Communication Concepts AN779 Application Note - Manual (Page 6)

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PRODUCT TRANSFERRED T
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AN779
6 RF Application Reports
The output matching is done with a transformer similar
to that described in the first part of this paper (Figures 4B,
4C). This transformer employs a multi-turn primary, which
can be provided with a center tap for the collector dc feed.
In addition to a higher primary inductance, more effective
coupling between the two transformer halves is obtained,
which is important regarding the even-order harmonic
suppression.
28-Volt Version
A 28-V version of this unit has also been designed with
the MHW592 and a pair of MRF401s. The only major change
required is the output transformer, which should have a 1:1
impedance ratio in this case. The transformer consists of
six turns of RG-196 coaxial cable wound on an Indiana
General F-627-8-Q1 toroid. Each end of the braid is
connected to the collectors, and the inner conductor forms
the secondary. A connection is made in the center of the
braid (three turns from each end) to form the center tap and
dc feed.
The MRF433 and MRF401 have almost similar input
characteristics, and no changes are necessary in the input
circuit, except for the series feedback resistors, which should
be 68 — 82 ohms and 1 W.
In designing the gain-leveling networks, another approach
can be taken, which does not involve the computer program
described in Reference 8. Although the input VSWR is not
optimized, it has proved to give satisfactory results.
R1 — 1 k Trimpot
R2 — 1 k Ohm, 1/4 W
R3 — Optional
R4, R5 — 5, 6 Ohms, 1/4 W
R6, R7 — 47 Ohms, 1/4 W
C1, C2, C5, C6, C7 — 0.01 µF Chip
C3, C4 — 1800 pF Chip
C8 — 10
µF/35 V Electrolytic
L1, L4, L5 — Ferrite Beads (Fair-Rite Products Corp.
#2643000101 or Ferroxcube #56 590 65/3B or equivalent)
L2, L3 — 10 µH Molded Choke
L6, L7 — 0.1 µH Molded Choke
Q1 — MJE240
Q2, Q3 — MRF433
H1 — MHW591
T1, T2 — 4:1 and 1:4 Impedance Transformers, respectively.
(See discussion on transformers.) Ferrite Beads are
Fair-Rite #2643006301 or equivalent)
C2
D1
R3
Q3
Q1
R2
RF OUT
50 Ω
RF IN
50 Ω
Q2
L6
C1
D2
R1
C4
R5
C3
R6
R4
L2
L3
R7
L7
T2
C7 C8
L4 V
CC
C6
T1
L5
L1
+
- C5
H1
Figure 8. Schematic and Components Parts list
* NOTE: Parts and kits for this amplifier are available from Communication Concepts Inc., 508 Millstone Drive, Beavercreek, Ohio 45434–5840
(513) 426–8600.