Electrical Engineering ⇒ Topic : Applications of Series Resonant Circuits

Applications of Series Resonant Circuits

We have seen that in a R-L-C series circuit, there is one frequency (called resonant frequency) at which the circuit offers minimum impedance. For all other frequencies, the circuit offers considerable impedance. Thus if alternating currents of various frequencies are supplied to such a circuit, only those close to the resonant frequency would pass through the circuit. This is the key point in series resonance. There are many applications of series resonance in electronics but only two are discussed below by way of illustration :

(1) As a wave trap. Fig. (A) shows a series resonant circuit connected to the antenna circuit of a radio receiver. This series resonant circuit will offer a very low impedance to any signal current whose frequency is equal to f_r (i.e. 1/2π√LC ). However, for all other frequencies, the circuit will offer a high impedance. Thus the circuit will provide a low impedance path to the ground for any signal current of frequency f_r. In this way, the circuit prevents the signal currents of undesired frequency from entering the radio receiver and thus acts as a wave trap.

                                                                        FIGURE (A)

(2) As a tuning circuit. A series resonant circuit is used for tuning purposes. Fig. (B) shows such an application in a radio receiver. The input signal comes from the antenna and induces a voltage E in L of the *series resonant circuit. The voltage across the capacitor becomes **V_C = QE where Q is the quality factor of the circuit. As the value of Q is generally large, the original signal received by the antenna increases many times in value and appears across C. The value of V_C is much more than that could have been obtained by direct transformer ratio. Thus the amplifier receives a greatly increased signal

                                                               FIGURE (B)