biased so that conduction occurs for much less than 180o . • Class C amplifiers are more efficient than either class A or push-pull class B and class AB, which means that more output power can be obtained from class C operation. • The output amplitude is a nonlinear function of the input, so class C amplifiers are not used for linear amplification. • They are generally used in radio frequency (RF) applications, including circuits, such as • oscillators, that have a constant output amplitude • modulators, where a high-frequency signal is controlled by a low-frequency signal. • Therefore, Class C amplifiers are also called Tuned Amplifiers. • An amplifier which amplifies a specific frequency ( or a narrow band of frequencies) is called a tuned voltage amplifier. • It has two purposes: • Selection of a desired radio frequency signal. • Amplification of the selected signal to a suitable voltage level. J-601-1448 , Lec#9 , Dec 2014
biased below cutoff with the negative VBB supply. • A class C amplifier is normally operated with a resonant circuit load, so the resistive load is used only for the purpose of illustrating the concept. J-601-1448 , Lec#9 , Dec 2014
of the transistor in a class C amplifier is low because it is on for only a small percentage of the input cycle. • To avoid complex mathematics, we will assume ideal pulse approximations. • Using this simplification, if the output swings over the entire load, the maximum current amplitude is Ic (sat) and the minimum voltage amplitude is Vce (sat) during the time the transistor is on. Check EXAMPLE 7–7! • The power dissipation during the on time is • The transistor is on for a short time, ton , and off for the rest of the input cycle. • The power dissipation averaged over the entire cycle is J-601-1448 , Lec#9 , Dec 2014
Because the collector voltage (output) is not a replica of the input, the resistively loaded class C amplifier alone is of no value in linear applications. • It is therefore necessary to use a class C amplifier with a parallel resonant circuit (tank). • The short pulse of collector current on each cycle of the input initiates and sustains the oscillation of the tank circuit so that an output sinusoidal voltage is produced. • The tank circuit has high impedance only near the resonant frequency, so the gain is large only at this frequency. J-601-1448 , Lec#9 , Dec 2014
curve is required to be as sharp as possible in order to provide a high selectivity. • A sharp resonance curve means that the impedance falls off rapidly as the frequency is varied above and below the resonant frequency. • A higher value of quality factor (Qo ) provides a higher selectivity but a smaller bandwidth and vice versa. J-601-1448 , Lec#9 , Dec 2014
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