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HIVE INF

RMATI

PRODUCT TRANSFERRED TO M/A–COM

1RF Application Reports

LINEAR AMPLIFIERS FOR MOBILE OPERATION

Prepared by: Helge O. Granberg

RF Circuits Engineering

INTRODUCTION

The three versions of the amplifier described here are

intended mainly for amateur radio applications, but are

suitable for other applications such as marine radio with

slight modifications.

100 W is obtained with two MRF455’s. MRF460 or

MRF453 is also adaptable to this design, resulting in

approximately 1.0 to 2.5 dB higher overall power gain than

the values shown. The MRF454 devices which can be

directly substituted with MRF458’s for slightly lower IMD,

deliver the 140 W, and two MRF421 devices are used in the

180 W version.

The use of chip capacitors results in good repeatability,

making the overall design suitable for mass production.

There are several precautions and design hints to be

taken into consideration regarding transistor amplifiers:

1. Eliminate circuit oscillation. Oscillations may cause over-

dissipation of the device or exceed breakdown voltages.

2. Limit the power supply current to prevent excessive dis-

sipation.

3. Adopt protective circuitry, such as fast acting ALC.

4. Ensure proper attachment of the device to a heatsink using

Silicone grease (such as Corning 340 or GC Electronics

8101) to fill all thermal gaps.

THE TRANSISTORS

The MRF421 with a specified power output of 100 W PEP

or CW is the largest of the three RF devices. The maximum

dissipation limit is 290 Watts, which means that the

continuous collector current could go as high as 21.3 A at

13.6 V operated into any load. The data sheet specifies 20 A;

this is actually limited by the current carrying capability of

the internal bonding wires. The values given are valid at a

25°C mount temperature.

The minimum recommended collector idling current in

Class AB is 150 mA. This can be exceeded at the expense

of collector efficiency, or the device can be operated in Class

A at an idling current of approximately one fourth the

maximum specified collector current. This rule of thumb

applies to most RF power transistors, although not specified

for Class A operation.

The MRF454 is specified for a power output of 80 W CW.

Although the data sheet does not give broadband

performance or IMD figures, typical distortion products are

[ – 31 to – 33 dB below one of the two test tones (7) with

a 13.6 V supply. This device has the highest figure of merit

(ratio of emitter periphery and base area), which correlates

with the highest power gain.

The maximum dissipation is 250 Watts, and the maximum

continuous collector current is 20 A. The minimum

recommended collector idling current is 100 mA, and like

the MRF421, can be operated in Class A.

The data sheet specification for the MRF455 is 65 W CW,

but it can be operated in SSB mode, and typically makes

– 32 to – 34 dB IMD) in reference to one of the two test tones

at 50 W PEP, 13.6 volts. It contains the same die as MRF453

and MRF460, but is tested for different parameters and

employs a smaller package. The MRF455/MRF453/MRF460

has a higher figure of merit than the two devices discussed

earlier. Due to this and the higher associated impedance

levels, the power gain exceeds that of the MRF454 and

MRF421 in a practical circuit. The minimum recommended

collector idling current is 40 mA for Class AB, but can be

increased up to 3.0 A for Class A operation.

It should be noted that the data sheet figures for power

gain and linearity are lowered when the device is used in

multi-octave broadband circuit. Normally the device input and

output impedances vary by at least a factor of three from

1.6 to 30 MHz. Therefore, when impedance correction

networks are employed, some of the power gain and linearity

must be sacrificed.

The input correction network can be designed with RC

or RLC combinations to give better than 1 dB gain flatness

across the band with low input VSWR. In a low–voltage

system, little can be done about the output without reducing

the maximum available voltage swing.

At power levels up to 180 Watts and 13.6 V, the peak

currents approach 30 A, and every 100 mV lost in the emitter

grounding or collector dc feed also have a significant effect

in the peak power capability. Thus, these factors must be

emphasized in RF power circuit design.

THE BASIC CIRCUIT

Figure 1 shows the basic circuit of the linear amplifier.

For different power levels and devices, the impedance ratios

of T1 and T3 will be different and the values of R1, R2, R3,

R4, R5, C1, C2, C3, C4 and C6 will have to be changed.

The Bias Voltage Source

The bias voltage source uses active components

(MC1723G and Q3) rather than the clamping diode system

as seen in some designs. The advantages are line voltage

regulation capability, low stand-by current, (1.0 mA) and

wide range of voltage adjustability. With the component

values shown, the bias voltage is adjustable from 0.5 to

0.9 Volts, which is sufficient from Class B to Class A

operating conditions.

In Class B the bias voltage is equal to the transistor V

and there is no collector idling current present (except small

collector-emitter leakage, I

CES

), and the conduction angle

is 180°.

MOTOROLA

SEMICONDUCTOR

APPLICATION NOTE

Order this document

by AN762/D

 Motorola, Inc. 1993

AN762

1 2 3 4 5 6 7 8 9 10

Inhaltsverzeichnis

Seite 1 - SEMICONDUCTOR

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COM1RF Application ReportsLINEAR AMPLIFIERS FOR MOBILE OPERATIONPrepared by: Helge O. GranbergRF Circu

Seite 2 - 2 RF Application Reports

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN76210 RF Application ReportsMotorola reserves the right to make changes without further notice to

Seite 3 - RF Application Reports

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7622 RF Application Reports Ω Ω

Seite 4 - GENERAL DESIGN CONSIDERATIONS

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7623RF Application ReportsThe measured output voltage variations of the bias sourcefrom zero to 1

Seite 5

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7624 RF Application ReportsA. B.Figure 3. Two Variations of the Input and Output Transformers (T1

Seite 6 - 6 RF Application Reports

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7625RF Application ReportsTable 1. Parts List*100 W Amplifier 140 W Amplifier 180 W AmplifierC1 5

Seite 7 - PERFORMANCE AND MEASUREMENTS

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7626 RF Application ReportsLoops can be provided for current probe measurements.L3L4T3C6C6CCdacbE

Seite 8 - 8 RF Application Reports

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7627RF Application ReportsThis number can be used to select a suitable heat sinkfor the amplifier

Seite 9

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7628 RF Application Reports     

Seite 10 - 10 RF Application Reports

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7629RF Application ReportsThe PCB layout below is a supplement to Figure 4 and may be used for ge

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