Complete Electronics Self-Teaching Guide with Projects

Transformers in Communications Circuits

14 In communications circuits, an input signal is often received via a long interconnecting wire (usually called a line) that normally has an impedance of 600 ohms. A typical example is a telephone line between two cities.

Question

Communications equipment works best when connected to a load that has the same impedance as the output of the equipment. What output impedance should communications equipment have? _____

Answer

600 ohms output impedance, to be connected to a 600-ohm line

15 Because most electronic equipment does not have a 600-ohm output impedance, a transformer is often used to connect such equipment to a line. Often, the transformer is built into the equipment for convenience. The transformer is used to “match” the equipment to the line, as shown in Figure 10.10.

Figure 10.10

To work correctly, the output of the transformer secondary coil should have a 600-ohm impedance to match the line. The output impedance of the transformer (measured at the secondary winding) is governed by two things. One of these is the output impedance of the equipment.

Question

What would you expect the other governing factor to be? _____

Answer

The turns ratio of the transformer. (The DC resistance of each coil has no effect, and you can ignore it.)

16 Figure 10.11 shows a signal generator with an output impedance of Z_G connected to ...