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Lesson: Chapter 11 — Electronics

The transistor at work 2 of 4

The signal amplifier

A transistor is also widely used as a signal amplifier — an amplifier of alternating current. Activity 11.4 amplifies an audio-frequency signal.

Activity 11.4

You will need: a 2SD400 transistor, a 22 kΩ carbon resistor, an 8 Ω speaker, a 0.1 µF capacitor, a 3 V battery holder and two cells, a project board, and an audio-frequency signal generator

  1. Build the circuit on the project board.
  2. First connect the signal generator straight to the speaker and set it so the sound is just audible.
  3. Then feed that small signal between A and B.

Figure to be added

Activity 11.4 — the amplifier: input A–B through 0.1 µF to the base, 22 kΩ from the supply to the base, the speaker LS in the collector lead, 3 V supply.

The observation

The sound is amplified and heard from the speaker. The 0.1 µF capacitor passes only the alternating signal to the base; the 22 kΩ resistor gives the base its 0.7 V bias.

A tune from a chip

A UM66 integrated circuit, a 220 Ω resistor and a 3 V battery make a simple circuit giving a musical audio signal (pin 1, signal; pin 2, supply +; pin 3, supply −). Its outputs X and Y can be fed into A and B of the amplifier.