What is the capacitor supposed to do?

Hi,
I am reading about Class D amplifier in the page: https://www.maximintegrated.com/app-notes/index.mvp/id/3977

I got stuck with this. Please help me out. Thanks.

Hi,

That capacitor is often called the "coupling capacitor" or "DC blocking capacitor". It is there to make sure that the output load sees only the AC part of the output signal and does not get any DC current. It would typically be a speaker which does not like DC current because it forces the cone to have a positional offset which means it can no longer be used over it's full range of travel.
In a few cases (probably not this one) it could be used as a frequency selective element that directs mostly the higher end of the audio spectrum to the speaker. We probably wont see this in a single speaker arrangement, but in a dual speaker (per channel for stereo) we might see the capacitor being used to direct the higher end spectrum to the tweeter, but then there would still be a larger capacitor for the main speaker.

Thank you, MrAl.

That capacitor is often called the "coupling capacitor" or "DC blocking capacitor". It is there to make sure that the output load sees only the AC part of the output signal and does not get any DC current.

Click to expand...

Well, I knew about coupling capacitor, however, in this case its position was confusing me. How about if I place the capacitor before the low pass filter?
I think both positions are OK, right?

In a few cases (probably not this one) it could be used as a frequency selective element that directs mostly the higher end of the audio spectrum to the speaker. We probably wont see this in a single speaker arrangement, but in a dual speaker (per channel for stereo) we might see the capacitor being used to direct the higher end spectrum to the tweeter, but then there would still be a larger capacitor for the main speaker.

Click to expand...

It seems to me that the function is a DC blocking! How this function differ from DC blocking?

anhnha said:

Thank you, MrAl.

It seems to me that the function is a DC blocking! How this function differ from DC blocking?

Click to expand...

Yes, it is DC blocking.

What MrAl intends to add is additional info that in composite speaker systems (Woofer, tweeter, midrange, ..etc), this capacitor also performs a part of a frequency selective network.

Ramesh

Hello Ramesh and anhnha,

Thank you Ramesh for clarifying that.

anhnha:
Theoretically the capacitor can be put in front of the Low Pass filter, but then the capacitor will be banged with changes from 0 to +Vcc which is not as good for the life of the cap as putting it on the output side. On the input side it gets banged with a pulse that goes from zero to +Vcc, while on the output it is a nearly clean sine wave (for a single frequency) which is much more gentle for the capacitor.