Reactance and Impedance

Impedance

Impedance, Z =  V  I

Resistance, R =  V  I

V = voltage in volts (V) I  = current in amps (A) Z = impedance in ohms () R = resistance in ohms ()

Impedance (symbol Z) is a measure of the overall opposition of a circuit to current, in other words: how much the circuit impedes the flow of current. It is like resistance, but it also takes into account the effects of capacitance and inductance. Impedance is measured in ohms, symbol . Impedance is more complex than resistance because the effects of capacitance and inductance vary with the frequency of the current passing through the circuit and this means impedance varies with frequency! The effect of resistance is constant regardless of frequency.
The term 'impedance' is often used (quite correctly) for simple circuits which have no capacitance or inductance - for example to refer to their 'Input Impedance' or 'output impedance'. This can seem confusing if you are learning electronics, but for these simple circuits you can assume that it is just another word for resistance.
Four electrical quantities determine the impedance (Z) of a circuit:
resistance (R), capacitance (C), inductance (L) and  frequency (f).
Impedance can be split into two parts:

• Resistance R (the part which is constant regardless of frequency)
• Reactance X (the part which varies with frequency due to capacitance and inductance)

Matching for Impedance

How important is impedance matching?

It can be shown mathematically that any source of power e.g. a receiver antenna, an audio amplifier or an amateur radio transmitter will deliver its maximum possible power output when the impedance of the subsequent load is equal to the internal impedance of that source. This is achieved through impedance matching.
Unfortunately in this situation the efficiency can only be 50% because half the power is consumed in the source.

Examples of Impedance Matching

Our first example is one of an old type transistor radio where in the audio stages the output speaker was a typical 8 ohms. Such stages are impedance matched by audio transformers as depicted in figure 1 below.

Fig 1 - schematic of impedance matching of transistor audio stages

Transformer for Audio

Audio transformers are "wide band" transformers. In essence a "transformer" is two or more windings coupled by a common magnetic field. It is this magnetic field which provides the means to pass voltages and currents from the primary winding to the secondary winding when alternating current flows.
Essentially the main purpose of an interstage audio transformer is to isolate the DC and couple the signal, with minimal loss. The transformer windings look like short circuits to DC, yet are seen as complex impedances to the AC signal.
Much which follows on the topic of audio transformers is of necessity somewhat over simplified to give a general overview. A transformer schematic is depicted in Figure 1 below.

Figure 1. - audio transformers schematic