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HIVE INF

RMATI

PRODUCT TRANSFERRED TO M/A–COM

1RF Application Reports

LOW-DISTORTION 1.6 TO 30 MHz SSB DRIVER DESIGNS

Prepared by: Helge O. Granberg

RF Circuits Engineering

GENERAL CONSIDERATION

Two of the most important factors to be considered in

broadband linear amplifier design are the distortion and the

output harmonic rejection.

The major cause for intermodulation distortion is

amplitude nonlinearity in the active element. The nonlinearity

generates harmonics, and the fundamental odd-order

products are defined as 2

f1–f2

, 2

f2–f1

, 3

– 2

, 3

– 2

, etc.,

when a two-tone test signal is used. These harmonics may

not always appear in the amplifier output due to filtering and

cancellation effects, but are generated within the active

device. The amplitude and harmonic distortion cannot really

be distinguished, except in a case of a cascaded system,

where even-order products in each stage can produce

odd-order products through mixing processes that fall in the

fundamental region.

This, combined with phase distortion

— which in practical circuits is more apparent at higher

frequencies — can make the distortion analysis extremely

difficult;

5,2

whereas, if only amplitude distortion was present,

the effect of IMD in each stage could easily be calculated.

In order to expect a low harmonic output of the power

amplifier, it is also important for the driving source to be

harmonic-free. This is difficult in a four-octave bandwidth

system, even at 10 – 20 watt power levels. Class A biasing

helps the situation, and Class A push-pull yields even better

results due to the automatic rejection of even harmonics.

Depending on the application, a full Class A system is

not always feasible because of its low efficiency. The

theoretical maximum is 50%, but practical figures are not

higher than 25% to 35%. It is sometimes advantageous to

select a bias point somewhere between Class AB and A

which would give sufficiently good results, since filtering is

required in the power amplifier output in most instances

anyway.

In order to withstand the high level of steady dc bias

current, Class A requires a much larger transistor die than

Class B or AB for a specific power output. There are

sophisticated methods such as generating the bias voltage

from rectified RF input power, making the dc bias proportional

to the drive level.

This also yields to a better efficiency.

20 W, 25 dB AMPLIFIER WITH LOW-COST

PLASTIC DEVICES

The amplifier described here provides a total power gain

of about 25 dB, and the construction technique allows the

use of inexpensive components throughout. The plastic RF

power transistors, MRF475 and MRF476, featured in this

amplifier, were initially developed for the CB market. The high

manufacturing volume of these TO-220 packaged parts

makes them ideal for applications up to 50 MHz, where low

cost is an important factor.

The MRF476 is specified as a 3-watt device and the

MRF475 has an output power of 12 watts. Both are extremely

tolerant to overdrive and load mismatches, even under CW

conditions. Typical IMD numbers are better than -35 dB, and

power gains are 18 dB and 12 dB, respectively, at 30 MHz.

The collectors of the transistors are electrically connected

to the TO-220 package mounting tab which must be isolated

from the ground with proper mounting hardware (TO-220 AB)

or by floating heat dissipators. The latter method, employing

Thermalloy 6107 and 6106 heat dissipators, was adapted

for this design. Without an airflow, the 6106 and 6107 provide

sufficient heat sinking for about 30% duty cycle in the CW

mode. Collector idle currents of 20 mA are recommended

for both devices, but they were increased to 100 mA for the

MRF475 and to 40 mA for the MRF476 to reduce the higher

order IMD products and to achieve better harmonic

suppression.

Figure 1.

Biasing and Feedback

The biasing is achieved with the well-known clamping

diode arrangement (Figure 2). Each stage has it own diode,

resistor, and bypass network, and the diodes are mounted

between the heat dissipators, being in physical contact with

them for temperature-tracking purposes. A better thermal

contact is achieved through the use of silicone grease in

these junctions.

The bias currents of each stage are individually adjustable

with R5 and R6. Capacitors C4 and C10 function as

audio-frequency bypasses to further reduce the source

impedance at the frequencies of modulation.

MOTOROLA

SEMICONDUCTOR

APPLICATION NOTE

Order this document

by AN779/D

 Motorola, Inc. 1993

AN779

1 2 3 4 5 6 7 8 9 10

Inhaltsverzeichnis

Seite 1 - SEMICONDUCTOR

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COM1RF Application ReportsLOW-DISTORTION 1.6 TO 30 MHz SSB DRIVER DESIGNSPrepared by: Helge O. Granber

Seite 2 - 2 RF Application Reports

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN77910 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–COMAN7792 RF Application ReportsR1, R4 — 10 Ohms, 1/4 WR2, R3 — 30 Ohms, 1/4 WR5, R6 — 82 Ohms, 3 W (N

Seite 4 - 4 RF Application Reports

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7793RF Application ReportsFigure 3. Examples of broadband transformers.Variations of these are us

Seite 5

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7794 RF Application ReportsXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX= Feedthrough Eyelets.= Terminal PinsR9Q

Seite 6 - 6 RF Application Reports

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7795RF Application Reports         η

Seite 7

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7796 RF Application ReportsThe output matching is done with a transformer similarto that describe

Seite 8 - REFERENCES

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7797RF Application ReportsThe amount of negative feedback is difficult to determine,as it depends

Seite 9

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7798 RF Application ReportsMeasurements and Performance DataThe output harmonic contents of this

Seite 10 - 10 RF Application Reports

ARCHIVE INFORMATION PRODUCT TRANSFERRED TO M/A–COMAN7799RF Application ReportsThe PCB layouts below are a supplement to Figures 5 and 9 and may be us

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