Complete Electronics Self-Teaching Guide with Projects

The Three-Transistor Switch

24 The circuit shown in Figure 4.19 uses three transistors to switch a load on and off. In this circuit, Q₁ is used to turn Q₂ ON and OFF, and Q₂ is used to turn Q₃ ON and OFF. The calculations are similar to those you performed in the last few problems, but a few additional steps are required to deal with the third transistor. Use the circuit diagram in Figure 4.19 to determine the answers to the following questions.

Figure 4.19

Questions

Assume that the switch is in position A.

A. Is Q₁ ON or OFF? _____

B. Is Q₂ ON or OFF? _____

C. Where is current through R₄ flowing? _____

D. Is Q₃ ON or OFF? _____

Answers

A. ON

B. OFF

C. Into the base of Q₃

D. ON

25 Now use the same circuit as in problem 24.

Questions

Assume that the switch is in position B.

A. Is Q₁ ON or OFF? _____

B. Is Q₂ ON or OFF? _____

C. Where is the current through R₄ flowing? _____

D. Is Q₃ ON or OFF? _____

E. Which switch position turns on the lamp? _____

F. How do the ON/OFF positions for the switch in the three-transistor switch differ from the ON/OFF positions for the switch in the two-transistor switch circuit? _____

Answers

A. OFF.

B. ON.

C. Through Q₂ to ground.

D. OFF.

E. Position A.

F. The positions are opposite. Therefore, if a circuit controls lamps with two transistors and another circuit controls lamps with three transistors, flipping ...