Fundamentals of RF and Microwave Transistor Amplifiers


By Inder J. Bahl

John Wiley & Sons

Copyright © 2009 John Wiley & Sons, Inc.
All right reserved.

ISBN: 978-0-470-39166-2


Chapter One

Introduction

Among electronic circuits, signal amplification is one of the most important radiofrequency (RF) and microwave circuit functions. The introduction of radar during World War II provided the first significant application requiring amplification of microwave signals. In recent times, the wireless communication revolution has provided an explosion of RF and microwave amplification applications. During the last two decades, amplifier technology has made tremendous progress in terms of devices (low noise and power), circuit computer-aided design (CAD) tools, fabrication, packaging, and applications. Low-cost power amplifiers for wireless applications are a testament to this explosion.

Early microwave amplifiers were the exclusive province of vacuum tube devices such as Klystrons, traveling-wave tube (TWT) amplifiers, and magnetrons. Today, microwave amplification is dominated by solid state amplifiers except for applications at high output powers ([??] 100 watts). Today, the most common vacuum tube application is the 900-watt microwave oven using a 2.45-GHz magnetron. The power levels achievable for tube amplifiers are on the order of [10.sup.3] higher than achievable for solid state amplifiers. The microwave oven magnetron, with a manufacturing cost of about $10 (~0.01/watt), has no solid state competition in sight. Likewise, today's $0.50/watt 900-MHz to 2-GHz cell phone solid state transistor amplifier and $0.30/watt 200-500-W L/S-band base station transistor power amplifiers have no tube competition.

Solid state amplifiers are of two general classes: those based on two-terminal negative resistance diode devices, and those based on three-terminal devices known as transistors. Early solid state amplifiers were dominated by two-terminal devices because diodes are typically much easier to fabricate than transistors. Quite an array of two-terminal amplifier designs have been introduced, including parametric amplification (varactor diodes), tunneling diodes, transferred electron diodes (Gunn and LSA diodes), and avalanche transit-time diodes (IMPATT, TRAPATT, and BARITT). Such diodes are used only for special amplifier functions.

1.1 TRANSISTOR AMPLIFIER

Today, solid state amplification is dominated by use of three-terminal transistors. Using a small voltage applied at the input terminal of the device, one can control, in an efficient manner, a large current at the output terminal when the common terminal is grounded. This is the source for the name transistor, which is a unification of the words transfer resistor.

Solid state transistors may be grouped into two categories: bipolar and unipolar devices. The bipolar devices are comprised of silicon (Si) bipolar junction transistors (BJTs) and silicon germanium (SiGe) and gallium arsenide (GaAs) heterojunction bipolar transistors (HBTs). The unipolar devices include Si metal oxide semiconductor field effect transistors (MOSFETs), GaAs metal semiconductor field effect transistors (MESFETs), and pseudomorphic high electron mobility transistors (pHEMTs). The switchover to three-terminal devices was largely due to cost. Diodes are typically less expensive to manufacture than transistors but the associated circuitry to achieve gain from a two-terminal device is much more expensive than that for a three-terminal device. For example, a transistor (without any matching network) connected between 50-ohm input and output terminals can provide 15-20 dB gain at radiofrequencies and 6-8 dB at 20 GHz. In addition, design of three-terminal amplifiers for stable operation and routine high-yield manufacturing is exceedingly simple.

Signal amplification is a fundamental function in all RF and microwave systems. When the strength of a weak signal is increased by a device using a direct current (DC) power supply, the device along with its matching and biasing circuitry is known as an amplifier. Here the DC power from the power supply is converted into RF power to enhance the incoming signal strength. If a device is a transistor, the signal is applied to the input terminal (gate/base) and the amplified signal appears at the output (drain/collector) and the common terminal (source/emitter) is usually grounded. The matching networks help in exciting the device and collecting the output signal more efficiently. Figure 1.1 shows a schematic representation of a single-stage transistor amplifier. Basic constituents are a transistor, input and output matching networks, bias circuitry, and input and output RF connections. The DC bias and RF connections may be made to connectors if housed in a fixture or to lead frame if assembled in a package depending on the amplifier fabrication scheme.

There are various types of amplifiers used at RF and microwave frequencies. Basic types consist of low-noise, buffer, variable gain, linear power, saturated high-power, high-efficiency, narrowband, and broadband amplifiers. The design of amplifiers requires essentially device models/S-parameters, CAD tools, matching and biasing networks, and fabrication technology. Each type mandates additional insights to meet required amplifier specifications. For example, a low-noise amplifier (LNA) needs a low-noise device and a low-loss input matching network while a power amplifier (PA) requires a power device and low-loss output matching network.

