
CHAPTER 20
Selectivity and Tuning
20.1 Need for R.F. Amplification
In Sec. 18.11 we saw how the effectiveness of detectors deteriorates
quite rapidly as the r.f. input is reduced much below about 1 V.
Signals picked up by a receiving antenna are seldom more than a few
millivolts and are often only microvolts, so (except from a powerful
sender at short range) they are too weak to operate a detector
satisfactorily. Amplification of the r.f. signals is therefore a
practical necessity in order to give a receiver adequate sensitivity,
which, for an a.m. sound receiver, is usually expressed as the input
microvoltage needed to yield a specified a.f ...