.A push pull amplifier can be made in Class A, Class B, Class AB or Class C configurations. The circuit diagram of a typical Class A push pull amplifier is shown above. Q1 and Q2 are two identical transistor and their emitter terminals are connected together. R1 and R2 are meant for biasing the transistors. Collector terminals of the two transistor are connected to the respective ends of the primary of the output transformer T2. Power supply is connected between the center tap of the T2 primary and the emitter junction of the Q1 and Q2. Base terminal of each transistor is connected to the respective ends of the secondary of the input coupling transformer T1. Input signal is applied to the primary of T1 and output load RL is connected across the secondary of T2.Quiescent current of Q2 and Q1 flows in opposite directions through the corresponding halves of the primary of T2 and as a result there will be no magnetic saturation. From the figure you can see the phase splited signals being applied to the base of each transistors. When Q1 is driven positive using the first half of its input signal, the collector current of Q1 increases. At the same time Q2 is driven negative using the first half of its input signal and so the collector current of Q2 decreases. From the figure you can understand that the collector currents of Q1 and Q2 ie; I1 and I2 flows in the same direction trough the corresponding halves of the T2 primary. As a result an amplified version of the original input signal is induced in the T2 secondary. It is clear that the current through the T2 secondary is the difference between the two collector currents. Harmonics will be much less in the output due to cancellation and this is results in low distortion.
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Class B push pull amplifier.
The Class B push pull amplifier is almost similar to the Class A push pull amplifier and the only difference is that there is no biasing resistors for a Class B push pull amplifier. This means that the two transistors are biased at the cut off point.The Class B configuration can provide better power output and has higher efficiency(up to 78.5%). Since the transistor are biased at the cutoff point, they consumes no power during idle condition and this adds to the efficiency. The advantages of Class B push pull amplifiers are, ability to work in limited power supply conditions (due to the higher efficiency), absence of even harmonics in the output, simple circuitry when compared to the Class A configuration etc. The disadvantages are higher percentage of harmonic distortion when compared to the Class A, cancellation of power supply ripples is not as efficient as in Class A push pull amplifier and which results in the need of a well regulated power supply.The circuit diagram of a classic Class B push pull amplifier is shown in the diagram below.
Class B push pull amplifier
Class B push pull amplifier
The circuit arrangement of the Class B push pull amplifier is similar to the Class A push pull amplifier except for the absence of the biasing resistors. T1 is the input coupling capacitor and the input signal is applied to its primary. Q1 and Q2 are two identical transistors and their emitter terminals are connected together. Center tap of the input coupling transformer and the negative end of the voltage source is connected to the junction point of the emitter terminals. Positive end of the voltage source is connected to the center tap of the output coupling transformer. Collector terminals of each transistor are connected to the respective ends of the primary of the output coupling transformer T2. Load RL is connected across the secondary of T2.
The input signal is converted into two similar but phase opposite signals by the input transformer T1. One out of these two signals is applied to the base of the upper transistor while the other one is applied to the base of the other transistor. You can understand this from the circuit diagram. When transistor Q1 is driven to the positive side using the positive half of its input signal, the reverse happens in the transistor Q2. That means when the collector current of Q1 is going in the increasing direction, the collector current of Q2 goes in the decreasing direction. Anyway the current flow through the respective halves of the primary of the T2 will be in same direction. Have a look at the figure for better understanding. This current flow through the T2 primary results in a wave form induced across its secondary. The wave form induced across the secondary is similar to the original input signal but amplified in terms of magnitude.