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4.3 Matching two unequal resistive impedances 181 Let us now look at how the load impedance affects the overall Q of the circuit shown in Figure 4.3-12. In this circuit the load impedance R L is no longer a very large value, as it was in the schematic of Figure 4.3-10, and hence it draws significant current thereby affecting the circuit response. Figure 4.3-12 Parallel RLC with lower load impedance How does this load affect our circuit then? Well first of all, we can draw the equivalent circuit for resonance calculations shown in Figure 4.3-13. Figure 4.3-13 Equivalent circuit for resonance calculations The resonant circuit sees an equivalent resistance R P which is the parallel combination of R S and R L as its actual load. By definition R P is smaller in value than R S and R L . Now let us consider the Q of this resonant circuit, in a similar fashion as we did in section 4.2.8 here is either the inductive or capacitive reactance since these values are the same at resonance. It is apparent that Q is directly proportional to and that increasing increases Q. It also evident that the same effect may be achieved if is kept constant and is decreased! This in turn shows that a large capacitor and a small inductor, would increase the Q of the circuit and hence its selectivity. The designer has therefore two options: 1) Selecting a optimum values of source and load impedance 2) Selecting appropriate values for L and C Usually however, source and load impedances are fixed hence the only control the designer has is over the values of C and L. One must be careful not to end up with impractical values though! So far we have assumed that our capacitors and inductors were lossless but in practice, the finite component Q must be taken into account. Let us now look at the effect of component Q on the performance of the circuit. V SOURCE C L R S R L C L (4.3-3) Conquer Radio Frequency 181 www.cadence.com/go/awr

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