Experiment 6 - The Common Collector Circuit

The next basic transistor circuit you examine is the common collector circuit. You find this circuit often in audio amplifiers, but also in many other applications.

Consider the circuit shown. The circuit converted to obtain a common collector circuit...

Common collector circuit. (Enlarge)

Circuit diagram of a common collector circuit.

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

• Pushbutton switch open...the transistor blocks and the LED goes out.
  (No current flows through T1 or LED1.)
• Pushbutton switch closed...the transistor switches through and the LED lights up.
  (A current flows through T1, R3 and LED1.)

This is not a surprising result when you look at the circuit, a common collector circuit does not invert the input signal!

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