Introduction to transistors

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from Getting Started in Electronics, by Forrest Mims


  • small components used either as switches or as amplifiers
  • have three pins: base, collector, and emitter


  • fast switching, since all electronic and no mechanical moving parts (unlike relays or manual switches)
  • great for DC-to-DC signal switching or amplification
  • not good for DC-to-AC switching or amplification
  • not good for very heavy loads (use a relay instead)

as switches

When wired up as switches, transistors allow low-power signal to turn on higher power signal

  • when the base of the transistor has 0V applied to it (i.e. grounded), the switch is open (i.e. it is off: no current flows from the collector to the emitter)
  • when the base has about ~0.6V applied to it, the switch is closed (i.e. it is on: current flows from collector to emitter).

This behavior useful for circuits with microcontrollers, since a microcontroller can easily be wired up to a transistor's base to turn a load on or off.

as amplifiers

When hooked up as amplifiers, transistors take a low-power signal and convert it to a higher-power signal

  • small variations in the current supplied to the base of the transistor lead to larger variations in the collector-to-emitter current.
  • the amount of amplified current from collector-to-emitter is throttled by the amount of current to the base, even if there is a higher voltage difference from collector-to-emitter.
  • in this way, transistors behave differently from resistors - current flow from collector-to-emitter is not merely linearly correlated with amount of voltage, as it is in resistors (a la Ohm's Law)
  • every transistor has a specific gain factor, indicating how much it amplifies the base signal. This is specified as the transistor's "β" (beta) or “hfe" factor, which you can find in the data sheet documentation for any given transistor model.

Example circuits

Transistor amplifying signal from solar cell(s). Note that the amount of voltage/current produced by a solar cell varies based on the type of cell used, and the amount of light hitting it. The current flowing through the higher power circuit will vary based on how much current hits the base of the transistor.

Transistor amplifying a sound signal. Note that microphones produce a very tiny amount of AC current, which may need to be preamplified and rectified before it can activate the base of the transistor.

Integrated circuits

Integrated circuits are effectively miniaturized circuits.

  • transistors were invented in 1940s
  • integrated circuits invented in 1958
  • transistors make up the largest percentage of components within any integrated circuit (IC, a.k.a. chip).
    • so you can think of most chips as being composed of lots of miniature switches hooked up together in complicated patterns
  • number of transistors in an integrated circuit has ballooned:
    • 1960's: a few
    • 1970's: a few thousand
    • 1980's: a few million
    • a typical microprocessor today contain several billion transistors
    • newer forms of transistors will be needed in order to be able to miniaturize them further
  • Moore's Law, penned in 1965, said that the number of transistors in an IC will double every year. This is still somewhat true today.


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