Communication Concepts EB104 Engineering Bulletin - Manual (Page 4)

ARCHIVE INFORMA
TION
ARCHIVE INFORMA
TION
EB104
4 RF Application Reports
The effect of R11 through R14 and R21 through R24 is
minimal and can be disregarded. Considering the standard
integers for T1 impedance ratio, 9:1 with its 5.55 ohms
secondary appears to be the closest. This sets the values
of R15 – R18 at 2.0 ohms each, which results in 2.5 dB gain
loss, and about 0.5 W would be dissipated in each resistor.
To be sure about the stability of the amplifier, a higher
standard value of 2.7 ohms was selected. As a trade-off,
the VSWR will peak slightly at 15 – 20 MHz, but still remain
below 2:1.
NOTE: Values of R15 – R18 were originally 1.0 ohm each
when the amplifier was designed. Since then some of the
parameters of MRF150 (especially G
FS
) have been
enhanced in production resulting in higher values required
for R15 – R18.
Negative feedback is derived from a winding in T2 through
R19 and R20. Its purpose is to equalize the load impedance
for T1 and reduce the amplifier gain at low frequencies. Since
the gate to source capacitance of a MOSFET is fairly
constant with frequency, the amount of feedback voltage is
inversely proportional to its reactance. This function should
be more or less linear, unless the inductive reactance of T1
is too low, or if resonances occur somewhere in the circuit.
No computer analysis (as in Reference 2) was performed
on the negative feedback system. Instead a simple approach
described in Reference 1 was taken, where the gain
difference between 2.0 and 30 MHz determines the feedback
voltage required to equalize the voltages of the secondary
of T1 at these frequencies. With an input impedance of
45 ohms at 2.0 MHz, and the feedback source delivering
15 V
(RMS)
, (P
out
= 600 W) the values of R19 and R20 will
be around 10 ohms each.
A ferrite toroid or a two hole balun type core can be used
for T2. Relatively low
µ
i material with high curie temperature
is recommended, since the minimum inductance requirement
for the dc feed winding is less than 2.0
µ
H. Depending on
the material, T2 can reach temperatures of 200 – 250
°
C,
which the wire insulation must also be able to withstand.
Several different output transformer configurations (T3) were
tried, including a transmission line type in Figure 5. Although
difficult to make, it has the advantage that low
µ
i, low loss
ferrite can be used with multiple turn windings. At this power
level, heat in the output transformer was a major problem.
High permeability materials, required in the metal tube and
ferrite sleeve transformers could not be used because of their
higher losses and low curie temperature. On the other hand,
low
µ
i cores with larger cross sectional areas were not readily
available. To reach the minimum inductance required for
2.0 MHz, two of these transformers, with low permeability
ferrite cores were connected in series. Both have 9:1
impedance ratios. Alternatively the secondaries can be
connected in parallel with twice the number of turns (6) in
each. C11 must withstand high RF currents, and must be
soldered directly across the transformer primary connec-
tions. Regular mica or ceramic capacitors cannot be used,
unless several smaller values are paralleled.
PERFORMANCE
Due to the mechanical proximity of the four MOSFET
devices, the RF ground of the circuit board is poor, and
results in 1.0 – 1.5 dB gain loss at 30 MHz, which can be
seen in Figure 6. The ground plane can be improved by
connecting all source leads together with a metal strap over
the transistor caps. Another method is to place solder lugs
under each transistor mounting screw, and solder each one
to the nearest source lead. In this case, the heat sink will
serve as the RF ground. Although the 3rd order IM distortion
is not exceptionally good, (Figures 6, 7) the worst case 5th
order products are better than – 30 dB at all frequencies,
Figure 5. Number of Turns Shown Is Not Actual