Message-ID: <00a601c1a7db$f9beec00$a22865d5@oemcomputer>
From: "Johan Bodin" <sm6lkm.jbeab@swipnet.se>
To: rsgb_lf_group@blacksheep.org
References: <89.1292de2e.2985b417@aol.com>
Subject: Re: LF: Envelope shaping and class D
Date: Mon, 28 Jan 2002 10:10:31 +0100
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John wrote:

> Interesting idea, Johan, but I believe you'll find that if you drive the 
> amplifiers with a sinewave, there is an efficiency penalty; whereas, if you 
> drive them with a square wave, you actually have made a PWM modulator.

Yes and yes.

I meant square wave drive (class D).

My idea was to combine the modulator and the P.A. in the same stage.
This would eliminate the need for a separate PWM modulator in the
P.A. power supply.

The switching waveform will look a bit "spikey" at low output power. Worst case
is when the output power is close to zero, the signal will look like a train of impulses
with alternating polarity. I guess the spectrum will then contain all odd harmonics
at the same amplitude as the fundamental, limited only by the switching times.

Assuming a Q of 6 in the series resonant output circuit (typical value, I think), this
circuit alone would attenuate the 3f component by 24dB. Not too bad.


73
Johan SM6LKM