RF and microwave amplifiers have the following characteristics: Band-limited RF response Less than 100% DC to RF conversion efficiency Nonlinearity that generates mixing products between multiple signals RF coupled and no DC response Power-dependent amplitude and phase difference between the output and input Temperature-dependent gain, higher gain at lower temperatures and vice versa

1.2 EARLY HISTORY OF TRANSISTOR AMPLIFIERS

The use of Si based bipolar transistors and GaAs based MESFET for amplifiers have been reported since the mid-1960s and early 1970s, respectively. Most of the initial work on Si based bipolar transistor amplifiers was below C-band frequencies, whereas GaAs based MESFET amplifiers were designed above L-band frequencies (see Appendix C for frequency band designations). Low-noise HEMTs were reported in the early 1980s. Internally matched narrowband MESFET power amplifiers working from S- through X-band were available during the 1980s and Ku-band amplifiers were introduced in the early 1990s.

The GaAs monolithic microwave integrated circuit (MMIC) amplifier was reported in 1976 and since then there has been tremendous progress in both LNAs and PAs. Some of the early development milestones in MMIC amplifiers are as follows:

X-band low-power GaAs MESFET amplifier in 1976 X-band GaAs MESFET power amplifier in 1979 K-band GaAs MESFET LNA in 1979 Q-band GaAs MESFET power amplifier in 1986 V-band GaAs HEMT LNA in 1988 X-band GaAs HEMT power amplifier in 1989 W-band HEMT LNA/power amplifier in 1992

1.3 BENEFITS OF TRANSISTOR AMPLIFIERS

Major benefits of transistor amplifiers versus tube amplifiers are smaller size, lighter weight, higher reliability, high level of integration capability, high-volume and high-yield production capability, greater design flexibility, lower supply voltages, reduced maintenance, and unlimited application diversity. Transistors have much longer operating life (on the order of millions of hours) and require much lower warming time. Solid state amplifiers also do not require adjustment in the bias or the circuit, as required in tubes, over long periods of operation.

In comparison to solid state diode amplifiers, transistor amplifiers have greater flexibility in terms of designing matching networks, realizing high-stability circuits, and cascading amplifier stages in series for high gain. The outstanding progress made in monolithic amplifiers is attributed to three-terminal transistors, especially on GaAs substrates. Monolithic amplifiers are fabricated on wafers in batches, and hundreds or thousands can be manufactured at the same time. For example, over 15,000 amplifiers, each having a chip size of 1 [mm.sup.2], can be obtained on a single 6-inch diameter GaAs wafer. Thus monolithic amplifiers have a great advantage in terms of the manufacturing cost per unit. In general, monolithic amplifiers will have advantage in terms of size and weight over hybrid integrated techniques. It is worth mentioning that the weight of an individual or discrete chip resistor or a chip capacitor or an inductor is typically more than an entire monolithic amplifier chip. Many of today's high-volume applications using amplifiers are in hand-held gadgets. Both hybrid and monolithic MIC technologies are used and considered reliable. However, a well-qualified MMIC process can be more reliable because of the much lower part counts and far fewer wire bonds.

1.4 TRANSISTORS

During the past two decades outstanding progress has been made in microwave and millimeter-wave transistors. The low-noise and power performance as well as the operating voltages have significantly been advanced. Among low-noise devices, the pHEMT is the most popular due to its low noise figure and high gain characteristics. Other devices for small-signal applications are MESFETs, MOSFETs, and SiGe HBTs. Today, a designer has several different types of power transistors available as discrete devices (in chip or packaged form) or as part of a foundry service to design power amplifier MMICs. Several solid state devices are being used to develop power amplifier (PA) circuits including BJTs, laterally diffused metal oxide semiconductor (LDMOS) transistors, MESFETs, or simply FETs, both GaAs and indium phosphide (InP) based HEMTs, GaAs based HBTs and silicon carbide (SiC) based FETs, and gallium nitride (GaN) HEMTs. Each device technology has its own merits, and an optimum technology choice for a particular application depends not only on technical issues but also on economic issues such as cost, power supply requirements, time to develop a product, time to market a product, and existing or new markets.

