The main aim of large signal amplifier also called Power Amplifier is to produce large amount of power to load. It takes power from the DC power supply connected to the output circuit and converts it into AC output power. This type of AC output power is controlled by the input signal.
Based on the conduction angle of a transistor, large signal amplifiers can be classified into four types namely class A, class B, class AB, and class C amplifiers.
Class A power amplifier is one in which the Quiescent point and input signal are selected such that output signal is obtained for a full input cycle. It conducts 0 to 360 degrees of input signal. The Q point is located at the centre between cut off and saturation points.
Class B power amplifier is one in which the output signal is obtained for half cycle of AC input signal. It conducts 0 to 180 degrees of input signal. The Q point is located at the cutoff point.
Class AB power amplifier is one in which the output signal is obtained for more than 180 degrees but less than 360 degrees of input signal. The Q point is located just above the x-axis but below the midpoint on a load line.
Class C power amplifier is one in which the output signal is obtained for less than a half cycle of input signal. The Q point is located just below the X- axis.
Class A power amplifier:
Class A power amplifiers can be classified in to two types namely
1) Series fed class A power amplifier
2) Transformer coupled class A power amplifier
Following figure (a) represents series fed class A power amplifier,
AC output power = Pac =vce * ic
But vce = Vm/√ 2 = (Vmax-Vmin)/2√2 =Vcc/2√2
Ic=Im/√2 = Imax – Imin/2√2 = Vcc/2√2 Rc
Pac = Vcc/2√2 * Vcc/2√2 Rc = Vcc 2/8 Rc
DC input power = Pdc = Vcc * IcQ
But IcQ =I max – I min/2 =Vcc/2Rc
Efficiency =η= Pac/Pdc *100 = (Vcc/8Rc) (2Rc/Vcc2)100 =25%
Thus the maximum efficiency in series fed class A power amplifier is 25%
Figure (a): Series Fed Class A power Amplifier
In order to increase the efficiency, transformer coupled class A power amplifier is used. For getting maximum AC output power, the primary winding of a transform is having high impedance and low resistance value but the secondary winding is having low impedance. Following figure (b) represents transformer coupled class A power amplifier.
AC output power= pac=vce*ic
But vce= (Vmax-Vmin)/2√2=2Vcc/2√2=vcc/√2
DC input power=Pdc=Vcc*IcQ
Efficiency=η= (Vcc*IcQ/2) (1/Vcc*IcQ)*100=50%
Thus the maximum efficiency in Transformer coupled class A power amplifier is 50%. But practically, the efficiency is always less than 50%.
Figure (b): Transformer Coupled Class A power Amplifier
Article Presented By
HOD : ECE Department
LAQSHYA Institute of Technology & Sciences