TABLE OF CONTENTS In all wireless applications, we need tuned or narrow-band amplifiers that provide gain at a certain frequency and in a narrow band around this frequency. The tuning frequency and the bandwidth depend on the area of application. Since the early days of the radio, resonance circuits have been the main components of tuned amplifiers, owing to their frequency-selective nature.
We have seen that the voltage gain of any amplifier is proportional to the total load impedance, which is the parallel equivalent of the external load and the output impedance of the amplifier. We also know that the impedance of a parallel resonance circuit is maximum at a certain frequency that is approximately equal to the natural frequency (or the resonance frequency) of the circuit. Then the easiest way to form a tuned amplifier is to use a parallel resonance circuit as the load of the amplifier such that the resonance frequency of the load together with the output impedance of the amplifier is equal to the desired tuning frequency. In some applications, the tuning frequency must be fixed as in the intermediate frequency (IF) amplifier of a receiver. But in some other applications, such as the input amplifier of a radio, the tuning frequency must be adjustable in a certain frequency band. To tune the frequency of an amplifier, the most commonly used way is to incorporate a suitable varactor into the resonance circuit. [8]
The bandwidth of the amplifier is certainly determined by the frequency...
