How Do Audio Power Amplifiers Work?

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If you were ever wondering exactly how power amplifiers work, then read on. I’m going to give you a very simple overview of some techniques which are used nowadays in audio amplifiers. Obviously, every manufacturer pretty much has refined their own technology. However, most of today’s amplifier technologies are all related to commonly known technologies.

First of all, let’s discuss the function of a power amplifier. The reason that amplifiers are needed is because a loudspeaker is not able to be driven directly from a regular source such as a DVD player. The signal which is delivered by the audio source is not strong enough to drive the speaker. Because speakers have very low impedances which means that the current that they requires fairly high, the signal which is provided to the speaker has to come from a source with very low internal resistance. All audio sources have a fairly high internal resistance and would easily take damage if the output was connected directly to a loudspeaker.

However, there are some loudspeakers out there which have a built in audio amplifier. The speakers are most common in the computer area. Computer speakers typically connect directly to a soundcard. The same is true for Bluetooth speakers. These types of speakers do not require an external power amplifier because the amplifiers built into the speakers.

The basic working principle of every audio amplifier is some power stage which is driven with a fairly small signal. The output of that power stage changes depending on the input signal. In the past, many power amplifiers would utilize tubes as the power stage. Tube amplifiers are less common nowadays but are still popular amongst audio fanatics. However, the bulk of audio amplifiers use transistors. Traditionally, the signal transistor was utilized which would connect to the positive power supply voltage via a load resistor. The more current the transistor would draw the lower the output voltage would become.

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The main problem with this arrangement is the low power efficiency. A lot of wattage is actually dissipated as heat inside the load resistor and the transistor. However, these amplifiers typically excel through great linearity which means the audio signal is not being distorted. The next generation of audio amplifiers was utilizing to transistors instead of one. That would eliminate the power resistor. By using a balanced voltage source, the power efficiency could be enhanced greatly. That is because the current would be flowing into ground will be drawn from ground. So when the amplifier is an idol, there would be virtually no current flow in thus no heat dissipation.

However, most modern audio amplifiers are based on a switching technology. Again, the power stage is made up of two power resistors, typically MOSFETs. These transistors are switched on and off. Thus the output signal will be changing between the negative and positive power supply rail. The trick is that this power stage is driven by a pulse-with modulated signal. This signal is being generated from the analog signal. Following the power stage is usually a low-pass filter which is designed to remove all higher-frequency signal components.