Hi,

Could someone tell me the reason for using a Low Pass Filter in a Class D Power Amplifier?

Also, what sort of a Low Pass Filter (Fisrt order, second order, active, passive) should be used when designing a Class D amp?

Thanks!

 

The filter type isn't that important, basically Class D uses a form of PWM, and needs the high frequency removed, which is the reason for the filter.

Here is a article I wrote for the AAC book...

Pulse Width Modulation

My blog...

Bill's Index

A very long while back I helped a user design one using a M-Derived filter (low pass). They are a stone cold dog to tune though.

 

The output from a Class D amplifier switches voltages on and off, i.e. it is a pulse-width modulated (PWM) signal. You don't want to feed this directly into the loudspeaker. This high frequency signal must be smoothed out using a Low Pass Filter (LPF).

It makes no sense using an active filter because that would require a linear power amplifier, defeating the whole purpose of a Class D amplifier.

A passive filter using an inductor and a capacitor would give you a second order filter.