Class D Amplifier Power Stage and Open Loop and Closed Loop Characteristics

[Akanimo]

I have posted this thread to gather any information that anyone feels is important about the design of Class D audio amplifier, both of the power stage and of the feedback loop. Please feel free to drop your comments. Any random comment in the subject is welcome.

--- Updated Nov 29, 2020 ---

References to good books, web pages and documents, and videos on the subject are also welcome.

 

[KlausST]

Hi,

I wonder whether there are polyphase class D amplifiers. Controlled by an FPGA with advanced algorithm to reduce noise while getting high upper cutoff frequency and high dV/dt but filtered analog output.

Klaus

 

[Audioguru]

I haven't designed a linear amplifier since excellent ones became available as ICs and are inexpensive.
Today, there are many excellent class-D amplifier ICs available, also inexpensive.
Why would anybody want to design or copy one? For school work?

 

[Akanimo]

Likening SMPS to the Class D amplifier, and owing to the fact that I have an understanding of SMPS independent of controller ICs, I believe I could get to understand the Class D amps pretty well too if some key factors are pointed out to me.

I am already digging into the subject and I have gotten hold of what happens in the power stage. I still need to understand how its feedback loop operates.

well, the major aim is that I might be able to digitize the amp along with other functionalities in a single chip.

 

[Akanimo]

For SMPS, we tend to desire a constant output voltage against all perturbations. For the Class D amp, we seem to desire the output voltage to mimic 'perturbations' (i.e. the changing input signal) and, in some cases, voltage feedback is taken at the switch-node contrary to what's done with SMPS.

This seem to make the transfer functions somewhat more complicated. Any idea what the transfer functions would be like?

 

[schmitt trigger]

Both TI and STMicro have app notes with plenty of good information.
Read them, a lot of your questions will be answered.

 

[Easy peasy]

the transfer function desired is simply Vo = Av x Vin, for any normal range of load R, you set Av.