Power amplifier are electronics amplifier that magnifies the input signal voltage magnitude or current magnitude or both in order to increase the power of the output signal. Power amplifiers are required to drive actuators like loud speaker, motors, RF transmitters etc that requires high power to operate. There are different classes of power amplifier which are classified according to conduction angle in the active region of operation. Conduction angle is the angle in degrees of the input signal cycle that results into output signal. These power amplifier classes are Class A, Class B, Class AB, Class C, Class D. Operation of class A power amplifier was explained in the tutorial How does Class A power amplifier work?.
Here class B power amplifier operation is explained. Class B power amplifier have conduction angle less than 180 degree. That is the transistor conducts for half of the input ac signal cycle that is 180 degree.
The circuit diagram of class B power amplifier with input and output signal waveform for a common collector(CC) BJT amplifier.
Class B power amplifier advantage over class A power amplifier is that class B power amplifier is more efficient class A power amplifier. More efficient here means that class B amplifier gives more output power for the same amount of input power. This is required for battery powered devices. The disadvantage of class B power amplifier over class A power amplifier is that class B power amplifier circuit is more difficult to implement.
Class B Push Pull Amplifier
Because we want to output signal to appear over all cycle of the input signal without clipping hence distortion, class B power amplifier is implemented in push pull configuration. In such push pull amplifier two transistors are used and one transistor conducts for half of input signal while the other transistor is off and the other transistor conducts for the other half of the input signal the other transistor is off.
The following circuit diagram shows push-pull class B amplifier using two emitter-followers.
In push pull amplifier, complementary transistor NPN which is transistor Q1 and PNP transistor which is transistor Q2 are used. In the above diagram the input signal is not DC biased and therefore the amplitude of the signal must be greater than the BE voltage(base-emitter voltage) to make the transistors conduct. The transistor Q1 conducts during the positive half cycle of the input signal while the transistor Q2 is off. Similarly, during the negative half cycle of the input signal the transistor Q1 is off while the transistor Q2 conducts. The overall effect is that the output signal appears over all 360 cycle of the input signal.
Crossover Distortion in Class B Push Pull Power Amplifier
There is chances in distortion in the output signal from a class B push pull power amplifier which is due to fact that there is a time interval in which both the complementary transistor does not conduct. This non-conduction time interval happens in this BJT based complementary power amplifier because the transistors conducts only when the input signal amplitude is greater than the base to emitter(BE) voltage added by the fact that during this time the other transistor if off. The non-conducting time interval is illustrated below.
Biasing the Push-Pull Amplifier
To avoid crossover distortion found in push pull class B power amplifier, both the complementary transistors have to be biased slightly above cutoff in absence of signal. The biasing required can be achieved using voltage divider and diode arrangement with capacitive coupling as shown below.
For stable bias, both the diodes D1 and D2 properties should be closely matched to the
properties of the transistor base-emitter junctions. Also the resistors R1 and R2 should of equal value. When resistors R1 and R2 are equal then the voltage at node A is \(V_{CC}/2\). If we assume that the characteristics of the transistors and diodes are identical, the voltage across diode D1 is equal to the \(V_{BE}\) of transistor Q1 and the voltage across diode D2 is equal to the Vbe of transistor Q2. Because of this the voltage at both transistor emitters is \(V_{CC}/2\), that is, \(V_{CEQ1}\) = \(V_{CEQ2}\) = \(V_{CC}/2\). And since both the transistors are biased near cutoff, \(I_{CQ} \approx 0\).
The above circuit diagram showed push pull class B power amplifier implemented capacitive coupling. The following circuit diagram shows push pull class B power implemented with transformer coupling.
