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Class D Switching Power Amplifiers and Power D/A conversion

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Old 9th December 2003, 01:37 PM   #21
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Lars,

I know you hate these discussion about what we exactly mean by which class, but I am the one that hates sloppy use of terms so please bear with me.
PWM is just ONE specific class of digital amplifiers. You can also have Puls Repetition Modulation, where the puls width remains constant but the repetition freq varies. I haven't seen it in commercial audio, but have seen it in SMPS circuits. Presumably there is also a technology with alternating polarity pulses, varying in length to modulate the audio. Don't know the exact name, something with Biphase Modulation.

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Old 9th December 2003, 01:57 PM   #22
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Quote:
The one i use is the integrator type, with no analog signal processing, and therefore no THD.
I agree with jannemann on the definition of PWM: A fixed frequency rectangular whose duty-cycle is varied.

So are you using a self-oscillating topology with either hysteresis or delay control (or both) ?
In this case what leads you to the assumption that there is only noise generated and no "real" harmonics ?

I know that delta-sigma modulation (i.e. one bit modulation with a fixed sampling rate) generates a "noise-floor" that is consisting of odd order harmonics of the input signal if this is consisting of a single sinusoidal signal. For anything else it looks much more complicated. It is interesting however that IMD seems to be quite low for delta-sigma modulation.

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Charles
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Old 9th December 2003, 02:24 PM   #23
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It all adds up to PWM, no matter if it is the number of pulses, the length, the time, delay, etc. etc. As long as you have only 1's and 0's to put out, the only way you can modulate the signal is by altering the (or modulating) the Pulse Width. Hence Pulse Width Modulation.

Does'nt need a fixed frequency to fit the definition. ...

I agree with you two guys completely, the class term should not be used in a sloppy manner. Therefore i will refrain from even using any 'class' terms. Since there seems to be such widely spread confusion about the definition of each class.

Maybe one of you 'definition of terms' experts could make a list (in this thread) with the precise definition of each amplifier class.

However i admit there is one exception to the PWM / 1-0 rule, namely Frequency Modulation. In theory it would be possible (but not nessescarily beneficial) to make a frequency resonant mode controlled power amplifier. A parallel LC circuit is exited with a squarewave of various frequency. At it's resonance the p-p voltage goes up, at other freq's it goes down. So you can control the output voltage by controlling the frequency of the square wave. Anyone gonna build one of these ??
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Old 9th December 2003, 02:31 PM   #24
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My topology in your terms would be:

Delay controlled.
(with some feedback filtering as used in DS converters)

But since the modulator is integrating (producing a triangular wave) you can also add an external clock signal to syncronize the amplifier to other amplifiers or the DAC.
In this case it is no longer free running, but locked to the clock generator. (Typically 300 kHz - 1 MHz).
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Old 9th December 2003, 03:11 PM   #25
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Quote:
Originally posted by Lars Clausen

Does'nt need a fixed frequency to fit the definition. ...
What definition? your own? I think you might be confusing
terminology with definition. Usually the name of something
does not tell the whole truth, since it would be unbearably
long and complex. As I remember the definition, I agree with
Jan and Charles that PWM means fixed frequency with
varying duty cycle, and of course just two output values.
I don't think I have any textbook to refer to, or at least none
I can find for the moment. However, searching the web for
"pulse width modulation" seems to give a very coherent
picture. All documents I looked at that gave some form of
definition at all agreed with what I said above. The following
has the crispest definition.
http://controls.ame.nd.edu/~bill/mic...in/node39.html
If you don't like maths, then just stick with what I said above.
Some documents also handled special cases like saturated
PWM, which seems not interesting for audio.

I think what you are after Lars might be some term that
encompasses all different technologies that has just two
output values, both pulse width modulation and pulse density
modulation and also other imaginable things like varying both
frequency and pulse length (guess there are no more
possibilities). I don't know if there is such a
term, but I agree such a term would be useful. If there is
already a term for this, someone will probably tell us soon,
and it would not be wise to introduce a new term if there is
already an established one.
Otherwise, what about just calling it a switching amplifier or
a binary amplifier?
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Old 9th December 2003, 03:32 PM   #26
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Quote:
Otherwise, what about just calling it a switching amplifier
That would also be fine with me. OTOH some people call all sorts of switching amplifiers simply class-D (while I personally use this term only for a PWM amp)


Regards

Charles
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Old 9th December 2003, 03:32 PM   #27
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Try this one:

R. Hauck; Binary coding techniques with emphasis on puls density modulation;
Proc. Soc. Photo-Opt. Instr. Eng. 700, 265-269 (1986)

Constant puls width, varying # of pulses per second.

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Old 9th December 2003, 05:30 PM   #28
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Old 9th December 2003, 05:47 PM   #29
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Jocko: If i understand right Class G is switching between different power supply levels, but using ananalog power stage ... right?

You are right about the distortions in the "********" amplifiers
( don't know what to call them anymore ) Just some don't have distortions harmonic to the audio signal, but random noise instead.
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Old 10th December 2003, 04:06 PM   #30
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Quote:
Originally posted by Lars Clausen
:However i admit there is one exception to the PWM / 1-0 rule, namely Frequency Modulation. In theory it would be possible (but not nessescarily beneficial) to make a frequency resonant mode controlled power amplifier. A parallel LC circuit is exited with a squarewave of various frequency. At it's resonance the p-p voltage goes up, at other freq's it goes down. So you can control the output voltage by controlling the frequency of the square wave. Anyone gonna build one of these ??
I think this would be an interesting experiment, but the efficiency would not be good. The frequency would have to go very high above the base frequency where full power output would be obtained. Otherwise, the dynamic range would be poor. Maybe I am off on this view, but was interested in taking a stab at the idea
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