Doherty Amplifier

Thu Aug 9 09:34:17 CDT 2007

Andre,

With the Taylor system, maybe I should have said 'lost art', but I was really thinking 'lost ark'.  Or holy grail!  One of the old timers had published something long ago about his supermodulated Taylor system, and nobody knew the details.

Without the extra transformer in the audio chain, audio can be better, although a well designed transformer system sounds fine.  (There are badly designed ones that sound terrible, even though they meet FCC distortion specs!)  A big heavy modulation transformer is difficult and expensive to replace when it fails, too.  So if efficiency is about the same, or better, then the high-tech tricks become attractive.  Nowadays AM broadcasters save so much money on AC power with the new switch-mode transmitters that the transmitters are paying for themselves.

Short wave systems have to move around according to the MUF, the LUF, the target coverage area and international agreements, and the high-tech analog designs are definitely harder to switch and adjust.  Some of the modern switch-mode designs overcome that, so I'm sure that they are used.  Short Wave broadcasting used to be big government operations, but now there are many little religious groups using old analog gear, some of it modified from medium-wave broadcast service.  I think Armstrong in Syracuse, NY does a lot of shortwave modification work, and I heard about a Doherty unit that was set up for muli-channel shortwave operation.

There is a difference between Ampliphase and Doherty.  Doherty is a class-B arrangement, similar to the Taylor system, that gets extra power out of the carrier tube during positive modulation by making a 1/4 wave line put a heavier load on it when the peak tube adds positive peak power.  Ampliphase essentially took two phase-modulated transmitters with opposite modulation polarity, and combined them into a steady-phase, amplitude modulated signal.  Ampliphase must have irritated Major Armstrong, because the phase modulation was produced using the Armstrong method, but the output was AM, so his patents must not have applied.

There is also a Terman-Woodyard high-efficiency grid modulated design, similar to Doherty, but the modulation is done by applying audio to grid bias.  Doherty was meant to be a linear RF amplifier.  The Taylor design was actually based on grid modulation, but using it for linear amplification seems little different.  I only heard about Doherty and Ampliphase in commercial operation.

The timing of the Amrad meeting on this subject unfortunately comes at the same instant as a super-crunch at work.  It always seems to happen this way.  I'll make it if I can.

   Bob, WA3WDR

  ----- Original Message ----- 
  From: Andre Kesteloot 
  To: Bob Bruhns 
  Cc: AMRAD Tacos ; andre.kesteloot at ieee.org 
  Sent: Thursday, August 09, 2007 9:41 AM
  Subject: Re: Doherty Amplifier

  Bob, 
  thanks for the clarifications on the Doherty amplifier.
  The RCA approach, named by them the "Ampliphase" sold fairly well in the AM (medium wave band), as I recall, but as I mentioned earlier, much to tricky in the HF band, where multiple QSY every night would often be necessary.
  Its great advantage was the fact that one did not have to produce a huge amount of Audio power to modulate the output stage, and yet audio quality could, be quite good. Hence one gained in power consumption, rack space, modulation transformers, etc. Maintenance was therefore easier
  73
  André

  ****************************

  Bob Bruhns wrote: 
Hi Andre,

I don't know if anyone sold a Taylor system transmitter.  The Taylor system was almost a "lost ark" in the AM community - people
kept asking about it, but the knowledge was lost, until someone gave me the right hint, and I found that patent.

There is a series-modulator system that is basically similar, but it does the trick at baseband, and provides modulated B+ to a
regular Class-C modulated stage.  You would think that this would have been used back in the old days, but the references I see are
recent.  One series device, a tube or a transistor, provides B+ from a supply that is a little higher than the unmodulated B+ level.
This device normally operates just below saturation, and it works as a series element that can reduce the voltage from there.  With
no modulation, it drops the voltage just a little bit to the unmodulated B+ level,  and this device handles the negative modulation
peaks.

A second device goes to a higher voltage supply, about 2.5 the unmodulated B+ level, or a litle higher.  This device is running
almost cut off.  When it is driven into conduction, it can raise the modulated B+ to 2X the unmodulated level or a bit higher.  This
device handles the positive peaks.

Efficiency is reasonably high at carrier level, dropping somewhat with modulation, but no big audio transformers or inductors are
required, and no special multi-tapped RF section is required either.  I have seen this system called a 'class-B series modulator'.
It is sometimes used for AM using transistors operating around the 50V level, to modulate solid-state class E RF amplifiers.

www.amfone.net/21stAM/class_b_series.htm

   Bob, WA3WDR

----- Original Message ----- 
From: "Andre Kesteloot" <andre.kesteloot at verizon.net>
To: "Bob Bruhns" <bbruhns at erols.com>
Cc: "AMRAD Tacos" <tacos at amrad.org>
Sent: Wednesday, August 08, 2007 10:31 PM
Subject: Re: Doherty Amplifier

Bob Bruhns wrote:
  An interesting alternative is the Taylor linear amplifier.

    Bob
thanks for the pointer
do you know who, if anyone, marketed this invention ?
73
André N4ICK
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