AN3589| Application Note
Power Amplifier Theory for High-Efficiency Low-Cost ISM-Band Transmitters
AUTOMOTIVE BASESTATIONS / WIRELESS INFRASTRUCTURE WIRELESS, RF, AND CABLE
Jul 20, 2005
APPLICATION NOTE 3589
Power Amplifier Theory for High-Efficiency Low-Cost ISMBand Transmitters
Low-cost ASK and/or FSK transmitter and transceiver ICs are key components in short-range radio systems designed to work in unlicensed industrial, scientific, and medical (ISM) bands between 300MHz and 450MHz. Applications for these short-range devices (SRDs) include remote keyless entry (RKE), tire-pressure monitoring (TPM), remote-control and security systems. System design, which addresses RF link budgeting, antenna design, battery life, and regulatory concerns, and other issues, often involves a tradeoff between the output power and the current drain of the transmitter. The power amplifier (PA) on Maxim's low-cost transmitters and transceivers (such as the MAX1472, MAX7044, MAX1479, and MAX7030/ MAX7031/MAX7032) possess a unique feature that allows the user to control the RF power/current-drain tradeoff while maintaining high efficiency. Managing that tradeoff is key to maximizing battery life for a particular application. The Maxim ICs require no modifications to handle this tradeoff; simply changing th
e load impedance presented to the PA can change the PA's output power and current drain. This application note includes a brief overview of power amplifier theory and presents simulation results for insight into the operation of the PA on all Maxim LFRF transmitters and transceivers.
Power Amplifier Overview
Class A, B, and C Amplifiers A class A amplifier is characterized by a combination of bias point and signal level where the device's average current drain does not change with the magnitude of the input signal. In Figure 1, M1 can be assumed to be a current source of magnitude IDC.
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