RF and IF Amplifiers
After a weak radio frequency (RF) signal has arrived at the antenna, it is channeled to the input terminals of the RF amplifier through a passive matching network. As we learned in Chap. 6, the matching network enables maximum power transfer of the receiving signal by equalizing the antenna impedance with the RF amplifier input impedance. After that, it is job of the RF amplifier to increase the power of the received signal and prepare it for further processing. In the first part of this chapter, we review the basic principles of linear baseband amplifiers and common circuit topologies. In the second part of the chapter, we introduce RF and intermediate frequency (IF) amplifiers. In order to clarify the difference between RF and IF amplifiers, we need to know that in most radio receiver topologies the incoming high-frequency signal is not shifted down to the baseband in a single step. Instead, for reasons that we discuss in detail in Chap. 9, frequency down-shifting inside radio receivers is usually done in one or more intermediate steps. RF amplifiers used at those lower frequencies are referred to as IF amplifiers. Aside from their operating frequency, for all practical purposes, there is not much difference between the schematic diagrams of RF and IF amplifiers. In this book, unless we need to specifically separate the two functions, we refer to all tuned amplifiers as RF amplifiers.