Message-ID: <007c01c0608e$da6ef060$775697d4@oemcomputer>
From: "Johan Bodin" <sm6lkm.jbeab@swipnet.se>
To: rsgb_lf_group@blacksheep.org
Subject: LF: SV: Re:amplifiers
Date: Thu, 7 Dec 2000 21:46:26 +0100
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Hi Andy and All

>The catch diodes clamp over-voltage spikes extremally well, in fact I
>don't see any spikes at all on the breadboard design.

Yes, the "totem-pole" configuration is sort of self-clamping if MOSFETs
are used. Did you add external (low Vf) diodes or do you rely on the
intrinsic diodes in the FETs?

>As far as I can see, the only potential device destroyer is a low
>impedance load or short circuit.
<snip...>
>By the look of the network, A detuned antenna will reflect back as an
>increased load impedance giving an inherent fail safe mechanism.

The lowpass filter between the amp' and the antenna can turn things upside
down when a severe mismatch occurs. For example, a simple pi filter with
a Q of 1 (C1 across, L along, C2 across, all reactances equal to line Z)
will "invert" the impedance like a lambda/4 length of transmission line.
In case the filter output is open (antenna way off resonance), L and C2 forms
a series resonant circuit which makes the filter input look like a dead short at
the design frequency. On the other hand, A short on the filter output causes the
input to look like an open circuit (C1 and L in parallel resonance)...
The impedance inverting properties of a simple pi filter can be useful when the
antenna is fed via a tap or link on the loading coil but for series fed antennas
I'd choose two cascaded pi filters (like the one on G3YXM web site).

73
Johan SM6LKM