A push pull amplifier is an amplifier which has an output stage that can drive a current in either direction through through the load. The output stage of a typical push pull amplifier consists of of two identical BJTs or MOSFETs one sourcing current through the load while the other one sinking the current from the load. Push pull amplifiers are superior over single ended amplifiers (using a single transistor at the output for driving the load) in terms of distortion and performance. A single ended amplifier, how well it may be designed will surely introduce some distortion due to the non linearity of its dynamic transfer characteristics. Push pull amplifiers are commonly used in situations where low distortion, high efficiency and high output power are required. The basic operation of a push pull amplifier is as follows: The signal to be amplified is first split into two identical signals 180° out of phase. Generally this splitting is done using an input coupling transformer. The input coupling transformer is so arranged that one signal in applied to the input of one transistor and the other signal is applied to the input of the other transistor. Advantages of push pull amplifier are low distortion, absence of magnetic saturation in the coupling transformer core, and cancellation of power supply ripples which results in the absence of hum while the disadvantages are the need of two identical transistors and the requirement of bulky and costly coupling transformers.
Class A push pull amplifier.
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.
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.
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.
Cross over distortion.
Cross over distortion is a type of distortion commonly seen in Class B amplifier configurations. As we said earlier ,the transistor are biased at cut off point in the Class B amplifier. We all know a Silicon transistor requires 0.7V and a Germanium diode requires 0.2V of voltage across its base emitter junction before entering in to conducting mode and this base emitter voltage is called cut in voltage. Germanium diodes are out of scope in amplifiers and we can talk about a Class B push pull amplifier based on Silicon transistors. Since the transistors are biased to cut off, the voltage across their base emitter junction remains zero during the zero input condition. The only source for the transistors to get the necessary cut in voltage is the input signal itself and the required cut in voltage will be looted from the input signal itself. As a result portions of the input wave form that are below 0.7V (cut in voltage) will be cancelled and so the corresponding portions will be absent in the output wave form too. Have a look at the figure below for better understanding.
Class AB push pull amplifier.
Class AB is another type of push pull amplifier which is almost similar to that of a Class A push pull amplifier and the only difference is that the value of biasing resistors R1 and R2 are so selected that the transistors are biased just at the cut in voltage (0.7V). This reduces the time for which both transistors are simultaneously OFF (the time for which input signal is between (-0.7V and +0.7V) and so the cross over distortion gets reduced. Of the above said classes Class A has least distortion, then Class AB and then Class B. Any way Class AB configuration has reduced efficiency and wastes a reasonable amount of power during zero input condition. Class B has the highest efficiency (78.5%), then Class B (between 78.5 to 50%) and then Class A (50%) .