Now we begin the study of transistor circuits. Transistors have three terminals. Usually one of the terminals is the input, another is the output, and the third is a common connection that is shared between the input and the output. Transistor circuits can increase the power of a signal. For this they require an additional DC power source. Circuits that increase power are called active circuits. By comparison, a passive circuit has loss. The filters we covered in the earlier chapters are examples of passive circuits. We will study several different active circuits. An amplifier increases the power of a signal without changing the frequency. In an oscillator, an output sine wave is generated without any input signal. Transistors can also be used in passive circuits. In Problem 5, we saw that a transistor could act as a fast switch, with either a low resistance between the output terminals or a high resistance, depending on the input voltage. We will also use a transistor as a variable attenuator to control the signal level.

Manufacturers can combine many transistors on one chip of silicon. These circuits are called integrated circuits, or ICs. Many thousands of different integrated circuits are available. One common type of IC includes several amplifiers cascaded one after another, so that the output signal is much larger than the input. These circuits are called op amps, short for operational amplifiers.