Over the holiday, K4GF (James Fisher) asked about BJT (bipolar junction
transistor) or FET (field effect transistor) devices to use in a 400 Watt
3.9 MHz amplifier. I looked at my catalogs, and here is a core dump of
sources. If you check these references, there should be more than enough
info to help you build up a 50 Volt amplifier for your application. We
certainly shouldn't be limiting ourselves to tubes with handles or without,
on this forum!
Using the old Hercules 444 amplifier power supply for 48 Volts DC will make
matching easier for you. I assume that the Hercules 444 usess that voltage,
and the Hercules II version is a 13 VDC amplifier?
The impedance looking into the collector (BJT) or drain (FET) will be
determined by the voltage using the standard formulas (see Solid State
Radio Engineering, by Krauss, Raab, Bostian; or any other good handbook
like RSGB series or ARRL or Bill Orr's). 12 volt devices have notoriously
low impedance, making transformation from a few Ohms up to 50 neccessary.
Of course, this isn't much of a problem with a single frequency amplifier,
and the 75 meter requirement is certainly narrowbanded enough that no
tuning is needed in a well designed unit (200 Khz out of 4 Mhz is about 5%
BW).
If you can pick a higher voltage device, the power gain can be improved
also. Most (but not all) of the devices listed below can give you 300-400
Watts PEP in a push-pull arrangement, with under 50 watts of drive. For
further advice, check the following databook literature and articles. I
recommend that you call Motorola and Philips for their books, as they are
free. If you aren't planning mobile operation, then the 50 Volt approach is
preferable. The current is a lot lower. Watch out about the rated output
power, as some of these are in class C or B, perhaps not linear enough for
SSB. You would have to back them out of saturation and with lower output.
Of course, there are 13 VDC and 28 VDC components to chose from also, but
to keep this message from being a megabyte, I will stick to 50 VDC parts.
--------------------------
Communications Concepts, Xenia, OH makes kits based on some of the Granberg
designs, see ads in back of QST, 73, etc.
RF Power FETs, their Characteristics and Applications, by Helge Granberg of
Motorola (2 parts) QEX Jan. 1989, Feb. 1989.
[Following are all listed in the Motorola handbook, RF Application Reports,
HB215, 1995:]
Motorola AN-860 application note, Power Mosfets versus Bipolar Transistors,
Granberg, 1982.
Motorola EB104 engineering bulletin, Get 600 Watts RF from Four Power FETS,
Granberg, 1983.
Motorola EB27A engineering bulletin, Get 300 Watts PEP Linear Across 2 to
30 MHz From this Push-Pull Amplifier, Granberg
[I skipped listing the MRF154 family of HOGFET's by Motorola, because of
their expense, and the difficulty of using such monster transistors for
your application. QST and QEX have carried a plethora of articles on the
1KW no tune amplifiers for mobile/portable use with these devices. My
advice.... stay away unless you have a big bank account.]
[Other related articles on big HF amplifiers:]
Switching MOSFETS suit linear 500W HF Amp, Microwaves and RF, by U. Lott,
Oct. 1984.
Power FETS for RF Amplifiers, RF Design, by G. Appel/Siliconix., Sept/Oct.
1982.
Stable Hexfet RF Power Amplifiers, QST Technical Correspondence, by Wes
Hayward, Nov. 1989.
Power FET Switches as RF Amplifiers, QST, by Doug DeMaw, April, 1989.
Philips Application Reports, Bipolar and MOS Transmitting Transistors, page
20-30.
(Single Sideband Applications, 1.6-28 MHz, Class A and AB)
Philips/Mullard report MCO8002 (same book as above), A Single Stage
Wideband Linear Amplifier for 400 Watts PEP Using BLW96 Transistors, 1980.
Philips/Nijmegen report NCO 8703 (same book as above), A Wideband Linear PA
for 300 W PEP with 2 MOS Transistors BLF177. (this one has 22 dB gain at
50 VDC!)
Advanced Power Technology note APT9501, A High Efficiency 400 Watt 13.56
MHz RF PA, presented at RF Expo West, 1995. (These are class C designs, but
maybe could use in linear fashion is backed off)
----------------------------
[Suitable 50 Volt rated devices might be as follows. It's your call as to
which one is best, based on the availability of transistors and their cost
- call RF Parts, Communcations Concepts or your local manufacturers rep or
distributor for this. ]
Motorola MRF429 BJT, 150 Watt
Motorola MRF 448 BJT, 250 Watt
Motorola MRF150 FET, 150 Watt, high gain.
Motorola MRF156, 150 Watt, higher gain, inexpensive plastic package!
Philips BLW96 BJT, 200 Watt
Philips BLF177 FET, 150 Watt, high gain
Polycore RF Fet (Polyfet) F1415, 150 Watt single ended
Polyfet F1430 300 Watt Push Pull
SGS-Thomson SD1726, 1727 both BJT 150 Watt PEP
SGS-Thomson SD1731, 1728, both BJT >200 Watt PEP, 50 VDC.
SGS-Thomson SD1920 FET, 150Watt, similar to MRF150 Motorola.
Advanced Power Technology ARF440, 441, FETs, 125Watt
There are also suitable Japanese transistors with 2SCxxxx numbers, such as
used in popular HF rigs,
such as from Toshiba, etc, Sorry I won't even begin to list these!
73,
John - K5PRO
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