why sawtooth instead of square wave for class D amplifier?

mik3ca · Sep 22, 2017

I ended up playing around with a couple mosfets, a LM393 comparator, NE555 timer, and other stuff and I made myself a decent amplifier, but the inputs I used for the comparator were a square wave running at about 1Mhz and the audio source itself via a capacitor. I also connected a resistor voltage divider to the same input and this divider made things work well. (example: I made the input at ultra low volume and can hear the same thing very loud at the output)

Now I'm curious. In my design, I used a square wave as an input to the comparator. On the web, the majority of the designs use a sawtooth or triangular wave input. I'm curious. Why is the triangular wave input preferred? What is the disadvantage with a square wave input?

Les Jones · Sep 22, 2017

With a triangular wave connected to one input of the comparator and the audio connected to the other input the output of the comparator will be a PWM signal whose duty cycle represents the instantanious level of the audio signal. I don't see how it could work with a square wave input.

Les.

AnalogKid · Sep 22, 2017

With no schematic it is impossible to say.

ak

alec_t · Sep 22, 2017

Les Jones said:
I don't see how it could work with a square wave input.

Perhaps parasitics / bandwidth-limitation caused some low-pass filtering whereby the edges got knocked off the square-wave so that it became an approximation of a triangular wave. That would result in crude PWM.

AnalogKid · Sep 22, 2017

AN LM393 probably is slow enough internally to triangle-ize, or at least trapezoid-ize a 1 MHz input.

ak

kubeek · Sep 22, 2017

In my opinion Les Jones is closest to the actual point being asked, but let´s see what mik3ca has to say about it.

MrAl · Sep 22, 2017

Hi,

I agree that Les provided the answer, but the other suggestions that the LM393 device is too slow are also good. I would definitely look at that because the slew rate could actually be defeating the purpose of creating a class D amp in the first place because the output can not be a clean PWM and thus the efficiency of the amp will not approach a true class D amp so all is for nothing.
A quick rough efficiency measurement would show some problems.

schmitt trigger · Sep 22, 2017

AnalogKid said:
AN LM393 probably is slow enough internally to triangle-ize, or at least trapezoid-ize a 1 MHz input.

ak

If the pullup resistor is large, this is exactly what will happen.
But without schematics or scope waveforms, we are blindfolded.

crutschow · Sep 22, 2017

mik3ca said:
On the web, the majority of the designs use a sawtooth or triangular wave input.

Correction.
All the designs use a sawtooth.
None use a square-wave.

mik3ca said:
Why is the triangular wave input preferred?

Because a true square-wave will not work to make a proper PWM signal from the audio input.

mik3ca said:
What is the disadvantage with a square wave input?

See above.

AnalogKid · Sep 22, 2017

MrAl said:
I agree that Les provided the answer, but the other suggestions that the LM393 device is too slow are also good.

I think Les and I said the same thing. Due to a non-refreshed page, I did not see his post before I wrote mine.

ak

AnalogKid · Sep 22, 2017

crutschow said:
Correction.
All the designs use a sawtooth.

Sure about that? A lot of them look like triangle-wave-based to me.

ak

Nigel Goodwin · Sep 23, 2017

crutschow said:
Correction.
All the designs use a sawtooth.

Correction to your correction

All designs use a triangle wave - a saw tooth would mean massive distortion - not the 100% of a square wave, but at least 50%.

AnalogKid · Sep 23, 2017

Nigel Goodwin said:
Correction to your correction
All designs use a triangle wave - a saw tooth would mean massive distortion - not the 100% of a square wave, but at least 50%.

Disagreement with your correction to his correction.

The only difference between saw and triangle waves is a faster leading edge. There still is a linear relationship between elapsed time into a wave cycle and the percentage of the total wave area under the curve. With a positive-going saw wave (narrow peak at the top) of 2 V p-p and incoming audio of 2 V p-p, both into a fast comparator, the range of output rectangular wave pulse widths still is 0% to 100% of the saw wave period (note - theoretically perfect comparator).

For low-distortion PWM, the pulse frequency should be much greater than the highest audio freq. In terms of instantaneous values, the comparator sees a fast periodic waveform (saw or tri) on one input, and a slowly varying DC level on the other. For an instantaneous audio waveform value of 1.5 V, the comparator output will be a 25/75 rectangular wave (or 75/25 depending on how the inverting and non-inverting inputs are assigned), for *either* carrier wave.

ak

alec_t · Sep 23, 2017

Nigel Goodwin said:
All designs use a triangle wave - a saw tooth would mean massive distortion

LTspice thinks there is very little difference in distortion for the two wave shapes.

Nigel Goodwin · Sep 23, 2017

AnalogKid said:
The only difference between saw and triangle waves is a faster leading edge. There still is a linear relationship between elapsed time into a wave cycle and the percentage of the total wave area under the curve.

I would disagree, a sawtooth has near instantaneous transition in one direction, and a slow ramp in the other.

But regardless, all class-D amps (of this type) use triangle waves - why would you want to use something that would be inferior?.

crutschow · Sep 23, 2017

AnalogKid said:
Sure about that? A lot of them look like triangle-wave-based to me.

ak

Yes, I meant sawtooth or triangle.

And, just to be clear, we are talking about the internal modulator waveform, not the waveform out of the modulator (which is a PWM square-wave).
I think there may be some confusion on that point.

crutschow · Sep 23, 2017

Nigel Goodwin said:
But regardless, all class-D amps (of this type) use triangle waves - why would you want to use something that would be inferior?.

How is a sawtooth inferior?
A critical factor in the distortion generated by the PWM modulator is the linearity of the ramp of either the sawtooth or the triangle-wave.
If they both are equally linear, then they will produce equal output distortion.

AnalogKid · Sep 23, 2017

Nigel Goodwin said:
I would disagree, a sawtooth has near instantaneous transition in one direction, and a slow ramp in the other.

Which is what I said

AnalogKid said:
The only difference between saw and triangle waves is a faster leading edge.

To clarify, for waves of the same amplitude and frequency, a saw wave has a much faster leading edge. Mucho incredibly zoomy faster. But the area under both curves is the same, which is why the modulation noise is the same.
ak

ronsimpson · Sep 23, 2017

crutschow said:
A critical factor in the distortion generated by the PWM modulator is the linearity of the ramp of either the sawtooth or the triangle-wave.

Agree that a nice linear ramp is better but, I enclose the PWM in a feedback loop, (error amp) so this distortion should be removed.

schmitt trigger · Sep 23, 2017

I believe we have managed to utterly confuse the OP............

why sawtooth instead of square wave for class D amplifier?

Member

Well-Known Member

Well-Known Member

Well-Known Member

Well-Known Member

Well-Known Member

Well-Known Member

Well-Known Member

Well-Known Member

Well-Known Member

Well-Known Member

Super Moderator

Well-Known Member

Well-Known Member

Attachments

Super Moderator

Well-Known Member

Well-Known Member

Well-Known Member

Well-Known Member

Well-Known Member

Similar threads

Privacy & Transparency

Privacy & Transparency