IF amplifier

An intermediate-frequency amplifier, or IF-amp, is a fixed radio-frequency amplifier commonly used in superheterodyne receivers. Such amplifiers generally are cascaded two or more in a row, with tuned-transformer coupling. The IF-amplifiers follow the mixer stage, and precede the detector stage. Double-conversion receivers have two sets of IF-amplifiers. The first set follows the first mixer and precedes the second mixer, and the second set follows the second mixer and precedes the detector, as shown in the diagram.

The IF-amplifier chain serves two main purposes: to provide high gain, and excellent selectivity. Gain and selectivity are much easier to obtain with amplifiersthat operate at a single frequency, as compared with tuned radio-frequency amplifiers.

Intermediate frequency

The intermediate frequency in a superheterodyne receiver is the output frequency from the mixer stage. The intermediate frequency of the superheterodyne is usually a fixed frequency. This makes it easy to obtain high gain and excellent selectivity because the IF amplifier stage, as mentioned above, can be tuned precisely for optimum performance at a single frequency.
Some superheterodyne recerivers use one intermediate frequency; these are calle single-conversion receivers. Other receivers have two intermediate frequencies. The incoming signal is heterodyned to a fixed frequency called the first IF, which is in turn heterodyned in a later stage to the second IF. The second IF is generally a low frequency, which fascilitates high selectivity. This type of reciver is called a double-conversion receiver or dual-conversion receiver. Some receivers even have three intermediate frequencies and are logically called triple-conversion receivers.
A high intermediate frequency, such as several megahertz, is preferable to a low IF for purposes of image rejection. However, a low IF is better for obtaining sharp selectivity. This is why double-conversion receivers are common, they provide the advantages of both a high first IF and a low second IF.