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4.2 Impedance and Admittance 169 4.2.6 The Ubiquitous Quality Factor, Q In this section we will define an important and useful quantity: the quality factor, Q. Several quality factors may be defined for reactive components and the circuits which contain them. They generally relate the energy loss versus the energy stored in reactive components during a signal cycle. Non-ideal capacitors and inductors will introduce losses since they dissipate energy in the parasitic elements which are introduced by their physical construction and packaging. The unloaded Q, , of a reactive component is defined as shown below This quality factor takes into account the energy dissipated in the component alone and not in the rest of the circuit. For ideal components would of course be infinite. For a real inductor we can represent the losses by means of a series resistor (Figure 4.2-29) Figure 4.2-29 Series R-L, the resistance is used to model power dissipation in a non-ideal inductor and define the unloaded Q of this series connection as The lower the series resistance , the lower the voltage drop across it and the power dissipated in it, hence the higher the Q! For a real capacitor we can represent the losses by means of a shunt resistor (Figure 4.2-30) Figure 4.2-30 Parallel R-C, the resistance is used to model power dissipation in a non-ideal capacitor and define the unloaded Q of such a parallel connection as The greater the shunt resistance , the less current will flow through it and the less power will be dissipated in it, hence the greater the Q. The unloaded Q of individual components does have an effect on the overall quality factor of a network and particularly on its frequency response! The quality factor Q is also very useful when it comes to carrying out series to parallel conversions of electrical network since it simplifies the maths considerably as we will see in the next section. Conquer Radio Frequency 169 www.cadence.com/go/awr

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