AN1526 Freescale Semiconductor / Motorola, AN1526 Datasheet

AN1526

Manufacturer Part Number

AN1526

Description

RF Power Device Impedances: Practical Considerations

Manufacturer

Freescale Semiconductor / Motorola

Datasheet

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SEMICONDUCTOR APPLICATION NOTE

Prepared by: Alan Wood and Bob Davidson

ABSTRACT

output device impedances for RF power transistors is

explained, together with the techniques for measuring these

parameters. How these parameters change under varying

load and bias conditions is examined, and the impact of these

variations is demonstrated in a practical broadband test

fixture design.

INTRODUCTION

of small–signal s–parameters, previously used for solving

small signal text book problems, assume these same

techniques are applicable to bipolar class–C and class–AB

power amplifier design. They consider the best match is

achieved by a simultaneous conjugate match of the input

and output. However, power amplifiers provide higher power

gain and better efficiency at the rated output power if the

output is purposely mismatched. An added benefit of doing

this is potentially unstable devices, conjugately matched, can

be operated stably under these more optimum mismatched

conditions.

impedances, naively assume the published impedances are

independent of operating point. They forever wonder why,

although they have designed their impedance transformation

networks to match the device “data book impedances,” they

have to “tweak” the circuit for optimum performance. This

is the basis for much of the black magic that surrounds RF

power amplifier design, but the reality is the circuit designer

is plagued with a paucity of good design data, and a lack

of adequate tools to make the initial design “foolproof.” This

paper intends to enlighten these engineers to the true

meaning of large–signal series equivalent device

impedances. We will also show that the output impedance

is, for the most part, under the control of the circuit designer,

and the input device impedance can be expected to change

depending upon the designers choice of output matching (or,

in some cases, intentional mismatching).

DEFINITIONS

acceptance in low power linear amplifier design.

Unfortunately, progress in large–signal power amplifier

design has been less substantial. Techniques have been

published over the years, e.g., large–signal s–parameters;

load–pull; and stability analysis using small–signal

s–parameters, but they have not gained wide spread

acceptance for a number of reasons, including the degree

REV 0

MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION

Motorola, Inc. 1991

The definition of large–signal series equivalent input and

Many first time RF power designers, brought up on a diet

More knowledgeable designers, familiar with large–signal

Small–signal s–parameters have gained a great deal of

Motorola, Inc., Semiconductor Products Sector

Phoenix, Arizona 85008

Freescale Semiconductor, Inc.

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of applicability and the ease and accuracy of the

measurements. The universal starting point for RF power

amplifier design remains the published large–signal

impedances. These techniques are discussed briefly below,

outlining their advantages and disadvantages.

Small–Signal S–Parameters

classic s–parameter [1] characterization and design methods

for small–signal linear devices. Data is usually available at

multiple collector bias voltage and current conditions over

a wide range of frequencies. The ease of making these

measurements accurately with modern network analyzers

has done a great deal for systemizing small–signal RF

amplifier design. The availability of software for analyzing

and optimizing the performance of broadband amplifiers and

establishing their stable operation has further improved the

design methodology. However, when the designer is asked

to step into the high power RF design world, he is

immediately confronted with several possible device

characterization methods. First of all, let’s understand the

term “high power.” As used in this paper, we are talking about

RF power amplifier devices with output powers from roughly

one watt to several hundred watts. At these power levels,

the small–signal s–parameters lose their usefulness in

determining appropriate source and load reflection

coefficients, to say nothing of the familiar gain and stability

circles or non–unilateral issues. This is because high power

class–C RF amplifiers are VERY non–linear. The industry

standard s–parameters are valid only for devices operated

in small–signal linear conditions. These parameters have

very limited use in high power applications. One exception

is presented by Frost, [2] in using the “large–signal

s–parameters” as an aid in the stability analysis process.

Hejhall, [3] also demonstrates the use of small–signal

s–parameters for stability analysis in FET power amplifier

design and shows their utility when large–signal impedances

are unavailable.

Large–Signal S–Parameters

ease of measuring small–signal s–parameters has led to a

characterization technique referred to in the literature as

“large–signal s–parameters.” Successful measurement and

usage of these parameters has been reported [4]–[12].

However, the authors are not aware of these parameters

being used successfully above a few watts of output power.

Measurement of these parameters is usually accomplished

by driving the device from a 50 ohm source to achieve a

collector or drain current comparable to that expected in

actual operation.

Small–signal RF designers are very familiar with the

The availability of network analyzers and the subsequent

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AN1526 Summary of contents

Page 1

... However, the authors are not aware of these parameters being used successfully above a few watts of output power. Measurement of these parameters is usually accomplished by driving the device from a 50 ohm source to achieve a collector or drain current comparable to that expected in actual operation. Go to: www.freescale.com Order this document by AN1526/D 1 ...

