|RF power amplifier|
|Wednesday, 13 September 2006|
Amplifiers produce from an input signal, an output signal having an increased magnitude (i.e., gain). An amplifier produces a constant output power at a higher level. Different amplifiers are known and produce various types of gains (e.g., voltage and/or current gain). Power amplifiers are classified into different groups: class A, class B, class AB, class C, class E, class F, and class D (sometimes referred to as digital amplifiers). The different classes of power amplifiers usually signify different biasing conditions. Each of these types of amplifiers, however, have significant problems when operating in the RF range. For example, class A and class AB amplifiers have very poor efficiency but reasonable linearity. Class C amplifiers are reasonably efficient but are only practical for phase modulation. Similarly Class E, F, and D amplifiers are typically only useful for phase modulation applications. Class E amplifiers have improved power efficiency when compared to C type amplifiers, but large voltage swings at their output limit their usefulness. Class F amplifiers exhibit relatively efficient switching characteristics with a repeating input signal. Power amplifiers are also divided into two different categories, linear and non-linear. Linear amplifiers (e.g. class A amplifiers and class B push-pull amplifiers), maintain high linearity, resulting in faithful reproduction of the input signal at their output since the output signal is linearly proportional to the input signal. In non-linear amplifiers (e.g. single-ended class B and class C amplifiers), the output signal is not directly proportional to the input signal. The resulting amplitude distortion on the output signal makes these amplifiers most applicable to signals without any amplitude modulation, which are also known as constant-envelope signals.
Modern wireless communication base stations transmit and receive radio frequency signals through the use of RF power amplifiers. Radio frequency signal is an electrical signal conveying useful information and having a frequency from about 3 kilohertz (kHz) to thousands of gigahertz (GHz), regardless of the medium through which such signal is conveyed. An RF signal may be transmitted through air, free space, coaxial cable, fiber optic cable, etc. An RF transmitter mixes the desired signal, known as the baseband signal, with an RF carrier frequency for transmission over the selected medium. An RF receiver then mixes the signal with the carrier frequency to restore the signal to its original frequency. RF transmission typically occurs at a single band for specific applications such as cellular phone transmissions. Typical cellular phone transmission bands include 800 MHZ and 1900 MHZ in the United States, and 900 MHZ and 1800 MHZ in most countries in Europe and Asia. Radio frequency devices transmit an information signal from one point to another by moving the information signal to a higher frequency range that is more suitable for transmission over the medium being used. This process is known as upconversion. In operation, a power amplifier circuit receives an RF signal in the transmit path of the communication device, amplifies the RF signal, and provides the amplified signal to an antenna. To meet system requirements, the RF antenna power output must be maintained substantially constant.
An amplifier is typically divided into an integrated circuit chip having the transistors and other circuitry associated with the amplifier and a number of off-chip components that filter the signal or provide impedance matching at a particular operating frequency. RF amplifier assemblies contain plural printed circuit boards on which components that process the RF signals are mounted. RF integrated circuits often include a voltage controlled oscillator on-chip. The oscillator generates a local oscillator signal, which is buffered by an RF buffer amplifier and applied to a polyphase filter. Quadrature output signals are provided to receive and transmit mixers. The polyphase filters have a loss of 6 dB from input to output and have a minimum phase error when they receive a sinusoidal input. The amplifier chip is interconnected in a circuit with certain off-chip components such as inductors, capacitors, resistors, and transmission lines used for controlling operation of the amplifier chip and providing impedance matching of the input and output RF signals. In a typical printed circuit board, the chip and associated components are all mounted on one side of the board with the opposite, substrate-side of the board being exposed. A typical power amplifier package employs paralleled LDMOS transistors formed on one or more dies, which is attached to a package substrate serving as both a heat sink and common ground element. Input (gate) and output (drain) package terminals, which are electrically isolated from the package, are connected to respective input and output terminals on the one or more die by many bond wires.
An RF power amplifier is required to have linearity over the range of power operation and efficiency. Linearity is the ability to amplify without distortion while efficiency is the ability to convert DC to RF energy with minimal wasted power and heat generation. Both these requirements are critical for modern wireless communication systems but mutually exclusive in nature. Amplifiers are rated at a maximum power output for use in different applications. The most efficient output for an amplifier typically occurs when operating at the highest rated output. Linear amplification is of particular importance in communication devices that transmit amplified signals having information encoded in the amplitude and phase of the signal. Signals transmitted from a source to a destination are often modulated and amplified before they are transmitted. Several existing and prospective wireless digital communication systems are based on modulation schemes with both varying amplitude and phase are often referred to as linear modulation schemes. Generally the linearity of radio frequency power amplifiers is important in preventing spectral spreading, particularly of spectrally efficient forms of digital modulation which do not use constant envelope techniques. However, amplifiers are often operated at an average power much lower than the highest rated output in order to achieve a linear output. Therefore, there is a tradeoff between efficiency and linearity. Various linearization methods are used to enable the use of more cost-effective and more power efficient amplifiers while maintaining an acceptable level of linearity. Feed-forward correction is routinely deployed in modern amplifiers to improve the linearity of the main amplifier with various input patterns.
