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4.2 Impedance and Admittance 167 In the circuit shown in Figure 4.2-24, the capacitor and inductor values are such that the modulus of the susceptance of the capacitor | | is greater than that of the inductor | | . The overall admittance of the L-C parallel is which means that the capacitor dominates and the current leads the voltage (Figure 4.2-24). Now if we consider the circuit shown in Figure 4.2-25, we can see that in this case | | | | This means that the parallel combination of capacitor and inductor is resonant and its admittance is zero! This means that the parallel L-C circuit acts as an open circuit at 1000 MHz and hence voltage and current are in phase just as they would be if no reactive elements were present. Figure 4.2-25 Parallel R-L-C with inductor dominating | | | | Now let us consider the circuit shown in Figure 4.2-26. In this circuit we have a source and a load impedance (RS and RL) connected by a parallel L-C circuit. Once the values of C and L are fixed, if we sweep the frequency across a wide range starting from DC, our parallel L-C will appear inductive ( | | > | | ) up to the resonant frequency 30 √ It will look capacitive for frequencies higher than , since | | < | | , and it will behave as an open-circuit (zero admittance) at the resonant frequency thereby preventing the signal from reaching the load. We will therefore experience a "band-stop" effect, as shown in Figure 4.2-27. This is confirmed by the voltage and current profiles at the resonant frequency, which are shown in Figure 4.2-28. This figure shows that, at resonance, the voltage across and the current through the parallel L-C are in phase but their amplitudes are very small! 30 Similarly to the previous case the resonant frequency satisfies the condition below | | | | 0 0.5 1 1.5 2 Time (ns) parallel_R_L_C -1 -0.5 0 0.5 1 -40 -20 0 20 40 p2 p1 Vtime(M_PROBE.VP1,1)[*] (L, V) parallel_R_L_C Itime(ACVS.V1,1)[*] (R, mA) parallel_R_L_C p1: Freq = 1000 MHz p2: Freq = 1000 MHz Freq = 1000 MHz V mA Conquer Radio Frequency 167 www.cadence.com/go/awr

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