What is a Push-pull Amplifier : Circuit Diagram and Its Working Principle

When the need for long-distance audio communication increased, it created the need to increase the amplitude of electrical signals to transmit them over long distances. Departments like telephone and telegraphy, duplex transmission, etc adopted various methods to amply the signals., but results remained unsatisfactory. It was around the year 1912 that the world was first introduced to the Amplifiers. These are devices that can amplify to increase the power of an input signal. In early amplifiers, vacuum tubes were used which are later replaced by transistors in the 1960s. There are many types of amplifiers based on the active circuits used to design them, based on their operation, etc… A power amplifier is designed to increase the power available to the load. Push-pull Amplifier is one of the power amplifiers.

What is a Push-pull Amplifier?

Push-pull Amplifier is a type of power amplifier. It contains a pair of active devices such as a complementary pair of transistors. Here one transistor supply’s the power to the load from the positive power supply and the other sink’s the current from the load to the ground.

These amplifiers are more efficient than the single-ended class-A amplifiers. The transistors present in this amplifier are anti-phased. The difference between the outputs of these two transistors is given to the load. The even-order harmonics present in the signal gets eliminated. This method reduces the distortion present in the signal due to non-linearity components.

These amplifiers are called Push-pull amplifiers because here one of the transistors Pushes the current in one direction while the other pull’s the current in another direction. In the push-pull amplifier, one transistor works during the positive half of the signal cycle while the other works during the negative half.

Circuit Diagram

The circuit of the Push-pull amplifier contains two transistors, an NPN and a PNP transistor, as active devices. These transistors are anti-phased. One transistor gets forward biased during the positive half cycle of the signal while the other during the negative half of the cycle. To divide the input signal into two identical signals 180 degrees out of phase, a center-tapped coupling transformer T1 is used at the source of the amplifier.

This amplifier can be constructed in different configurations such as Class-A, Class-B, and Class-AB Push-pull amplifiers. The circuits designed for these classes are different.

Circuit Diagram for Class-A Push-pull Amplifier

The Class-A amplifier contains two identical transistors Q1 and Q2. The emitter terminals of these two transistors are connected together. Resistors R1 and R2 are used for biasing the transistors. One transistor has to be forward-biased during the positive half-cycle of the signal while the other during negative half-cycle.

Class-A-Push-pull-Amplifier
class-a-push-pull-amplifier

The collector terminals of these two transistors are connected to the two ends of the primary winding of the output transformer T2. The base ends of these two transistors are connected to the secondary winding of the input transformer T1. The power supply is connected between the center tap of the primary of T2 and the emitter junction of the Q1, Q2.

The load is attached to the secondary of the transformer T2. The quiescent current from Q1 and Q2 flows in the opposite direction through the halves of the primary of T2. This cancels out the magnetic saturation in the circuit.

Circuit Diagram for Class B Push-pull Amplifier

There are no biasing resistors R1 and R2 in the Class-B amplifier. Here the two transistors are biased at the cut-off points. The transistors do not consume any power during the ideal conditions. Thus, the efficiency of the class B Push-pull amplifier is higher than the Class-A Push-pull amplifier.

Circuit Diagram of Class AB Push-pull Amplifier

This circuit is similar to the Class A Push-pull amplifier. But unlike Class A in Class AB biasing resistor values are selected such that the transistors Q1 and Q2 are biased just above the cut in voltages. This arrangement reduces the time during which the transistors will be simultaneously OFF. Thus, the cross over distortion is reduced in the Class AB amplifier.

Push-pull Amplifier Working

The output stage of this amplifier can drive the current in both directions through the load. It contains two anti-phased transistors Q1 and Q2. The input coupling transformer T1 divides the input signal into two identical halves, every 180 degrees out of phase. One transistor gets forward biased during the positive half-cycle and passes the current. The other transistor stays reverse biased during the positive half cycle. This condition is reversed when the negative half cycle is applied to the transistors.

The collector currents I1 and I2 from Q1 and Q2 flows in the same direction through the corresponding halves of the primary of the transformer T2. This induces an amplified output of the input signal in the secondary of the T2 transformer. Thus, the current through secondary of T2 is the difference between the collector currents of the transistors.

Advantages

The output of the Push-pull amplifier is the difference between the collector currents of the two transistors. This eliminates the harmonics in the output. This method also reduces distortion. Class B amplifier has high efficiency and can work in limited power supply conditions. The class-B amplifier has simple circuitry and its output does not contain even harmonics. Cross over distortion is reduced in the Class AB amplifiers.

Applications

Some of the applications of the Push-pull Amplifiers are as follows-

  • These amplifiers are used in RF systems.
  • In digital systems, these amplifiers are used due to their low cost and smaller design.
  • These are used for audio amplification on TV, Mobile phones, Computers.
  • In long-distance communication systems where low distortion is required, these amplifiers are used.
  • These are used with loudspeakers.
  • For amplification of radio frequency signals.
  • In power electronic systems Push-pull amplifiers are used.

FAQs

1. Why is it called a Push-pull Amplifier?

This Amplifier has two transistors in the circuit. One of the transistors push the current towards output during positive half-cycle of the input signal The other transistor pulls the current towards the output during the negative half-cycle of the input signal Thus, the amplifier is called as Push-pull Amplifier.

2. What is a Complimentary Push-pull Amplifier?

The use of a transformer makes the design of the push-pull amplifier bulky. To remove this disadvantage, two transistors, an NPN and a PNP, which are complementary to each other are used at the input stage of the Push-pull amplifier. This design is known as the Complimentary Push-pull Amplifier.

3. What is Push-pull?

The push-pull output stage is designed using two complementary transistors that alternatively supply current load and absorb current from the load.

4. Why Push-pull Amplifier is used?

A push-pull amplifier is usually preferred to amplify the signals without distortion.

5. Which amplifier has the highest efficiency?

The Class B Push-pull Amplifier has the highest efficiency of 78.9 %.

Besides transistors, vacuum tubes are also used as the active elements in these amplifiers. Nowadays transformers are very rarely used at the output stage of the amplifiers. In the symmetrical push-pull, each output pair mirrors the other. Here the NPN of one half is mirrored with the PNP of the other. Similarly, there are quasi-symmetrical, super symmetrical, square law Push-pull depending upon their output circuits.  Here is a question for you, what is the main function of an amplifier?

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