Message-ID: <00ab01c0c396$70a2c4c0$aeb21bca@xtr743187>
From: "Vernall" <vernall@xtra.co.nz>
To: rsgb_lf_group@blacksheep.org
References: <001b01c0c355$f2388240$42ff7ad5@default>
Subject: LF: Re: Aerial Diplexer for 136
Date: Fri, 13 Apr 2001 09:17:21 +1200
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Alan,

OK on the absorption arm of a diplexer:

> Hi all, Dick PA0SE, remarked some time ago that a diplexer with an
> absorbtive load for the unwanted harmonics might help to tame some of the
'
> FET-eating'  TXs.

Harmonic power that goes to the absorber would detract from the potential
high efficiency with hard switching mosfets.

I was amazed to witness how small a heatsink was needed on a 180 kHz
pushpull mosfet PA built by Richard VK7RO, which used a driver chip from the
TC442x series.  It appears to be very stable and very efficient.  I hope to
build a similar but bigger "digital PA" using those driver chips, with one
driver chip per pushpull pair of mosfets.  I assume that the "DC coupled low
impedance drive", which avoids transformers or chokes in the gate circuit,
greatly reduces the scope for parasitic oscillation.  I'm also likely to use
0.1 ohm source resistors in the final to give guaranteed current sharing,
with some degenerative feedback and also providing easy metering or scope
probe display of individual mosfet current.

So the point of this reply is to flag that taming the final can lead to very
high efficiency, whereas a diplexer/absorber load in the output is lossy
(the absorber load may need a bigger heat sink than the mosfets).

73, Bob ZL2CA