[sdiy] Low voltage synthesis?

Mike Bryant mbryant at futurehorizons.com
Sat Jan 15 18:23:21 CET 2022

I thought most high power r.f. transmitters are Class C or Class G/H.  Never heard of one using class D but I suppose it’s possible nowadays.

Can’t say much about our design, but there’s a sprinkling of GaN devices in the right places.

As for the complex impedance of the load, as I said we have to calibrate it, and it’s doing this regularly during operation.  Worst load are actually those piezo tweeters – they seem to have an ever changing impedance depending on volume, temperature, phase of the moon, what have you.

From: Richie Burnett [mailto:rburnett at richieburnett.co.uk]
Sent: 15 January 2022 14:00
To: Synth DIY; Mike Bryant
Subject: Re: [sdiy] Low voltage synthesis?

Out of interest, how do you deal with distortion resulting from the load current free-wheeling through the body diodes during the switching dead-time without resorting to the use of global feedback?

This mechanism, and the variations in the frequency response of the output LC filter with different loudspeaker impedances were the two hardest problems to sort out in an "all digital Class-D amplifier" from my limited experience.

BTW, "Class-D" means different things to different people... There are RF power amplifiers used in big broadcast transmitters that use "Class-D" power amplifiers to efficiently generate many kilowatts of carrier power in the low Megahertz range. These switch directly at the RF carrier frequency followed by a filter to yield a nice clean sinusoidal output. They don't use PWM techniques to synthesise analogue waveforms like a Class-D audio amplifier or a motor drive inverter does. (There's also Class E, Class DE etc, used for radio applications like broadcast, induction heating, plasma generation, etc.)


---- Mike Bryant wrote ----

>  But Class D amps are analog, right? :-)

Mine aren’t !  STM32F730 MCU driving an IRF2003 driving pairs of paralleled n type power MOSFETs (I use F20N60s).  Voltages might vary but most definitely ON or OFF :-)
A small calibration signal is taken back to the STM32’s A-D to model the output stage behaviour, but this isn’t used in real time, i.e. no negative feedback.

From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Mattias Rickardsson
Sent: 14 January 2022 20:35
To: Synth DIY
Subject: Re: [sdiy] Low voltage synthesis?

cheater cheater via Synth-diy <synth-diy at synth-diy.org<mailto:synth-diy at synth-diy.org>> skrev:
 I wonder what's being used in places like
mobile phones or wireless earbuds for example, they can't be running
+/-5V op amps or LV op amps with major crossover distortion.

I've used some mobile codec from Dialog to pump up the jam in a low-voltage application with mainly positive supplies, check out their stuff for some typical insight.

Mike Bryant <mbryant at futurehorizons.com<mailto:mbryant at futurehorizons.com>> skrev:
Nowadays mobile phone amplifiers are all digital - the digital audio (I2S possibly) digital signal is converted to a PDM signal which drives a class D amplifier and a simple LC reconstruction filter on the output.
Some years ago they did use DACs (e.g. Wolfson devices) followed by a Class D amp, until someone realised going digital -> analogue -> digital -> analogue was a nonsense.

But Class D amps are analog, right? :-)


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