Application Note
AN4001
Application Note
300 Watt Class E Amplifier Using MRF151A
Rev. 01262010
2
V
dd
Po ≈ 0 . 577
(1)
R
V
d d
(2)
(3)
I
=
d c
1 . 7 3 4 R
V
= 3 . 56 V
Dpeak
dd
1 . 110 Q
(4)
(5)
X =
R
Q − 0 . 67
0 . 1836
0 . 81 Q
B =
(1 +
)
2
R
Q
+ 4
DESIGN AND SIMULATION
MRF 151A was chosen for this application because of its 50V operation capability as well as the low
DS-ON (~0.2ohm). Figure 2 shows the Level 1 SPICE model used in conjunction with Agilent’s ADS simulation
R
software to optimize the Class E circuit. To ease computations equations 1.0, 4.0, and 5.0 were re-arranged to
solve for C, R, Lo, Co as a function of frequency f, output power Po, drain voltage Vdd, and inductor Q. When
these variables are set to 81.36MHz, 300 watt, 48V, and 5 respectively the calculated values are C = 92.4pF, R
= 4.4ohm, Lo = 54nH and Co = 88.3pF. One issue that arises from these results is that the required shunt ca-
pacitance C = 92.4pF is smaller than the output capacitance of MRF151A which, per the data sheet, is 220pF.
This implies that a class E amplifier would not operate optimally. The maximum frequency of operation for opti-
mal, class E performance, for a particular capacitance value is given by:
0.0292
(6)
fmax
=
RCout
For a Cout = 220pF the maximum frequency for optimal performance is ~30MHz. Since the desired
frequency of operation is 81.36 MHz, the ratio f/fmax is 2.7. According to [2], the obtainable efficiency for f/fmax,≈
2.7 is approximately 82%, which is still an attractive number. The calculated component values were used as a
starting point in the simulation and varied in order to maximize output power and minimize DC current. Another
constraint used in this optimization was the instantaneous drain voltage which was capped at 125V, which is
the breakdown voltage of MRF151A. Figure 3 shows the optimized circuit and Figures 4,5, and 6 show the
resulting voltage and current waveforms on the transistor drain, voltage across the load resistor R, and DC cur-
rent.
On the input side, the gate is matched to 50 ohm using conjugate impedance values. ADS can be used
to easily perform this task. A 25 ohm resistor has been added in shunt to improve the bandwidth and stability.
The results suggest a power output of 319 watts and an efficiency of 84.1%. Also, the peak drain volt-
age is 120.6 V.
A sinusoidal signal has been used to drive the MRF151A circuit. It is possible to shape the drive signal
in order to increase efficiency, however, that is beyond the scope of this paper.
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is considering for development. Performance is based on target specifications, simulated results,
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and/or prototype measurements. Commitment to develop is not guaranteed.
PRELIMINARY: Data Sheets contain information regarding a product M/A-COM Technology
Solutions has under development. Performance is based on engineering tests. Specifications are
typical. Mechanical outline has been fixed. Engineering samples and/or test data may be available.
Commitment to produce in volume is not guaranteed.
Visit www.macomtech.com for additional data sheets and product information.
M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make
changes to the product(s) or information contained herein without notice.
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