Experiment 3 - The Common Emitter Circuit

The first basic circuit you examine is the common emitter circuit. This circuit corresponds to the normal operation of a transistor.

Consider the circuit shown. The circuit of the Experiment 1 is readily changed to obtain a proper common emitter circuit...

Common emitter circuit. (Enlarge)

Circuit diagram of a common emitter circuit.

The circuit is designed so that when pressing the push-button switch S1 of the transistor is saturated and completely switches through...

• Pushbutton switch open...the transistor blocks and the LED lights up.
  (A current flows through R3 and LED1.)
• Pushbutton switch closed...the transistor switches through and the LED goes out.
  (The current flows through R3 and T1.)

This is the opposite outcome of the first experiment, a proper common emitter circuit inverts the input signal!

But what about the gain of the current in a common emitter circuit...?