Complete Electronics Self-Teaching Guide with Projects

Turning Off the Transistor

11 Up to now, you have concentrated on turning the transistor ON, thus making it act like a closed mechanical switch. Now you focus on turning it OFF, thus making it act like an open mechanical switch. If the transistor is OFF, no current flows through the load (that is, no collector current flows).

Questions

A. When a switch is open, are the two terminals at different voltages or at the same voltage? _____

B. When a switch is open, does current flow? _____

C. For a transistor to turn OFF and act like an open switch, how much base current is needed? _____

Answers

A. At different voltages, the supply voltage and ground voltage.

B. No.

C. The transistor is OFF when there is no base current.

12 You can be sure that there is no base current in the circuit shown in Figure 4.6 by opening the mechanical switch.

Figure 4.6

To ensure that the transistor remains OFF when the base is not connected to the supply voltage, you add a resistor (labeled R₂ in Figure 4.7) to the circuit. The base of the transistor connects to ground or 0 volts through this resistor. Therefore, no base current can possibly flow.

Figure 4.7

Questions

A. Why doesn't current flow from the supply voltage to the base-emitter junction? _____

B. How much current flows ...