Electronics Components: Transistors as a Magic Potentiometer

By Doug Lowe

A transistor within an electronic circuit works like a combination of a diode and a variable resistor, also called a potentiometer or pot. But this isn’t just an ordinary pot; it’s a magic pot whose knob is mysteriously connected to the diode by invisible rays, kind of like this:

When forward voltage is applied to the diode, the knob of the magic pot turns much like the needle on a voltmeter. This changes the resistance of the potentiometer, which in turn changes the amount of current that can flow through the collector-emitter path.

Note that a magic potentiometer is wired so that when bias voltage increases, resistance decreases. When bias voltage decreases, resistance increases.

Besides being connected to the diode by invisible rays, the magic pot is magic in one more way: Its maximum resistance is infinite. Real-world potentiometers have a finite maximum resistance, such as 10 kΩ or 1 MΩ, but the magic pot has infinite maximum resistance.

With this knowledge of the magic pot’s properties in mind, you can visualize how a transistor works. There are three positions that the magic knob can be in, which correspond to these three operating modes for a transistor.

  • Infinite resistance: When there’s no bias voltage, the magic pot’s knob is spun all the way in one direction, providing infinite resistance. Thus no current flows through the transistor. This state is called cut-off because current is cut off. No amps for you!

    Actually, remember that the base of the transistor is like a diode, which means that a certain amount of forward voltage is required before current begins to flow through the base. The magic pot stays at its infinite setting until that voltage — usually about 0.7 V — is reached. This state is called cut-off because current is cut off. No amps for you!

  • Some resistance: As the bias voltage moves past 0.7 V, the diode begins to conduct, and the invisible rays start turning the knob on the magic pot. Thus current begins to flow. How much current flows depends on how far the bias voltage has caused the knob to turn.

  • No resistance: Eventually, the bias voltage turns the knob to its stopping point, and there’s no resistance at all. Current flows unrestricted through the collector-emitter circuit. You can continue to increase the bias voltage, but you can’t lower the resistance below zero! This state is called saturation.