Why parallel resonance is termed as rejecter circuit?

Answer 1

THE PARALLEL rlc CIRCUIT IS CALLED A REJECTOR CIRCUIT BECAUSE IT REJECTS DOWN THE CURRENT. THE REASON IS AT RESONANCE THE IMPEDENCE OF THE CAPACITOR BECOMES EQUAL TO THAT OF THE INDUCTOR SO NO CURRENT FLOWS. AT LOW FREQUENCY THE CAPACITIVE REACTANCE IS LOW SO ALL THE CURRENT FLOWS THROUGH THE INDUCTOR AND WHEN THE FREQUENCY IS HIGH ALL THE CURRENT WILL FLOW THROUGH THE CAPACITOR BECAUSE AT THAT POINT THE REACTANCE OF THE CAPACITOR IS LOW. SO WE OBTAIN A V-SHAPED GRAPH WITH THE PEAK OF V INDICATING THE REJECTION OF CURRENT IN PARALLEL R-L-C CIRCUIT CIRCUIT,AT RESONANCE,IMPEDANCE IS MAXIMUM AND CURRENT IS MINIMUM.HENCE, SUCH A CIRCUIT WHEN USED IN RADIO STATIONS IS KNOWN AS REJECTOR CIRCUIT BECAUSE IT REJECTS OR TAKES MINIMUM CURRENT OF THAT DESIRED FREQUENCY TO WHICH IT RESONATES.(THIS RESONANCE IS OFTEN REFERRED TO AS CURRENT RESONANCE BECAUSE THE CURRENT CIRCULATING BETWEEN THE TWO BRANCHES IS MANY TIMES GREATER THAN THE LINE CURRENT TAKEN FROM THE SUPPLY.THE PHENOMENON OF PARALLEL RESONANCE IS OF GREAT PRACTICAL IMPORTANCE BECAUSE IT FORMS THE BASIS OF TUNED CIRCUITS IN ELECTRONICS.)A PARALLEL R-L-C CIRCUIT HAS THE PROPERTY OF SELECTIVITY I.E.IT CAN SELECT THE DESIRED FREQUENCY FOR AMPLIFICATION OUT OF A LARGE NUMBER OF FREQUENCIES SIMULTANEOUSLY IMPRESSED UPON IT.FOR INSTANCE IF A MIXTURE OF FREQUENCIES INCLUDING RESONANT FREQUENCY IS FED TO THE INPUT THEN MAXIMUM AMPLIFICATION OCCURS FOR THE RESONANT FREQUENCY.FOR ALL OTHER FREQUENCIES ,THE CIRCUIT OFFERS VERY LOW IMPEDANCE AND HENCE THESE ARE AMPLIFIED TO A LESSER EXTENT AND MAY BE THOUGHT AS REJECTED BY THE CIRCUIT.