Amplifier output efficiency is defined as the ratio between the RF output power and the input (DC) power. A major source of power amplifier inefficiency is power dissipated in the transistor. PA efficiency is a vital parameter in portable radio equipment as it directly affects talk time. Unfortunately, efficiency of the base station amplifier is inversely related to its linearity. Compared to modulation schemes having only phase or frequency modulation, linear modulation schemes provide higher spectral efficiency for a given throughput. The linear power amplifier is driven by a direct current (DC) input voltage, provided for example by a battery in the transmitter, and the efficiency of the power amplifier is given by the ratio of the output power to the DC input power. RF power amplifiers are generally designed to provide maximum efficiency at the maximal output power. When the power amplifier produces an output power that is less than the maximal output power, the efficiency of the power amplifier may be significantly reduced. Since the amplitude of an RF output signal of the power amplifier varies greatly with communication distance, efficiency improvement can be considered in two ways: efficiency improvement at a maximum output signal and efficiency improvement at a low output signal. To achieve a high degree of linearity, the amplifiers are biased to operate in the class A or slight class AB. Maximum AC to DC efficiency achievable for class A operation is 50%, whereas that of a class AB amplifier is between 50 and 78.5%. The closer the particular class AB operation is to class A, the lower the maximum efficiency.
Amplifiers are used in a variety of applications requiring small signal amplification. Low distortion in amplifier output is of particular importance in applications requiring linear processing or reproduction of signals containing information in the amplitude and phase of a signal. Amplifier output should exhibit low distortion in amplitude and phase to maintain integrity of this information upon amplification. The linearization of RF power amplifiers is inherently difficult to achieve as RF power amplifiers use a large number of non-linear devices which become more and more nonlinear at increasingly higher output power levels. In practice, high power RF devices will generate substantial unwanted intermodulation distortion (IMD) products which appear as spurious signals at the output of the RF power amplifier. Wideband radio frequency (RF) power amplifiers must commonly amplify signals on multiple, adjacent carriers with high gain and linearity. Intermodulation distortion (IMD) is a well-known problem in such amplifiers, stemming from third-order (and higher-order) interaction between different carriers. Feed-forward correction is used to isolate the distortion generated by the main amplifier on a feed forward path. The distortion is provided to a correction amplifier on the feed forward path which amplifies the distortion. The distortion on the feed forward path is combined with the distortion on the main signal path to cancel the distortion on the main signal path. Predistortion techniques are commonly used to improve the performance of RF power amplifiers. Predistortion techniques distort the input signal prior to amplification by taking into account the transfer function characteristics for the amplifier. The desired amplified signal is achieved from the predistorted input signal by intentionally distorting the signal before the amplifier, so the non-linearity of the amplifier can be compensated.
The increase in linearity of the RF power amplifier causes a decrease in power rate, thereby causing a reduced battery run-time problem and a heat generation problem in the mobile phone. In a mobile phone, an RF power amplifier consumes much more power than other elements. Signal amplification requires corresponding power that generates heat in the amplifier which must be suitably dissipated for protecting the amplifier and associated electronic components. Therefore, the RF power amplifier chiefly causes a reduction in battery run-time and it generates heat which must be controlled to avoid degrading amplifier performance and reliability. Hence, the use of elaborate heat sinks and fans become a necessary by-product of the high linearity system. However, these measures add to the cost, size and weight of the base station equipment. RF power amplifiers employed in modern wireless communication systems must be efficient to avoid unnecessary power dissipation and heat. This requirement is increasing in importance driven by shrinking volumes of deployment facilities (e.g. cellular base stations), as well as reduction in heat exchanger size (or smaller air conditioning units), and reduction in operating noise levels due to cooling fans, as well as other factors. In cell phones, the PCB board of amplifier s mounted in a corresponding housing of the cellular phone in proximity to other electronic circuits therein, suitable accommodations are required for dissipating the heat and protecting the various electronic components thereof.
Wireless devices typically transmit and receive data through the air on high frequency electromagnetic waveforms, though some systems, such as satellite dishes and pagers simply receive, and others merely transmit. Since RF circuits operate at high signal frequencies, electromagnetic radiation is created which can interfere with other components of the cellular phone, or with other electronic devices within the transmission range of the phone. A cellular phone requires suitable shielding against electromagnetic interference (EMI). Various wireless communication devices such as cellular telephones, radios, and wireless modems require RF amplifiers to amplify the signal received from an antenna or the like. The amplifier must also provide an impedance match to the antenna.