Complete Electronics Self-Teaching Guide with Projects

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, R_B was given, and you had to find the value of collector current and voltages. Now, do the reverse. Start with the current through R_C, and find the value of R_B that turns the transistor ON and permits the collector current to flow.

Figure 4.1

Question

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

Answer

The base and collector currents

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

Questions

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.

Answers

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 In this figure, note that the resistor symbol has been replaced by the symbol for an incandescent lamp.

Figure 4.2

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.

Questions