The Transistor as an Amplifier

Turning a weak signal into a stronger one.

To amplify, the transistor is first biased so that it sits in the middle of its active region. A small AC signal added to the base then makes the collector current vary in step, and a resistor in the collector turns that varying current into a larger varying voltage.

In the common-emitter arrangement the output is inverted compared with the input. A voltage-divider bias network and an emitter resistor keep the operating point steady, and coupling capacitors pass the signal while blocking DC.

• Bias sets the quiet operating point.
• Voltage gain is output swing divided by input swing.
• Too much input signal distorts the output.

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