Class B Audio Power Amplifier

 Here a class B audio power amplifier with 2N3904 and 2N3906 transistors is presented and its working principle explained. 2N3904 is NPN BJT and 2N3906 is PNP BJT which is used at the output stage of the audio amplifier. These transistors are readily available and with some other passive components found in junkbox this audio amplifier can be build in no time. Usually audio amplifier op-amp IC such as LM386 audio amplifier ICare used but sometimes you may not have such IC(Integrated Circuit) and you may want to build one with discrete components and here is one way you can build one. The presented audio amplifier is also very efficient and simple to build. 

The audio amplifier circuit diagram is shown below.

audio amplifier circuit diagram

How the audio amplifier circuit works?

In the above circuit diagram, the audio either from microphone or other source such as PC or mobile enters the audio amplifier via the input labelled Vin. The R1 is a current limiting resistor and C1 is a coupling coupling capacitor. The first 2N3904 transistor Q1 acts as a gain stage that amplifiers the incoming small signal into its base. The second 2N3904 transistor Q2 acts as a constant voltage source for the next push pull circuit. The NPN transistor 2N3904 Q3 and the PNP transistor 2N3906 Q4 forms the class B push pull amplifier. The resistor R3 provides a negative feedback path from the output to the input that stabilizes the gain and bias of the gain circuit. The circuit can be powered with 5V to 15V power source.

We can perform simulation to test the circuit in electronics design software such as Proteus. The circuit is fed with 10mV amplitude and 1KHz sinusoidal signal representing an audio signal. The following shows the test pins on the circuit and signal waveform of the input, the signal after the gain stage and the output signal.

testing audio amplifier circuit

signal waveforms

We can test the audio circuit with actual audio input signal from an electret microphone.and using a loudspeaker at the output of the audio amplifier. The following shows the circuit with microphone and 8ohm speaker(4ohm speaker also applicable).

audio amplifier testing

For real time simulation and testing the circuit you need to plug in microphone into the PC microphone jack. After that you need to run the simulation and speak into the microphone. The audio will be amplified and heard from the speaker. The following video shows this real time circuit testing process and result.

 The following shows the audio signal waveform at various stages in the circuit.

audio waveform

So here we have shown how you can build your own simple class B audio amplifier with discrete components. General purpose discrete transistors 2N3904 and 2N3906 complementary transistors were used for this purpose. One can also build audio amplifier with LM741 op-amp.

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