MM0704, Lesson 1
The reactance curve (figure 1-8B) shows that, for applied frequencies below the circuit's resonant frequency, the circuit
is inductive, while above resonance, it is capacitive. To decide what kind of reactance is presented when the applied
frequency is above the resonant frequency, think in terms of the total current drawn. Is it leading or lagging current?
Since the parallel LC circuit offers a very high impedance at resonance, it is of primary importance where a relatively
large voltage change is desired as the circuit approaches resonance. It is, therefore, desirable to use a generator of high
internal impedance. In figure 1-9, R is the internal impedance of the generator. If R is very small, then change in the
current drawn by the LC circuit produces little change in the voltage drop across R, and the voltage across LC remains
practically constant as the circuit is varied through resonance. If R is large, any change in current produces large
changes in voltage across R and results in large voltage changes across the resonant circuit.
Figure 1-9. Parallel Resonant Circuit with Generator (R).
Unlike the series resonant circuit, note that increasing the R of the generator improves the selectivity of the circuit
Figure 1-10. Voltage Curves for Parallel Resonant Circuit