non-linearity in their amplification characteristics, and this causes major problems, especially for processing analogue signals. These problems can be minimised by using properly dimensioned feedback. There are also other nasty side effects that also occur, depending on the selected configuration, such as the notorious problem of crossover distortion.
Especially with large amplifiers, heat generation is another factor that must be taken into account. It can lead to far-reaching thermal effects, such as drift of the quiescent current set-ling and thermal modulation distortion.
Final amplifiers are normally classified according to the configuration of the output stage. This largely determines their efficiency and quality, since the output stage is where the actual power amplification takes place.
The various amplifier configurations are designated using the letters of the alphabet, although the letters do not say anything about how they work. It all just started with the first letter of the alphabet.
Reprinted Url Of This Article: http://www.seekic.com/blog/project_solutions/2011/08/02/THAT'S_CLASS__audio_amplifiers_from_A_to_T_(1).html
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