Question

Draw the circuit diagram of class AB power amplifier and describe its working.

Answer 1

Circuit diagram:

Circuit Description: The circuit consists of two center-tapped transformers T1 and T2, two identical transistors Q1 and Q2 , Resistor R and diode D. The DC voltage developed across the diode D is connected to the bases of both the transistors through the secondary winding of the input transformer. This voltage acts as DC bias for the transistors because it is equal to cut-in voltage and they will conduct for complete half cycleperiod of the input to eliminate the cross-over distortion.

WORKING: 

i. When there is no a.c. input signal is applied both the transistors Q1& Q2 are cut off. Hence no current is drawn from VCC. 

ii. DURING POSITIVE HALF CYCLE: The base of the transistor Q1 is positive and that of Q2 is negative. 

iii. As a result of this Q1 conducts, while the transistor Q2 is OFF. DURING DURING NEGATIVE HALF CYCLE: The base of the transistor Q2 is positive and that of Q1 is negative. As a result of this Q2 conducts, while the transistor Q1 is OFF. 

iv. Thus at any instant any one transistor in the circuit is conducting. Then the output transformer joins these two halves & produces a full sine wave in the load resistor.

OR

Circuit diagram:- 

Circuit operation:- 

Resistor R1, R2 are chosen to provide biasing to the transistors Q1, Q2, input transformer T1 provides phase splitting function in which two voltages are out of phase with each other. VCC is tied to the transistor collectors through the centre tapped output transformer T2. Re is stabilized resistor. 

When positive half cycle of the input signal is applied, the base of Q1 becomes positive and base of Q2 negative. Therefore Q1 is ON and Q2 is OFF. As transistors Q1 and Q2 are biased just above cut off. Therefore as positive input cross zero, collector current ic1 starts flowing through Q1, through transformer T2 as shown and ic2 = 0. A positive sinusoidal voltage will appear across load. 

When negative half cycle is applied across input the base of Q1 becomes negative while the base of Q2 is positive. Therefore Q1 is off and Q2 conduct, as soon as input cross zero, negative sinusoidal voltage will appear across load.