Low Power Push Pull Audio Amplifier

LOW-POWER PUSH PULL AUDIO AMPLIFIER

This is a low noise amplifier, which is only really suitable for 32 ohms headphone usage. It should not be used to drive a 4 ohm or 8 ohm loudspeaker. Doing this, would definitely result in potentially fatal overheating of the small transistors which are employed in this design. (NPN: 2N3904 PNP: 2N3906)

It's a viable contender to the popular but noisy LM386 package integrated circuit amplifier. You will notice that the output contains a 10 ohm resistor. This is placed there as a safeguard against potential mishaps involving the use of low-ohm loudspeakers.

CIRCUIT DESCRIPTION

From left to right, we have a signal generator which represents the input to the amplifier. Following this, there is a 5K ohm potentiometer which must be log-law. Then, we see a simple RC filter network which rolls off all frequencies above 10 KHz. This results in the amplifier having severe attenuation of radio frequencies, which otherwise would waste power. It is also a noise reducer.

The input to the first transistor is achieved via the 1 µF ceramic capacitor. This transistor is a type of current source and serves as the sole source for voltage amplification. There is a feedback resistor of 100 K ohms. The DC bias is provided by this and acts as negative feedback, stabilising the amplifier.

After this, there are the two current amplifier transistors. The bases of each of these are separated by signal diodes, such that we obtain a 1.4 volts difference to compensate for the internal volt drop within each of these transistors. You will also note that there is a 4.7K current limiting resistor from VCC to these diodes.

The circuit is shown to be powered by 12 volts, which could be between 6 and 20 volts.