TABLE OF CONTENTS Amplifier circuits are used to increase the level of the very small signals (1 V or less) to which a receiver must respond so that these signals can be demodulated and produce output of a useful level (on the order of volts). Such circuits may amplify at the received radio frequency, at any of the IFs, or at the lowest frequency to which the signal is transformed. This frequency is generically referred to as baseband frequency, but in specific cases, audio frequency (AF), video frequency (VF), or another notation appropriate for the particular application may be used.
Because of the wide range of signals to which a receiver must respond, the input device at least must operate over a wide dynamic range (120 dB or more). The input device should be as linear as possible so as to minimize the generation of IM products from the strongest signals that must be handled. Therefore, the number of strong signals should be minimized by restricting the receiver bandwidth at as low a gain level as possible. Thus, the gain should be low prior to the most narrow bandwidth in the receiver. It is not always possible to narrow the bandwidth adequately at RF, and so RF amplifiers are especially subject to many strong interfering signals. If there is more than one RF amplifier, the later stages encounter stronger signal levels, and the first mixer generally encounters the strongest interferers. On the other hand, mixers often have poorer NFs than amplifiers, and...
