Message-ID: <3467.200012081849@gemini>
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Organization: University of Hertfordshire
To: rsgb_lf_group@blacksheep.org
Date: Fri, 8 Dec 2000 18:52:52 +0000
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Subject: LF: Re:amplifiers
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Dear LF Group,

Andy Wrote:
> >The parasitic diode is no good in this configuration for catching the
> >spikes from what I call the transient soak choke, I use separate clamp
> >diodes up to the rails effectively 'crosing over' the centre tapped choke
> >- see the circuit in the new LF Experimenters Book.


I think this is the main function of the "transient soak" chokes in the 
Decca design - diverting the current into the fast diodes rather than 
through the Mosfet diodes during the switching transient. There is 
not much chance of cross conduction through the mosfets, since 
the transformer coupled gate circuit inherently prevents the gate 
drive from overlapping. 

Johan wrote: ........Current switching and Voltage switching.
> Full and half H-bridges (the Decca TX has this configuration I guess?) are
> voltage switching amplifiers where the drain voltage is a square wave and
> the fundamental frequency energy is sucked out by a series resonant
> circuit to the load. 

The Decca design is indeed a voltage-switching full bridge - but the 
series resonant tank circuit has a high impedance everywhere 
except the output frequency, so it won't see a capacitive load at 
the harmonics - mine seems quite happy running into a 2 x pi 
section low pass filter. There are some fairly nasty looking spikes 
which seem to be produced by the drive circuit that reach the 
output one way or another otherwise - although I'm not sure if they 
are actually there, or just being coupled into the output monitor 
wiring, which runs through one of the PA compartments.

Cheers, Jim Moritz
73 de M0BMU