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