Class B Power Amplifiers

CLASS-B POWER AMPLIFIER

The main drawback of class A amplifier is that all of the supply power is dissipated in the transistor when there is no input.

The output power is obtained for one half cycle of input only. The collector current flows for 180° only. The Q-point is adjusted to be in cutoff region i.e I_B = 0, V_CE = V_CC

The transistor is in conduction state during positive half cycle of the signal and in cutoff state during negative half cycle of the signal. Therefore, the output signal is highly distorted.

Since the transistor conducts for one half cycle of the input, the power dissipation of these class B amplifiers are very less. Hence efficiency is increased.

Advantages

1. Impedance with load is possible

2. Second harmonic is cancelled automatically

3. Zero power dissipation

4. Higher efficiency than class A amplifiers

Disadvantages

Crossover distortion is present in the output waveform, since the transistor is biased at cutoff region, the waveform is distorted near zero crossings.

Push Pull Amplifier - If both the transistors are of same type (npn or pnp)

Complementary symmetry - If one of the transistor is NPN and other is PNP or vice versa

CLASS B PUSH PULL POWER AMPLIFIER

If the two transistors of same type (either pnp or npn) are used then the amplifier is called class-B Push-Pull Amplifier.

Construction

The push-pull amplifier uses two centre tapped transformers. For getting full input signal we use push pull circuit. In push-pull amplifiers, two transformers are used, one at the input and the other at the load side. The transformer on the input side is known as driver transformer and the transformer on the load side is called as output transformer. Both are centre tapped transformers. Consider the transformers coupled push pull amplifier shown in Fig.5.12.

Operation

When V_in = 0, both transistors, T₁ and T₂ are cut off. For positive half input cycle, T₁ goes ON and current i, flows in the direction as shown in Fig.5.12.

For negative half cycle of input, T₂ goes ON and a current i₂ flows in the direction as shown in Fig.5.13.

When V_in= 0, V_CC appears across both the transistors. Since de resistance of transformer winding is negligible, dc loadline is almost vertical.

Fig. 5.14 shows load lines for class B pushpull amplifier.

When V_in is zero, resistance R'_L is present in i₁ to i₂. Current i₁ is taken as positive and i₂ is in anticlockwise and is negative. The combination of these two half cycles is shown in Fig.5.15.

Direct current supplied by the battery is

where I_m - Peak value of half cycle of i_C (conducted by T₁ or T₂)