Question

To eliminate cross over distortion in Class B power amplifier, the circuit should have

• A. Two complementary transistors to conduct in alternate half cycles
• B. Two complementary transistors to conduct in full cycles
• C. Two same transistors to conduct in two full cycles
• D. Two same transistors to conduct in alternate half cycles

Hint:

Crossover distortion occurs in Class B amplifiers at the zero-crossing of the signal.
It is eliminated by biasing the transistors slightly into conduction (Class AB operation), ensuring a smooth transition between the two transistors.
This typically involves using biasing diodes or a voltage divider to provide a small quiescent base current.

Answer 1

The Correct Option is A

To eliminate crossover distortion in Class B power amplifier, the circuit should have

Explanation: 

Crossover distortion is a type of distortion that occurs in Class B amplifiers because each transistor conducts for only one half-cycle of the input signal. When the signal crosses the zero-point, one transistor turns off while the other has not yet turned on, creating a "dead zone" that leads to distortion near the zero-crossing point of the output waveform.

The problem arises because in a Class B amplifier, each transistor only conducts for half of the input signal cycle (positive or negative half). When the input signal crosses zero, there is a brief period when neither transistor is conducting. This creates a discontinuity and results in crossover distortion, especially at the zero-crossing point of the output waveform.

To eliminate or reduce this distortion, the most common solution is to bias the transistors slightly into conduction even when no input signal is present. This ensures that there is no point where both transistors are simultaneously off. As one transistor is conducting, the other will smoothly take over conduction, eliminating the dead zone and preventing distortion.

This configuration is known as Class AB operation. By biasing both transistors slightly on, the circuit operates in a way that allows continuous conduction, preventing crossover distortion.

The circuit uses two complementary transistors that conduct in alternate half cycles of the input signal. For a typical push-pull Class B amplifier, this means:

• One transistor conducts during the positive half of the input signal.
• The other transistor conducts during the negative half of the input signal.

To further refine the system and prevent distortion, slight biasing (typically about 0.5V to 0.7V for silicon BJTs) is applied to both transistors. This bias ensures that both transistors remain in partial conduction even when no signal is applied, thus bridging the gap between the two conduction periods and eliminating the crossover distortion.

The result of this configuration is that both transistors conduct over a small portion of the signal's cycle, effectively filling the "dead zone" where no transistor was conducting. This is achieved by biasing the transistors slightly into conduction, making the transition between the two transistors' conduction periods smooth.

The solution involves using two complementary transistors to conduct in alternate half cycles, which is the correct configuration to eliminate crossover distortion.

Therefore, the correct answer is: Two complementary transistors to conduct in alternate half cycles.

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