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Abstract
In order to properly configure a radio transceiver, designers must understand the performance parameters and tradeoffs of the RF amplifiers being used. A multitude of system requirements must be considered to choose an optimal amplifier from the many devices available by suppliers in the marketplace. Amplifier choices have greatly increased for RF designers since early days where only GaAsFET and Silicon Bipolar technologies were available. Performance tradeoffs of GaAs MESFET, InGaP HBT, and GaAs HFET technologies targeted at wireless and wireline basestation driver amplifier applications are discussed. Measured ACPR and PAE performance is shown for 1/2-, 1-, and 2-Watt amplifiers utilizing the various process technologies.

Introduction
In a standard base-station transmitter application, the various types of RF amplifiers used in the chain can be broken up into three categories: small-signal buffer amplifier stages, driver amplifier stages, and power amplifier stages (Figure 1). Although the main focus of this paper is with driver amplifier stage applications, key parameters are fundamentally similar for RF designers looking to choose a device to amplify a signal.

Figure 1. Simplified Base Station Transmitter Block Diagram

Obviously, the small-signal RF performance such as gain and return loss is what most engineers will look for with an RF amplifier -- especially for the small-signal amplifier stages. Then depending on where the amplifier is used in the chain and whether the amplifier is used in the Rx/Tx path, device linearity, compression, and noise figure would be the key parameters. For digital radio applications, linearity can be described as spectral regrowth of the modulated (as ACPR, ACLR, or EVM) signal rather than the traditional figure of merit with third order intercept point (OIP3) which is ideally independent of output signal level. Because these characterizations are dependent upon the signal output power level, graphs with respect to output power are often shown on suppliers' datasheets rather than a direct numerical value.

Other RF performance parameters that designers often consider are the device's operational bandwidth, gain flatness across the frequency of operation, RF stability, the amplifier's performance over temperature, and depending on the application, power added efficiency (PAE). All other things being equal, subtle considerations designers often make are with the reliability of device (also known as MTTF) and the ease of designing or matching with the device.

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