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Build Your Own Transistor Radios by Ronald Quan

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Chapter 17 Intermediate-Frequency Circuits

The intermediate-frequency (IF) amplifier and filter determine selectivity and sensitivity for an amplitude-modulated (AM) superheterodyne radio. In commercially made radios, some of the first transistor radios needed two stages of IF amplification and three IF filters to match the sensitivity and selectivity of vacuum-tube receivers. Transistor radios made with only one IF amplifier generally performed worse in sensitivity (and selectivity) than the ones with two IF amplifiers. However, there are some ways to bring up the sensitivity of the radio by using longer antenna coils.

While IF amplifiers provide amplification, often an IF amplifier also serves to provide some type of automatic gain control. ...

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