Impedance matching
Martin Hartley Jones · Cambridge University Press eBooks · 1995
Introduction The subject of impedance matching is frequently surrounded by an undeserved aura of mystery. When a circuit fails to work as well as expected, impedance mismatch is often blamed in a rather vague sort of way, but correcting the problem may be regarded more as witchcraft than science. The purpose of this chapter is to outline the principles and practice of impedance matching and, in so doing, to attempt to dispel any associations of mystery and magic! Input impedance Any electrical device which requires a signal for its operation has an input impedance. Just like any other impedance (or resistance in d.c. circuits), the input impedance of a device is a measure of the current drawn by the input with a certain voltage across it. For example, the input impedance of a 12 V light bulb rated at 0.5 A is 12/ 0.5Ω, or 24 Ω. The bulb is a clear example of impedance because we know that there is nothing but a filament to consider. The input impedance of a circuit such as bipolar transistor amplifier might seem to be more complicated. At first sight, the presence of capacitors, resistors and semiconductor junctions in a circuit makes the input impedance difficult to assess. However, any input circuit, however complicated, may be resolved into the simple impedance shown in fig. 5.1.