Low power class d amp turorial

This article is intended for electronic hobbyist or students alike who are interested in learning the basic operating principle of these audio amplifiers popularly known as class D amp. These types of switching amps are ideally suited for battery operated gadgets and are popularly being used in cell phones, set top boxes, and most MP3 players due to its inherently high efficiency and low power consumption as opposed to linear amps. The sample circuit bellow is for low power headphone amplifier but can also drive small speakers as well.

The basic operating principles of these audio amps are similar to those of switching DC to DC converters utilizing PWM (pulse width modulation). But when used in audio applications, the output is purely in a clock pulses being modulated on its duty cycle in proportion to audio signal as a modulator.

Let’s say at chosen clock frequency of 200 kHz, the quiescent output pulses are at 50% duty cycle without audio but being modulated to +/- 50 when strong audio is present. This will yield an output of from 0 to 100% centered at 50.

Fig. 1 shows the waveform at 50% no audio.

In a real world scenario, audio modulations should not necessarily swing to 100% but practically less than that. In this case, a 10 to 90% centered at 50 are common to avoid distortions.

Fig. 2 shows the waveform at 90% large signal swing.

Fig. 3 shows the waveform at 10% at minimum signal swing.

Fig. 4 shows the composite signal containing 1 KHz audio sine wave as the modulator, and the 200 KHz pulse width modulated carrier.

Fig. 5 shows the “magnified” view of the modulated carrier signal on it’s rising and falling contours of the sine wave.

The waveforms shown above were done in a circuit simulator using LTspice IV with its integral signal generator configured to produce PWM. The next thing I’m going to show you is how to implement it in real circuit but the big question is, where will I get a low cost and readily available chip that can provide a decent sound that can satisfy my taste in music quality?

Well, this question is somewhat subjective and varies from person to person. So, the measurements and data that I may get are strictly for personal evaluation only and not for commercial application purposes.

Another thing I would like to emphasize is to use “non-audio chip” as the primary source of PWM signal as opposed to expensive and hard to get high speed opamps or comparators.  So, as the name of this site implies, I will use exotic circuits but relatively low cost and readily available IC’s that are not intended for audio applications but are specifically made by chip makers for dc to dc converters. Well, this doesn’t mean that the music quality is not good enough but if you will try it, you’ll be surprise. Dont worry guys, this will not necessarily put your learning at risk but it will merely stretch your skill beyond your existing one. So, stand-by please for the following demo circuit soon……….

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Class D amp using 555 timer