Push-Pull Class A Power Amplifier

So far, we have seen two types of class A power amplifiers. The main problems that should be dealt with are low power output and efficiency. It is possible to obtain greater power output and efficiency than that of the Class A amplifier by using a combinational transistor pair called as Push-Pull configuration.

In this circuit, we use two complementary transistors in the output stage with one transistor being an NPN or N-channel type while the other transistor is a PNP or P-channel (the complement) type connected in order to operate them like PUSH a transistor to ON and PULL another transistor to OFF at the same time. This push-pull configuration can be made in class A, class B, class C or class AB amplifiers.

Construction of Push-Pull Class A Power Amplifier

The construction of the class A power amplifier circuit in push-pull configuration is shown as in the figure below. This arrangement mainly reduces the harmonic distortion introduced by the non-linearity of the transfer characteristics of a single transistor amplifier.

In Push-pull arrangement, the two identical transistors T₁ and T₂ have their emitter terminals shorted. The input signal is applied to the transistors through the transformer T_r1 which provides opposite polarity signals to both the transistor bases. The collectors of both the transistors are connected to the primary of output transformer T_r2. Both the transformers are center tapped. The V_CC supply is provided to the collectors of both the transistors through the primary of the output transformer.

The resistors R₁ and R₂ provide the biasing arrangement. The load is generally a loudspeaker which is connected across the secondary of the output transformer. The turns ratio of the output transformer is chosen in such a way that the load is well matched with the output impedance of the transistor. So maximum power is delivered to the load by the amplifier.

Circuit Operation

The output is collected from the output transformer T_r2. The primary of this transformer T_r2 has practically no dc component through it. The transistors T₁ and T₂ have their collectors connected to the primary of transformer T_r2 so that their currents are equal in magnitude and flow in opposite directions through the primary of transformer T_r2.

When the a.c. input signal is applied, the base of transistor T₁ is more positive while the base of transistor T₂ is less positive. Hence the collector current i_c1 of transistor T₁ increases while the collector current i_c2 of transistor T₂ decreases. These currents flow in opposite directions in two halves of the primary of output transformer. Moreover, the flux produced by these currents will also be in opposite directions.

Hence, the voltage across the load will be induced voltage whose magnitude will be proportional to the difference of collector currents i.e.

$$(i_{c1} - i_{c2})$$

Similarly, for the negative input signal, the collector current i_c2 will be more than i_c1. In this case, the voltage developed across the load will again be due to the difference

$$(i_{c1} - i_{c2})$$

As $i_{c2} > i_{c1}$

The polarity of voltage induced across load will be reversed.

$$i_{c1} - i_{c2} = i_{c1} + (-i_{c2})$$

To have a better understanding, let us consider the below figure.

The overall operation results in an a.c. voltage induced in the secondary of output transformer and hence a.c. power is delivered to that load.

It is understood that, during any given half cycle of input signal, one transistor is being driven (or pushed) deep into conduction while the other being non-conducting (pulled out). Hence the name Push-pull amplifier. The harmonic distortion in Push-pull amplifier is minimized such that all the even harmonics are eliminated.

Advantages

The advantages of class A Push-pull amplifier are as follows

• High a.c. output is obtained.

• The output is free from even harmonics.

• The effect of ripple voltages are balanced out. These are present in the power supply due to inadequate filtering.

Disadvantages

The disadvantages of class A Push-pull amplifier are as follows

• The transistors are to be identical, to produce equal amplification.
• Center-tapping is required for the transformers.
• The transformers are bulky and costly.