two stage audio amplifier.

INTRODUCTION:

In order to successfully troubleshoot any electronic circuit to the component level, one must have a good understanding of each component’s function within the context of that circuit. Transistor amplifiers are no exception to this rule. The following schematic shows a simple, two-stage audio amplifier circuit:

CIRCUIT DIAGRAM:

 

COMPONENTS:

The 0.47 μF capacitor connected to the microphone

The 220 k Ω and 27 k Ω resistor pair

The 4.7 μF electrolytic capacitor connected across the 1.5 k Ω resistor

The 33 μF electrolytic capacitor connected to the speaker

The 47 μF electrolytic capacitor connected to the power supply rail

Two transisitor’s of bc 547

Resistors.

WORKING:

The 0.47 μF capacitor connected to the microphone: passes (AC) audio signal, blocks DC bias voltage from reaching microphone

The 220 k Ω and 27 k Ω resistor pair: sets DC bias voltage for first transistor stage

The 4.7 μF electrolytic capacitor connected across the 1.5 k Ω resistor: bypasses (AC) audio signal around emitter resistor, for maximum AC voltage gain

The 33 μF electrolytic capacitor connected to the speaker: couples (AC) audio signal to speaker while blocking DC bias voltage from speaker

The 47 μF electrolytic capacitor connected to the power supply rail: “decouples” any AC signal from the power supply, by providing a low-impedance (short) path to ground.

CONCLUSION:

Incidentally, this circuit makes a good intercom” amplifier for a student project. Using a small dynamic speaker for the microphone, and another speaker (or audio headset) on the receiving end of a long cable connected to the amplifier output, students can easily talk between two rooms in a building, or even between buildings.