The specifications of different types of power amplifiers are tabulated below-
Class-A Power Amplifier
(i) Output current flows during the entire cycle of the A.C. input signal.
(ii) The operation of the amplifier is restricted to smaller central region of the load line,so that it can operate in the linear region of the load line.
(iii) The circuit efficiency of the amplifier circuit is defined as the ratio of A.C. power delivered to the load to the total power supplied by the D.C. source.
Class-B Power Amplifier
(i) Output current flows only for one-half of the cycle of the input signal.
(ii) The transistor dissipates no power with zero input signal.However,it increase with the increase in the amplitude of the input signal.
(iii) The overall efficiency of the circuit is higher than that of case-A amplifier. Its maximum value has been obtained to be equal to 78.5%.
Class-C Power Amplifier
(i) Output current flows only during a part of the positive or negative half-cycle of the input signal.
(ii) The output signal does not resemble the input signal because it consist of narrow pulses.
(iii) The class-C amplifier is the most efficient power amplifier and its overall efficiency under certain conditions may approach even hundred percent
(i) Highly stabilized gain.
(ii) Reduction in non-linear distortion.
(iii) Increased bandwidth,i.e.,improved frequency response.
(iv) Increased stability of circuit.
(v) Less amplitude distortion.
(vi) Less frequency distortion and less phase distortion.
(vii) Increases input impedance and decreases output impedance. In other words,the input and output impedance and decreases can be modified as required.
(viii) Reduces noise and less harmonic distortion.
(ix) The transfer gain Af of the amplifier with feedback can be stabilized against variations of the h-parameters of the transistor.
(x) The significant improvement in the frequency response and in the linearity of operation of the feedback amplifier compared with that of the amplifier without feedback.
(i) If the feedback signal (voltage or current) is applied in such a way that it is in phase with the input signal and thus increases it,then it is called a positive feedback.
(ii) it is a regenerative or direct feedback.
(iii) Positive feedback increases the gain of amplifier.
(iv) It produces excessive distortion.
(v) Positive feedback is used in oscillators.
(i) If the feedback signal (voltage or current) is applied in such a way that it is out of phase with the input signal and thus decreases it,then it is called negative feedback.
(ii)It is a degenerative or inverse feedback.
(iii) Negative feedback reduces gain of the amplifier.
(iv) It generates less distortion.
(v) Negative feedback is frequently used in small-signal as well as with large-signal amplifier circuits.