Low-pass filter in a Class D audio amplifier design

Discussion in 'Homework Help' started by star, Feb 21, 2007.

  1. star

    Thread Starter Member

    Dec 18, 2006
    19
    0
    Hi,

    I'm designing a simple class D audio amplifier at the moment, and have been reading up on what each section of the amplifier does. As I understand it, the low-pass filter at the last section of the circuit design changes the PWM square-wave signal into a smooth sinusoidal signal again. What I don't understand is how it does that? I was under the impression that all filters did was to allow/block waves with past a threshold frequency which depends on whether it is a high-pass, low-pass or band-pass filter. And does the filter play any other roles in this amplifier?

    Any help there would be appreciated. Thanks in advance.
     
  2. Distort10n

    Active Member

    Dec 25, 2006
    429
    1
    Well, my knowledge of Class D is nil. But I believe the point behind the modulation scheme is to keep the current in the load low and thus power dissipation low as well.

    But onto your square wave question. What is a square wave made of? Theoretically it is made up of an infinite number of odd harmonic sine waves which you can see if you trodded through its Fourier series.

    So if you had a 40 kHz square wave running through a low pass filter that has a cutoff of say 100 kHz, you are going to eliminate the higher order harmonics from the square wave and what will be 'left over' are the fundamental and first harmonic. It will look more like a sine wave.

    I hope I explained that correctly.
     
  3. star

    Thread Starter Member

    Dec 18, 2006
    19
    0
    Ahh.. I see what you mean. It makes perfect sense to me. Is this why some passive first order low-pass filters will produce a sine wave that's slightly distorted?

    As for my question about other roles of the filter, don't worry... but does anyone else have any ideas about this?
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
  5. star

    Thread Starter Member

    Dec 18, 2006
    19
    0
    Nope.. I haven't come across that white paper yet. Thanks for the link though - it was quite useful.
     
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