Complete Electronics Self-Teaching Guide with Projects

Alternative Base Switching

28 In the examples of transistor switching, the actual switching was performed using a small mechanical switch placed in the base circuit of the first transistor. This switch has three terminals and switches from position A to position B. (This is a single-pole, double-throw switch.) This switch does not have a definite ON or OFF position, as does a simple ON-OFF switch.

Question

Why couldn't a simple ON-OFF switch with only two terminals have been used with these examples? _____

Answer

An ON-OFF switch is either open or closed, and cannot switch between position A and position B, as shown earlier in Figure 4.19.

29 If you connect R₁, R₂, and a switch together, as shown in Figure 4.20, you can use a simple ON-OFF switch with only two terminals. (This is a single-pole, single-throw switch.)

Figure 4.20

Questions

A. When the switch is open, is Q₁ ON or OFF? _____

B. When the switch is closed, is the lamp ON or Off? _____

Answers

A. OFF

B. ON

30 When the switch is closed, current flows through R₁. However, at point A in Figure 4.20, the current divides into two paths. One path is the base current I_B, and the other is marked I₂.

Question

How could you calculate the total current I₁? _____

Answer

31 The problem now is to choose ...