The class AB complementary pair transistor audio amplifier is a configuration that works using the push-pull audio power amplification method. The class AB amplifier is a combination of class A and class B amplifiers. Here, the transistors are biased in such a way that they conduct for more than 180 degrees and less than 360 degrees of the input waveform. It has good distortion-less amplification as well as high DC to AC conversion efficiency.
The forward voltage drop at the PN junction of 0.7 volts at the transistor base is reduced by biasing the transistors accordingly. Thus the transistors will conduct for more than 50% or 180 degrees of the input cycle. It minimizes the problem of crossover distortion that affects the class B operation. The biasing is done using a voltage divider network or a series connected diode arrangement.
Working of Class AB Audio Power Amplifier
Three transistors are used here to construct this audio power amplifier. Two complementary transistors, one NPN and the other PNP transistors are used as push pull output power amplifiers and another NPN transistor is used as the driver. There are two forward biased diodes in series, which are used to bias the output transistors. A small current constantly flows through the network of resistors and diodes such as R1, D1, D2 and R2 that provides the required voltage for the biasing. When no input voltage is given, the point between the two diodes will remain zero.
As the current flows through the biasing network, there is a forward bias voltage drop of approximately 0.7 volts across the diodes, which is applied to the base-emitter junction of the push pull transistors. The voltage drop across the silicon diodes gives a biasing voltage of 0.7 volts for the NPN transistor and -0.7 volts for the PNP transistor. This constant voltage drop across the two diodes, biases the transistor above the cut-off. There are less biasing voltage changes that can happen due to temperature variation as the electrical characteristics of the diodes are closely matched with that of the base emitter junction of the transistor. This compensates for variations in temperature at the base-emitter junction and thereby helps to prevent crossover distortion. This amplifier provides about 200 mW of power into an 8 ohms load using a 9 volts battery.
Applications of Class AB Audio Amplifier
Working of Class AB Audio Power Amplifier
Three transistors are used here to construct this audio power amplifier. Two complementary transistors, one NPN and the other PNP transistors are used as push pull output power amplifiers and another NPN transistor is used as the driver. There are two forward biased diodes in series, which are used to bias the output transistors. A small current constantly flows through the network of resistors and diodes such as R1, D1, D2 and R2 that provides the required voltage for the biasing. When no input voltage is given, the point between the two diodes will remain zero.
As the current flows through the biasing network, there is a forward bias voltage drop of approximately 0.7 volts across the diodes, which is applied to the base-emitter junction of the push pull transistors. The voltage drop across the silicon diodes gives a biasing voltage of 0.7 volts for the NPN transistor and -0.7 volts for the PNP transistor. This constant voltage drop across the two diodes, biases the transistor above the cut-off. There are less biasing voltage changes that can happen due to temperature variation as the electrical characteristics of the diodes are closely matched with that of the base emitter junction of the transistor. This compensates for variations in temperature at the base-emitter junction and thereby helps to prevent crossover distortion. This amplifier provides about 200 mW of power into an 8 ohms load using a 9 volts battery.
Applications of Class AB Audio Amplifier
- This amplifier is used for audio amplification in low powered battery operated radio receivers.
- It is suitable as an audio amplifier in radio and communication receivers used for portable operation.
- The class AB amplifier is preferred in audio amplifiers that have least distortion such as multimedia players, intercoms, TV sound systems, line drivers etc.
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