R.F. and I.F. Amplifiers
Author: J.B. Hoag
Radio-frequency amplifiers are often designed to amplify only one frequency, together with those frequencies in the immediate neighborhood. Thus they serve the dual purpose of an amplifier and of a band-pass filter, strengthening the input voltage and simultaneously selecting a restricted band of frequencies. They use condensers and coils in their grid and plate circuits, tuned to the desired frequency. By changing the inductance or capacitance of these resonant circuits, one may select first one and then another frequency from a complex input voltage, and amplify and use it alone, to the exclusion (more or less) of all the others. In this way a receiver is made to pick out only one of several stations that are " on the air " at the same time. An amplifier which will amplify only an extremely narrow range of frequencies is said to be highly selective, while one which amplifies a broader band of frequencies is said to be broadly tuned. R.F. amplifiers are used in both transmitters and receivers.