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Re: [Amps] LDMOS availability

To: <>
Subject: Re: [Amps] LDMOS availability
From: "Jeff DePolo" <>
Date: Wed, 7 Jun 2017 12:21:00 -0400
List-post: <>
> Cant quote you the device numbers,  but have seen KW plus 
> MOSFETS around since the
> middle to late 1990s.    When I purchased this AM station 
> property there 
> was a Harris
> 50KW chugging along,  it used 50 MOSFET modules,  50 for RF 
> and 50 for 
> modulation.

Harris DX50 probably?  But if it were a DX50, the design was such that the
modules weren't operated separately for RF and modulation.  The amplifier
modules all ran at RF, and they were individually switched on/off via the
modulation encoder board based on the incoming audio.  The sampling rate was
several hundred kHz (varied based on operating frequency).  On each module
there were eight (?) TO-247AC package MOSFET's that operated class D.
That's a simplified overview, but you get the idea.

Anyway, the FET's weren't anything special, other than being capable of a
few hundred volts (the supply ran around 250VDC) and fast switching.  They
were nothing like the type, class, package, power handling, circuitry, etc.
of modern RF LDMOS devices and the amplifiers built around them operating
class AB, so no real comparison between this design and a multi-band HF
linear amplifier like we're used to in the ham world.  

And there were 8 devices per module, and somewhere around 150 modules total
in a 50 kW transmitter (most being "final" amplifier modules, a few being
drivers/pre-drivers), so well over 1,000 TO-247AC devices involved to make
that 50 kW of medium-wave AM (probably capable of more like 60-70 kW at
+140% mod).

Regardless, the design was elegant in its simplicity, and for the most part,
aside from a few quirks, the DX series was (I should say "is" as we still
maintain a few) a very reliable rig.  I'm still waiting for one to come out
of service for a 160m conversion project (not a 50 kW model mind you), but
because of how reliable they are, I might be waiting a few more decades...

                                        --- Jeff WN3A

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