Communication Concepts AN758 - Manual

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1 RF Application Reports
A TWO-STAGE 1 kW SOLID-STATE LINEAR AMPLIFIER
Prepared by: Helge O. Granberg
RF Circuits Engineering
INTRODUCTION
This application note discusses the design of 50 W and
300 W linear amplifiers for the 1.6 to 30 MHz frequency band.
Both amplifiers employ push-pull design for low, even
harmonic distortion. This harmonic distortion and the 50 Vdc
supply voltage make the output impedance matching easier
for 50-Ohm interface, and permits the use of efficient 1:1
and 4:1 broadband transformers.
Modern design includes integrated circuit bias regulators
and the use of ceramic chip capacitors throughout the RF
section, making the units easily mass producible.
Also, four 300 W modules are combined to provide a 1
to 1.2 kW PEP or CW output capability. The driver amplifier
increases the total power gain of the system to approximately
34 dB.
Although the transistors employed (MRF427 and
MRF428) are 100% tested against 30:1 load mismatches,
in case of a slight unbalance, the total dissipation ratings
may be well exceeded in a multi-device design. With high
drive power available, and the power supply current limit set
at much higher levels, it is always possible to have a failure
in one of the push-pull modules under certain load mismatch
conditions. It is recommended that some type of VSWR
based protective circuitry be adapted in the equipment
design, and separate dc regulators with appropriate current
limits provided for each module.
The MRF428 is a single chip transistor with the die size
of 0.140 x 0.248 ″, and rated for a power output of 150 W
PEP or CW. The single chip design eliminates the problem
of selecting two matched die for balanced power distribution
and dissipation. The high total power dissipation rating
(320 W) has been achieved by decreasing the thermal
resistance between the die and the mount by reducing the
thickness of the BeO insulator to 0.04
″ from the standard
0.062 ″, resulting in R
θJC as low as 0.5 °C/W.
The MRF427 is also a single chip device. Its die size is
0.118 x 0.066
″, and is rated at 25 W PEP or CW. This being
a high voltage unit, the package is larger than normally seen
with a transistor of this power level to prevent arcing between
the package terminals.
The MRF427 and MRF428 are both emitter-ballasted,
which insures an even current sharing between each cell,
and thus improving the device ruggedness against load
mismatches.
The recommended collector idling currents are 40 mA and
150 mA respectively. Both devices can be operated in Class
A, although not specified in the data sheet, providing the
power dissipation ratings are not exceeded.
GENERAL DESIGN CONSIDERATIONS
Similar circuit board layouts are employed for the four
300 W building block modules and the preamplifier. A
compact design is achieved by using ceramic chip
capacitors, of which most can be located on the lower side
of the board. The lead lengths are also minimized resulting
in smaller parasitic inductances and smaller variations from
unit-to-unit.
Loops are provided in the collector current paths to allow
monitoring of the individual collector currents with a clip-on
current meter, such as the HP-428B. This is the easiest way
to check the device balance in a push-pull circuit, and the
balance between each module in a system such as this.
The power gain of each module should be within not more
than 0.25 dB from each other, with a provision made for an
input Pi attenuator to accommodate device pairs with larger
gain spreads. The attenuators are not used in this device
however, due to selection of eight closely matched devices.
In regards to the performance specifications, the following
design goals were set:
Devices: 8 x MRF428 + 2 x MRF427A
Supply Voltage: 40 – 50 V
η, Worst Case: 45% on CW and 35% under two-tone
conditions
IMD, d
3
: – 30 dB Maximum (1 kW PEP, 50 V and 800 W
PEP, 40 V)
Power Gain, Total: 30 dB Minimum
Gain Variation: 2.0 – 30 MHz:
± 1.5 dB Maximum
Input VSWR: 2.0:1 Maximum
Continuous CW Operation, 1 kW: 50% Duty Cycle,
30-minute periods, with heatsink temperature < 75 °C .
Load Mismatch Susceptibility: 10:1, any phase angle
Determining the figures above is based on previous perfor-
mance data obtained in test circuits and broadband amplifi-
ers. Some margin was left for losses and phase errors
occurring in the power splitter and combiner.
THE BIAS VOLTAGE SOURCE
Figure 1 shows the bias voltage source employed with
each of the 300 W modules and the preamplifier. Its basic
components are the integrated circuit voltage regulator
MC1723C, the current boost transistor Q3 and the
temperature sensing diode D1.
MOTOROLA
SEMICONDUCTOR
APPLICATION NOTE
Order this document
by AN758/D

Motorola, Inc. 1993
AN758