Figure 4.13. One end of an AC generator connects to ground. The other end connects through C 1 to the base of an NPN transistor. The base connects through R 1 to plus 12 volts. The base also connects through R 2 to ground. The collector connects to plus 12 volts. The emitter connects through R 3 to ground. The emitter also connects through C2 to one side of a speaker. The other side of the speaker goes to ground. End verbal description.