Hows does a transistor amplify sound?

[Al Skierkiewicz]

WH,
This is kind of hard subject to explain when one does not understand the electronic basics of voltage and current in resistance. When a voltage is connected across a resistance current flows. Also if something causes a current to flow through a resistance, a voltage is developed. In your simple microphone transistor amplifier model, the microphone generates a voltage when sound occurs near the microphone element. But the transistor is a current controlled device so the microphone voltage is connected across a resistance to generate a small current. This current, is in turn, connected to the transistor base lead. However, it still will do nothing until another current is generated in the collector to emitter path. To make that current, a power supply voltage (i.e. a battery) is then connected across another resistor and the collector/emitter circuit. The small current coming into the base then can control the current in the collector/emitter circuit causing it to increase and decrease relative to the incoming signal from the microphone. This varying current then generates a varying voltage across the resistor that was used to generate the collector/emitter current. Please remember that there are two separate currents at play here. The current flowing into the transistor base and the current flowing in the collector/emitter circuit. This is an amplifier in it's simplest form, a base resistor, a collector resistor, the transistor, a battery and a input capable of generating base current.
To confuse things, if you have started to understand schematics, you might find an audio amplifier that has many resistors, and transistors. These additional elements allow the signal to be amplified several hundred times while making the circuit stable under all conditions.
The ARRL Radio Amateurs Handbook is an excellent place to begin your quest of understanding. Every public library has one on the shelf and they are available from the ARRL directly at http://www.arrl.org/shop/The-ARRL-Ha...cover-Edition/ For $49.95 it could make a nice Christmas gift and it comes with a searchable CD.

[Dick Linn]

Search Youtube for "how transistor works". You'll find some helpful stuff, including the basics: http://www.youtube.com/watch?v=ZaBLiciesOU