HEMTs have the highest frequency of operation, lowest noise figure, and high power and PAE capability. Due to the semi-insulating property of GaAs substrates, the matching networks and passive components fabricated on GaAs have lower loss than on Si. The GaAs FET as a single discrete transistor has been widely used in hybrid microwave integrated circuit (MIC) amplifiers for broadband, medium-power, high-power, and high-efficiency applications. This wide utilization of GaAs FETs can be attributed to their high frequency of operation and versatility. However, increasing emphasis is being placed on new devices for better performance and higher frequency operation. HEMT and HBT devices offer potential advantages in microwave and millimeter-wave IC applications, arising from the use of heterojunctions to improve charge transport properties (as in HEMTs) or pn-junction injection characteristics (as in HBTs). HEMTs have a performance edge in ultra low-noise, high-linearity, and high-frequency applications. The MMICs produced using novel structures such as pseudomorphic and lattice matched HEMTs have significantly improved power and power added efficiency (PAE) performance and high-frequency (up to 280 GHz) operation. The pHEMTs that utilize multiple epitaxial III-V compound layers have shown excellent millimeter-wave power performance from Ku- through W-bands. HBTs are vertically oriented heterostructure devices and are very popular as low-cost power devices when operated using a single power supply. They offer better linearity and lower phase noise than FETs and HEMTs.

On the other hand, bipolar transistors require only a single power supply, have low leakage, low l/f noise or phase noise, and are produced much cheaper on Si. The SiGe HBTs have the low-cost potential of Si BJTs and electrical performance similar to GaAs HBTs. Thus discrete silicon BJTs, SiGe HBTs, and MOSFETs have an edge over GaAs FETs, HEMTs, and HBTs in terms of cost at low microwave frequencies. For highly integrated RF front ends, GaAs FETs and HEMTs are superior to bipolar transistors and Si substrate based devices due to high performance multifunction devices and lower capacitive loss, respectively. The electrical performance and cost trade-offs between Si and GaAs generally favor silicon devices due to single power supply operation and lower cost, whereas GaAs based devices are preferred due to superior low-noise and power (high breakdown voltage) performance and high-frequency operation.

Many of these transistors are available as discrete devices as well as a foundry to design monolithic amplifiers. Discrete transistors are available in die form and in plastic and/or ceramic packages. The ceramic package devices are for high-frequency and high-power applications. Plastic packaged transistors are for low-cost and high-volume applications.

1.5 DESIGN OF AMPLIFIERS

The design of RF and microwave amplifiers has several facets impacting their performance. The most important factors include the selection of semiconductor technology, device models, circuit architecture and design methodology, matching networks, packaging, and thermal management. Thus amplifier design becomes an art, to meet several often conflicting requirements, and an experienced designer will outperform beginners.

The design of an amplifier for a particular application and frequency range is quite complicated in the sense that it has to meet physical, electrical, thermal, and cost requirements. Salient features of an amplifier design are given in Figure 1.2. The amplifier performance requirements in terms of frequency band, gain, noise figure, power output, PAE, linearity, and input and output VSWR are determined by the device sizes, the circuit design topology, matching networks, the number of gain stages, the aspect ratio for the devices between the stages, design methodology, fabrication technology, and packaging. More often it involves trade-offs in terms of size, electrical performance, reliability, and cost. The amplifier designs are normally performed using device S-parameters, linear and nonlinear models, and matching component models.

Design of amplifiers, in a broad sense, falls into two categories: low noise and power. In a low-noise amplifier, the transistor's input is matched for optimum noise figure and the transistor's output is conjugately matched to 50-[OMEGA] system impedance for maximum gain and return loss (RL). In a power amplifier, the 50-[OMEGA] system impedance is matched to a required load at the transistor's output for maximum power and the transistor's input is conjugately matched for maximum gain and RL. In linear amplifiers the input and output are matched for better linearity. Thus, in an amplifier, the device's input is either matched for minimum noise or maximum gain or linearity and output is matched for maximum gain or optimum power and PAE or linearity. The matching networks are comprised of distributed and lumped elements. In an amplifier, the supply voltage (drain or collector) is applied through an RF choke or through the biasing circuit, which is usually an integral part of the matching network.

A low-noise or small-signal amplifier is designed using a device's noise model or noise parameters and S-parameters. The amplifier design must be conditionally stable. Narrowband power amplifier design can be carried out using a device's source-pull and load-pull data to design the amplifier's input and output matching circuits. This technique provides approximate electrical performance such as gain, power, and power added efficiency calculated by using measured small-signal S-parameters of the active device. However, for broadband applications, the aforementioned technique is very involved. Accurate nonlinear models for active devices provide a more suitable technique by using nonlinear computer-aided design (CAD) tools. These models help in determining matching networks over the desired bandwidth of the amplifier and also assist in simulating large-signal performance such as gain, VSWR, [P.sub.1dB], PAE, [P.sub.sat], TOI or ACPR or EVM, and harmonic levels. All these terms are defined in Chapter 3. These models also provide accurate solutions for multistage power amplifiers. The amplifier design must be conditionally stable, and also odd-mode, parametric, and low-frequency oscillation conditions must be prevented. However, in power amplifiers, unconditional stability is generally desired.

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