Linearization Of RF Power Amplifiers Research Articles

On design of linear RF power amplifier for CDMA signals

In this paper, we analyze the nonlinear effect of an RF power amplifier on CDMA signals and then derive the direct relation between the power transistor's traditional nonlinearity parameter, the third order intermodulation coefficient (IP3), and an estimate of the out-of-band emission levels for CDMA signals which is used in IS-95 [1] for the control of the amplifier's nonlinearity. This result will be useful to system and component engineers in design of RF power amplifiers for CDMA wireless systems. © 1998 John Wiley & Sons, Inc. Int J RF and Microwave CAE 8: 283–292, 1998

An IC for linearizing RF power amplifiers using envelope elimination and restoration

This paper presents a monolithic CMOS implementation of an envelope elimination and restoration linearization system that improves the linearity of efficient radio-frequency power amplifiers. The linearization IC, which occupies 4.3 mm/sup 2/ when implemented in a 0.8-/spl mu/m CMOS technology, consists of a limiter, envelope detectors, and delta-modulated switching power supply. This circuit was used to linearize advanced mobile phone system cellular power amplifiers transmitting North American digital cellular (NADC) waveforms. Measurements show that the linearized outputs meet the spectral mask and phase distortion requirements of NADC. The linearization system can improve the overall efficiency from 36 to 49%, while increasing the maximum linear output power from 26.5 to 29.5 dBm.

RF power amplifier linearization through amplitude and phase predistortion

This paper presents a novel technique for power amplifier linearization in digital microwave radio systems. The proposed technique is based on the use of a predistortion circuit, whose AM/AM and AM/PM responses are separately implemented as polynomial approximations of the respective responses of the ideal linearizer. The proposed scheme is shown to attain superior performance in comparison with other well-known predistortion structures, such as those based on the cancellation of third or fifth order distortion, with no substantial aggravation in implementation complexity.

Intermodulation distortion in Kahn-technique transmitters

The Kahn Envelope Elimination and Restoration (EER) technique implements a linear RF power amplifier (PA) by combining nonlinear, but efficient, RF and AF power amplifiers. For signals with high peak-to-average ratios, the average efficiency of a Kahn-technique transmitter can be three to four times that of a transmitter that employs conventional linear RP PAs. Since switching-mode amplifiers are employed, the linearity of an EER transmitter depends upon parameters such as the bandwidth of the envelope modulator and the differential delay between envelope and phase signals. This paper determines the relationship between these parameters and IMD levels and verifies the predictions by laboratory measurements. The results can be used to determine the requirements for the components of a Kahn-technique transmitter.

Adaptive digital linearization of RF power amplifiers

Adaptive digital linearization of RF power amplifiers

Characterizing the effects of nonlinear amplifiers on linear modulation for digital portable radio communications

The design and performance of linear modulation techniques and RF power amplifiers are discussed. Various types of power amplifier are used in the comparison, using simulation, of out-of-band radiation and power efficiency. The designs focus on high-frequency silicon bipolar power amplifiers suitable for low-power portable radio applications. Two different 4-QAM (4-level quadrature amplitude modulation) schemes are compared. Power efficiency as high as 40% can be achieved using a dynamically biased amplifier operating near saturation. The effects of nonlinear distortions from power amplifiers on adjacent channel interference as well as bit and block error rate performance are also investigated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Efficiency of Doherty RF Power-Amplifier Systems

A Doherty system combines the outputs of two or more linear RF power amplifiers (PAs) through an impedance-inverting coupler such as a quarter-wave transmission line. At low output levels, the first PA operates linearly, reaching saturation (and maximum efficiency) at some transition voltage below the system peak-output voltage. At higher output levels, the first PA remains saturated and the second PA operates linearly. The instantaneous efficiency and power characteristics of a Doherty system are derived using ideal class-B RF PAs so that the results can easily be scaled for use with real-world PAs. The average efficiency and maximum-efficiency transition points are then determined for a variety of amplitude-modulated signals. The Doherty amplifier can be considerably more efficient than a conventional class-B linear PA. For example, the 28-and 8.9-percent average efficiencies of a class-B PA with Rayleigh-distri buted envelopes with 10-and 20-dB peak-to-average ratios are improved to 60 and 48 percent, respectively, by a two-stage Doherty system. The addition of a third stage further improves the efficiencies to 70 and 66 percent, respectively.

Efficiency of Outphasing RF Power-Amplifier Systems

The outphasing technique (LINC) combines two nonlinear RF power amplifiers into a linear RF power-amplifier (PA) system. The two PA's are driven with signals of different phases, and the phases are controlled so that the addition of the PA outputs produces a system output of the desired amplitude. However, the resultant time-varying impedances presented to the PA's alter their dc power consumption and efficiency. Power and efficiency characteristics are derived for both simple (transformer-coupler) and Chireix (transmission-line-coupler with shunt reactance) outphasing systems using saturated class-B PA's. Simple outphasing systems have the efficiency characteristic of a linear class-B PA. Through proper selection of the shunt reactance, the efficiency of a Chireix outphasing system can be maximized at a specific output amplitude. The average efficiency with various amplitude-modulated signals is determined as a function of shunt reactance. Selecting the shunt reactance to fit the signal can improve efficiency by as much as a factor of 2.

Detection techniques for nondestructive second-breakdown testing

The development of nondestructive second-breakdown measuring techniques and equipment is described. Such apparatus is essential to the analysis of the "safe area of operation" of transistors for device development and particular circuit application. The present techniques for detecting second breakdown in both the forward-biased and the reverse-biased mode are discussed. When a transistor is operated in the forward-biased mode, hot spots may develop which produce second breakdown if temperature extremes are reached before this condition is recognized. With existing detection methods, the device may already have been damaged before this condition is recognized. Techniques for electronic detection of hot spots before such degradation occurs are illustrated and compared to phosphorus ultraviolet hot spot measurements. When these new detection techniques are used, transistors can be characterized for forward-biased "safe area of operation" for applications such as audio output stages, series regulators, and class AB linear RF power amplifiers. Another form of second breakdown occurs under reverse-biased conditions, when the transistor is in primary breakdown. Because of the small areas involved, second breakdown occurs much more rapidly in this mode than under forward-biased conditions. An accompanying oscillation in the control electrode has proved to be a reliable detection mechanism. This work was performed primarily on multiple-diffused power transistors, in which reverse-biased second breakdown is a major concern. Because a "safe area of operation" in the reverse-biased mode can be established by this test, reliable circuits can be designed for such applications as high-speed inverters, solenoid drivers, servo controls, and UHF class C power amplifiers. Life-test results have proven this test procedure to be a dependable and consistent method for establishing a "safe area of operation" for transistors. Illustrations of the new RCA second-breakdown equipment based on the research described are presented.

Distortion Reducing Means for Single-Sideband Transmitters

The need for reduced levels of intermodulation distortion products from linear rf power amplifiers is increasing as the state of the art advances. Two basic means of reducing distortion to levels better than is obtainable from available tubes are shown. One is direct rf feedback and the other is envelope distortion cancelling modulation. Some of the problems and limitations as well as advantages and performance characteristics are presented to aid the reader in evaluating these methods. RF feedback is a superior method, although useful amounts of distortion reduction can be achieved using envelope distortion cancelling modulation. A circuit of an amplifier combining both methods is shown which produces better performance than using either alone.