Page 2

Freescale Semiconductor, Inc. Devices with output power ratings above a few watts have input reflection coefficient magnitudes very close to one, requiring drive levels far beyond the capability of a standard network analyzer to merely turn the device on, if ...

Page 3

Freescale Semiconductor, Inc. devices is a much stronger function of load impedance than shown for this small device. Device impedances published by vendors of RF power transistors should only be used as an approximation for a first cut circuit design. ...

Page 4

Freescale Semiconductor, Inc. Table 3. MRF873 Impedance Data Computed from S–Parameters (I I/P Simul. Freq. Input Z Conjugate. (MHz Match 11 806 0.95 + j3.15 0.48 – j3.19 838 1.00 + j3.31 0.50 – j3.41 870 1.07 + ...

Page 5

Freescale Semiconductor, Inc. specified on Motorola data sheets (or any other vendor’s data sheet known to the authors) is the impedances at the harmonic frequencies presented to the device. In most cases the first shunt capacitance combined with ...

Page 6

Freescale Semiconductor, Inc. 870 MHz, Class– 12.5 VDC 2.4 Watts CE in Figure 1. MRF873 Gain Surface vs Fundamental Load Impedance 870 MHz, Class–C, V Figure 3. MRF873 Return Loss Surface vs 6 For More Information ...

Page 7

Freescale Semiconductor, Inc. Load Impedance Chart Z0 = 3.0 Ohms 806 MHz, Class– 12.5 VDC Figure 4. MRF873 Constant Gain Contours Load Impedance Chart Z0 = 3.0 Ohms 806 MHz, Class– 12.5 VDC, ...

Page 8

Freescale Semiconductor, Inc. Load Impedance Chart Z0 = 3.0 Ohms 838 MHz, Class– 12.5 VDC Figure 8. MRF873 Constant Gain Contours Load Impedance Chart Z0 = 3.0 Ohms 838 MHz, Class– 12.5 VDC, ...

Page 9

Freescale Semiconductor, Inc. Load Impedance Chart Z0 = 3.0 Ohms 870 MHz, Class– 12.5 VDC 2.4 Watts CE in Figure 12. MRF873 Constant Gain Contours Load Impedance Chart Z0 = 3.0 Ohms 870 MHz, Class–C, V ...

Page 10

Freescale Semiconductor, Inc. Chart Z0 = 3.0 Ohms Figure 16. Load–Pull Impedances Presented to MRF873 at 870 MHz Figure 18. Photograph of CS–12 Impedance Measurement Probe Figure 20. Photograph of MRF873 Broadband 10 For More Information On This Product, Figure ...

Page 11

Freescale Semiconductor, Inc. C1, C15 — Tantalum C2, C14 — 1000 pF, 350 V, Unelco C3, C12 — 43 pF, 100 Mil, ATC Chip Capacitor C5, C13 — 91 pF, Mini–Unelco C4, C11 — 0.8 – ...

Page 12

Freescale Semiconductor, Inc. 12 For More Information On This Product, Figure 24. Load–Pull Test Setup V = 12.5 VDC Watts CE out Figure 25. Broadband Performance of MRF873 Production Test Fixture MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION Go to: ...

Page 13

Freescale Semiconductor, Inc. Figure 26. MRF873 Data Book Input and Output Impedances MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION For More Information On This Product 12.5 VDC Watts CE out Go to: www.freescale.com 13 ...

Page 14

Freescale Semiconductor, Inc. CONCLUSIONS For an RF power transistor we have demonstrated that the input and output large–signal device impedances are not only frequency dependent, but are also determined by the operating conditions of the device. Because of the wide ...

Page 15

Freescale Semiconductor, Inc. ( Leighton Chaffin, and J. G. Webb, “RF Amplifier Design with Large–Signal S–Parameters,” IEEE Transactions on Microwave Techniques, MTT–21, 809–814, December 1973. ( Mazumder, “Two–Signal Method of Measuring the Large–Signal S–Parameters ...

Page 16

... ASIA/PACIFIC: Motorola Semiconductors H.K. Ltd.; Silicon Harbour Centre, 2 Dai King Street, Tai Po Industrial Estate, Tai Po, N.T., Hong Kong. 852–26668334 Technical Information Center: 1–800–521–6274 HOME PAGE: http://www.motorola.com/semiconductors/ 16 For More Information On This Product, are registered trademarks of Motorola, Inc. Motorola, Inc Equal MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION Go to: www.freescale.com AN1526/D ...

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