What is advantage and disadvantage of impedance coupling?

Answer 1

The term impedance coupling can be used in two ways.

1) Example, coupling a signal from the output of one stage

of an amplifier, to the input of the next stage. For low power

as under about 0.1 watt, RC coupling is cheap and adequate

as little power is dissipated in the collector resistor of a

bipolar amplifier. However at higher power levels impedance

coupling is more efficient, but also more expensive. An

inductor simply replaces the resistor used for RC coupling.

Unlike a resistor the inductors impedance is frequency dependent.

This varies the level of coupling voltage available over the amps

bandwidth, and needs to be accounted for at the design stage.

An advantage is that by the addition of a parallel capacitor,

an LC circuit can be formed to produce the basis for a band

pass amplifier. Further by providing a tapped inductor; coupling,

tuning and impedance matching functions can be obtained.

Transformer coupling is a form of impedance coupling and

can provide galvanic isolation as well as matching.

2) Unbalanced circuits, that is those having a 'high' or 'signal' lines

and a common ground,chassis or earth; have the disadvantage

that undesired signal coupling can occur when the higher current

signals produce small voltage drops along the common ground.

To minimize these undesired voltage variations grounds on PCBs

are given wide low resistance tracks. Also by not mixing analog

and digital signal grounds, the adverse effects of coupling are reduced.

Furthermore, at higher frequencies a narrow length of track also has

inductance, and capacitance to nearby conductors. There components

provide the possibility of additional undesired impedence coupling.

Balanced circuits which do not have one line of any signal grounded

overcome the disadvantages of the simpler unbalanced circuits.