Power Amplifiers are essential in the any communication transceivers. They essentially as the name says amplifies the power of the input signal to a desired level at the output stage. The output stage may be a loudspeaker in which case it is called class AB audio amplifier or even an antenna. There are different classes of power amplifiers based basically upon the how much a transistor conducts in a full cycle. For example a class A amplifier is one in which the transistor conducts for full cycle, class B is one in which the amplifier conducts for half a cycle of the input whereas class AB is one in which the amplifier conducts for a cycle between half a cycle and full of a cycle. Here we show a class AB amplifier circuit and simulate it using Altium Designer.
Circuit Description:
In the circuit above, two complementary transistors 2N3904 which is a NPN transistor and 2N3906 which is a PNP transistor was used. Two diodes 1N4148 are used to prevent damaging the transistors from thermal runaway and to bias the transistors along with the two 3.9Kohm resistors. Two coupling capacitors(see amplifier design tutorial) are used at the input and output of the class AB amplifying circuit. An input sinusoid signal of 0.5V amplitude and 1KHz is applied to the amplifier and a supply of 20V VDC is applied as shown in the circuit schematic. The 10ohm resistor is the load resistor which is equivalent to a loudspeaker resistance.
Simulation:
The simulation was set up for transient analysis with run time 20ms and step size of 10us(micro second). The input(Vin) and output(Vout) voltage signal was plotted and the result is shown below-
From the waveform chart above we can see that the input signal peak to peak voltage is 1V whereas the output signal peak to peak voltage is 0.8V.
Similarly the clipped waveform from the upper and the lower transistors for this class AB amplifier is shown below-
Thus the class AB amplifier circuit shown above works as per the simulation.
Class AB Amplifier Circuit Schematic
The schematic diagram of a temperature compensated class AB audio amplifier driving load such as loudspeaker is shown below- |
| Fig: Class AB Power Amplifier Circuit Schematic |
Circuit Description:
In the circuit above, two complementary transistors 2N3904 which is a NPN transistor and 2N3906 which is a PNP transistor was used. Two diodes 1N4148 are used to prevent damaging the transistors from thermal runaway and to bias the transistors along with the two 3.9Kohm resistors. Two coupling capacitors(see amplifier design tutorial) are used at the input and output of the class AB amplifying circuit. An input sinusoid signal of 0.5V amplitude and 1KHz is applied to the amplifier and a supply of 20V VDC is applied as shown in the circuit schematic. The 10ohm resistor is the load resistor which is equivalent to a loudspeaker resistance.
Simulation:
The simulation was set up for transient analysis with run time 20ms and step size of 10us(micro second). The input(Vin) and output(Vout) voltage signal was plotted and the result is shown below-
 |
| Fig: Input Output Signal Waveform of a Class AB Power Amplifier |
From the waveform chart above we can see that the input signal peak to peak voltage is 1V whereas the output signal peak to peak voltage is 0.8V.
Similarly the clipped waveform from the upper and the lower transistors for this class AB amplifier is shown below-
 |
| Fig: Clipped Signals from complimentary transistors of a class AB Power Amplifier |
Thus the class AB amplifier circuit shown above works as per the simulation.
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