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ARCHIVE INFORMATION

1RF Application Reports

GET 600 WATTS RF FROM FOUR POWER FETs

Prepared by: Helge Granberg

Circuits Engineer, SSB

This unique push-pull/parallel circuit produces a power

output of four devices without the added loss and cost of

power splitters and combiners. Motorola MRF150 RF power

FET makes it possible to parallel two or more devices at

relatively high power levels. This technique is considered

impractical for bipolar transistors due to their low input

impedance. In a common-source amplifier configuration, a

power FET has approximately five to ten times higher input

impedance than a comparable bipolar transistor in a common

emitter circuit. The output impedance in both cases is

determined by the dc supply voltage and power level. The

limit to the number of FETs that can be paralleled is dictated

by physical, rather than electrical restrictions, where the

mutual inductance between the drains is the most critical

aspect, limiting the upper frequency range of operation. The

magnitude of these losses is relative to the impedance levels

involved, and becomes more serious at lower supply

voltages and higher power levels. Since the minimum

mounting distance of the transistors is limited by the package

size, the only real improvement would be a multiple die

package. For higher frequency circuits, these mutual

inductances could be used as a part of the matching network,

but it would seriously limit the bandwidth of the amplifier. This

technique is popular with many VHF bipolar designs.

In paralleling power FETs another important aspect must

be considered: If the unity gain frequency (f

) of the device

is sufficiently high, an oscillator will be created, where the

paralleling inductances together with the gate and drain

capacitances will form resonant circuits. The feedback is

obtained through the drain to gate capacitance (C

rss

), which

will result in 360° phase shift usually somewhere higher than

the amplifier bandwidth. Thus, the oscillations may not be

directly noticed in the amplifier output, but may have high

amplitudes at the drains. This can be cured by isolating the

paralleling inductance, which consists of the dc blocking

capacitors (C7 – C10, Figure 2) and their wiring inductance

from the gates. Low value noninductive resistors which do

not appreciably affect the system gain can be used for this

purpose.

CIRCUIT DESCRIPTION

Figure 2 shows a detailed schematic of the 600 W RF

FET amplifier. It can be operated from supply voltages of

40 to 50 depending on linearity requirements. The bias for

each device is independently adjustable, therefore no

matching is required for the gate threshold voltages. Since

the power gain of a MOSFET is largely dependent on the

drain bias current, only g

matching is required, and it can

be only ± 10%.

PHOTO

Figure 1. Photograph of the 600 Watt 2.0 – 30 MHz

MOSFET Linear Amplifier

The circuit board was designed to allow several different

gate biasing configurations (Figure 3). In circuit “a”, which

is used in the amplifier described here, D1 serves a purpose

of preventing positive voltage from getting fed back to the

bias source in case of a draingate short in a FET. This

protects the other three devices from gate overvoltage. C1

– R2 combination establishes an RF shunt from the gate

to ground, which is necessary for stabilization. R4 could also

be used for this purpose, but it would have to be a relatively

low value, resulting in unnecessary high current drain from

the bias supply. Normally R4 is only a dc return to ground,

which is required with D1 preventing an open circuit in one

direction. R3 is a low value resistor to prevent parasitic

oscillations in a parallel FET circuit, as discussed earlier.

Variations “b” and “c” are basically the same, except for R2,

which can be used to control the amount of RF rectified by

D1. In addition to blocking the dc in one direction, D1 can

be used for proportional biasing, in which the bias voltage

increases with RF drive. This allows the initial idle current

to be set to a lower than normal value, increasing the system

efficiency.

MOTOROLA

SEMICONDUCTOR

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by EB104/D



 Motorola, Inc. 1993

EB104

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ARCHIVE INFORMATION 1

Summary of Contents

Page 1 - ARCHIVE INFORMATION

ARCHIVE INFORMATIONARCHIVE INFORMATION1RF Application ReportsGET 600 WATTS RF FROM FOUR POWER FETsPrepared by: Helge GranbergCircuits Engineer, SSBThi

Page 2

ARCHIVE INFORMATIONARCHIVE INFORMATION2 RF Application ReportsR1 – R5 — 10 k TrimpotR6 — 1.0 k/1.0WR7 — 10 OhmsR8 — 2.0 kR9, R21, R22, R23, R24 —

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ARCHIVE INFORMATIONARCHIVE INFORMATION3RF Application Reportsb.a.c. d.Figure

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ARCHIVE INFORMATIONARCHIVE INFORMATION4 RF Application ReportsThe effect of R11 through R14 and R21 through R24 isminimal and can be disregarded.

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ARCHIVE INFORMATIONARCHIVE INFORMATION5RF Application Reports                 

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ARCHIVE INFORMATIONARCHIVE INFORMATION6 RF Application ReportsC7C12denotes feed-through eyeletsC8C2 C1C14L2R19R21J1R22To R25C15IC1R23R24R4C11C11C

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ARCHIVE INFORMATIONARCHIVE INFORMATION7RF Application ReportsNOTE: Not to ScaleFigure 9. Circuit Board Photo Master

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ARCHIVE INFORMATIONARCHIVE INFORMATION8 RF Application ReportsREFERENCES1.“A Two-Stage 1.0 kW Solid-State Linear Amplifier,”AN-758, Motorola Semi

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ARCHIVE INFORMATIONARCHIVE INFORMATION9RF Application ReportsMotorola reserves the right to make changes without further notice to any products h

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