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73 Amateur Radio - Free and Open Source Software
73 Amateur Radio - Free and Open Source Software
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Num ber 11 on your Feedback CItro<br />
The 220 MHz DMOS<br />
Linear Amplifier Project<br />
Follow-up to January's 220 MHz transverter project.<br />
by Robert E. Bloom W6YU Y<br />
F<br />
o r those dedicated home-brewers who<br />
have just gotten warmed up on the<br />
transvener project (Jan . '89 73) , here is another<br />
item to apply the iron to: the DMOS<br />
linear power amplifier.<br />
Chassts<br />
This amp is constructed on a IO ~ x 3 % ~ x<br />
'YI " radiating fin heat sink. The linear output<br />
of this amplifier exceeds 60 Walts RMS on<br />
CW power. The heat sink is somewhat larger<br />
than required, and runs quite cool. The amplifier<br />
is contained in an attached box made<br />
from double-sided ma terial 1_ %" deep.<br />
About a third of the box is empty. If you plan<br />
to make this state-of-the-art, cool linear amplifier,<br />
you may want to use a different heat<br />
sink.<br />
cc ,<br />
C:'\'<br />
~/..<br />
"<br />
'r,- -t,<br />
'" 40, 1"<br />
I~:<br />
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UT<br />
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r" ee<br />
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•<br />
,I.<br />
Basic Circuit Description<br />
The linear power amplifier package consists<br />
of two stages: a poly core F-1202, 20<br />
Watt MOS power driver and a MIA COM<br />
PHI DU·126OT UMOS 60 Watt ampl ifier.<br />
These are both 12 volt transistors, but you<br />
can operate them at much higher voltages if<br />
you observe the specifications. Both of these<br />
companies manufacture a variety of units ,<br />
from 2 Walt s to 200 Watts, in frequencies to<br />
1.4 GHz. See the sidebar for a description of<br />
MOS power.<br />
• •<br />
DMOS Power Amplifier<br />
The only similarity to printed circuitry in<br />
the amplifier unit was the removal of four<br />
short 1,4" wide strips of foil at the input and<br />
,j,,,<br />
1 "<br />
(<br />
,'., " .., 0<br />
I ,<br />
...<br />
-L ,I, -L<br />
;;'-" ;;'-"<br />
A<br />
.".......<br />
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,~<br />
r----j----j-f------'<br />
". DU-02G,n<br />
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". '-00<br />
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re~(..~",<br />
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FiKu r~ J. Schematictor th~ 220 to 225 MH: D-MOSFETlinear po...er amplifiu .<br />
38 73 Amateur Radio • June, 1989<br />
J·w<br />
output ofthe two DMOS transistors. It probably<br />
was not necessary to do it this way but I<br />
wanted the strips to look inductive, such as in<br />
an Lcnetwork. I didn't know at the time that<br />
all but the input circuit ofthe driver was going<br />
to be heavily loaded with Unelco or Underwood<br />
low-inductive capacitors. The placement<br />
ofthese capacitors is responsible for the<br />
amplifier' s outstanding purity and stability.<br />
The relatively narrow bandwidth or selectivity<br />
of a tuned design compared 10 a broadbanded<br />
design, along with the purity of the<br />
linear RF output, made output fi llers unnecessary.<br />
Despite several tuned stages preceding the<br />
UMOS amplifier, [ thought it prudent 10 include<br />
a seven-segment filter at the input ofthe<br />
amplifier driver stage. Why not put it at the<br />
output? This is because silver mica " dog bone"<br />
capacitors will not hand le the 60 Watts of RF<br />
current nonnally available. If you have the<br />
larger, more specialized capacitors required,<br />
you may wish to put them in your unit.<br />
It's best to layout the filter in a zigzag<br />
fashion so the inductors don't couple with<br />
each other. The best place for the filter is<br />
along the side panel. allowing space between<br />
the inpct gale of the transistor and the ONe<br />
input coaxial bulkhead connector. If you<br />
want the filter in the output end with higher<br />
current capacitors, you will probably have to<br />
compromise space with the antenna connector<br />
and the receiver ONC connector on the<br />
wall atthat end of the unit.<br />
Strip Line Technique<br />
Small strips of single-sided PC board<br />
material are used as input and output induetors<br />
to the power FETs. The small strips of<br />
PC material not only make the inductance for<br />
the Lnetwork, but also allow for the placement<br />
of non-inductive capacitors and other<br />
components.