Circuit basics (3) — Transistor example

State of Transistor
  •  If there is zero current though it, the transistor is in “CUTOFF” state.
  • If there is maximum current through it, called “saturation”.
  • Anywhere between saturated and cutoff is called “active” mode.
transistor_3
An example of common Emitter
transistor_6
1. Solar cell serves as a light sensor: by using a solar cell to provide 1V (> 0.7V),
  • With the solar cell darkened, the transistor behave as an open switch between collector and emitter.
  • When the solar cell is powered, electrons will flow from the emitter through to the base, up to the left side of the lamp, back to he positive side of the battery —A closed switch
2. Common emitter can e used to produce an output voltage derived from the input signal, rather than a specific output current.
transistor_7 
  • With the solar cell darkened, the transistor behave as an open switch between collector and emitter. there is maximum voltage drop between collector and emitter.
  • When the solar cell is fully powered, minimum voltage drop between collector and emitter.
Note:
  • For an amlifier to work properly, it must be biased on on the time (not just when a signal is present).
    • “On” means current is flowing through the transistor (VB = 0.6 ~ 07V). Usually, a CD circuit is used to achive the biasing.
Common emitter:
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