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HIVE INF

RMATI

PRODUCT TRANSFERRED TO M/A–COM

1RF 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

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

: – 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

1 2 3 4 5 6 ... 15 16

Inhaltsverzeichnis

Seite 1 - SEMICONDUCTOR

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COM1RF Application ReportsA TWO-STAGE 1 kW SOLID-STATE LINEAR AMPLIFIERPrepared by: Helge O. GranbergR

Seite 2 - 2 RF Application Reports

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN75810 RF Application ReportsThe main difference is at 2 MHz — and it was decidedthat the 29 dB of

Seite 3

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN75811RF Application ReportsFigure 14. 1 kW Linear Amplifier showing the inputpower divider in the

Seite 4 - 4 RF Application Reports

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN75812 RF Application Reports 

Seite 5

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN75813RF Application Reports°     

Seite 6 - THE DRIVER AMPLIFIER

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN75814 RF Application ReportsNot to scaleFigure 25. Board Layout of the Power Combiner Transmissio

Seite 7

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN75815RF Application ReportsREFERENCES1. Ruthroff: Some Broad Band Transformers, IRE, Volume47, Au

Seite 8 - 8 RF Application Reports

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN75816 RF Application ReportsMotorola reserves the right to make changes without further notice to

Seite 9 - THE OUTPUT COMBINER

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7582 RF Application Reports      

Seite 10 - 10 RF Application Reports

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7583RF Application Reports  Ω    Ω      Fig

Seite 11 - MEASUREMENTS

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7584 RF Application Reports   Ω

Seite 12 - 12 RF Application Reports

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7585RF Application ReportsAlthough omitted from the preliminary calculations, the2 x 5 nH induct

Seite 13

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7586 RF Application ReportsThe 1:4 output transformer is not the optimum in this case,but it is t

Seite 14 - 14 RF Application Reports

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7587RF Application ReportsThe input transformer is equal to what is used with thepower amplifier,

Seite 15 - REFERENCES

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7588 RF Application ReportsOptionalInputAtten-uatorQ1 Q2R4R2R3R1L4L3T3C7C6C8CCT2EE EEBC11 C13Q3L2

Seite 16 - 16 RF Application Reports

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7589RF Application Reports Ω Ω Ω Ω   Ω Figure 10. Four

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