Summary

In this chapter, you learned about the transistor switch and how to calculate the resistor values required to use it in a circuit.

  • You worked with a lamp as the load example because this provides an easy visual demonstration of the switching action. All the circuits shown in this chapter work when you build them on a breadboard, and the voltage and current measurements are close to those shown in the text.
  • You have not yet learned all there is to transistor switching. For example, you haven't found out how much current a transistor can conduct before it burns out, what maximum voltage a transistor can sustain, or how fast a transistor can switch ON and OFF. You can learn these things from the data sheet for each transistor model, so these things are not covered here.
  • When you use the JFET as a switch, it does not switch as fast as a BJT, but it does have certain advantages relating to its large input resistance. The JFET does not draw any current from the control circuit to operate. Conversely, a BJT will draw current from the control circuit because of its lower input resistance.

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