Turning the Transistor On

1 Start by examining how to turn a transistor ON by using the simple circuit shown in Figure 4.1. In Chapter 3, RB was given, and you had to find the value of collector current and voltages. Now, do the reverse. Start with the current through RC, and find the value of RB that turns the transistor ON and permits the collector current to flow.


What current values do you need to know to find RB? _____


The base and collector currents

2 In this problem circuit, a lamp can be substituted for the collector resistor. In this case, RC (the resistance of the lamp) is referred to as the load, and IC (the current through the lamp) is called the load current.

A. Is load current equivalent to base or collector current? _____
B. What is the path taken by the collector current discussed in problem 1? Draw this path on the circuit.
A. Collector current
B. See Figure 4.2. In this figure, note that the resistor symbol has been replaced by the symbol for an incandescent lamp.

3 For the transistor switch to perform effectively as a CLOSED switch, its collector voltage must be at the same voltage as its emitter voltage. In this condition, the transistor is said to be turned ON.


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