TOP RLC Circuit Questions and Answers with explanation – 2022

Ques.41. The impedance of a parallel resonance circuit is _____

  1. Small
  2. Zero
  3. No impedance
  4. Infinity large

Answer.4. Infinity large

Explanation

In a parallel resonance circuit, the total current is zero, the impedance of the parallel RLC circuit is infinitely large.

The parallel resonant circuit has a minimum current near resonance and hence the maximum impedance at that frequency. Hence the circuit will absorb maximum power from a high impedance voltage source at the resonant frequency.

 

Ques.42. The condition of the parallel resonant circuit is

  1. Inductive and capacitive susceptance is equal
  2. Minimum capacitive susceptance
  3. Voltage is maximum
  4. All of the above

Answer.4. All of the above

Explanation

Conditions at resonance in parallel RLC circuit:

  1. It behaves like a purely resistive circuit since net susceptance is equal to zero.
  2. Inductive and capacitive susceptances are exactly equal.
  3. Admittance is minimum and equal to conductance, i.e., Y = G.
  4. The current is minimum and is equal to VG.
  5. The applied voltage and current are in phase and the power factor of the circuit is unity.
  6. The voltage is maximum and is equal to I/G
  7. Current flowing through inductor is equal to current flowing through the capacitor, i.e., IL = IC

 

Ques.43. The parallel resonant LC circuit is also called as a _____.

  1. Tank Circuit
  2. Power circuit
  3. Isolated circuit
  4. None of the above

Answer.1. Tank Circuit

Explanation

At resonance, a parallel tuned circuit appears to have infinite resistance, draws no current from the source and thus has infinite impedance, and acts as an open circuit However’ there is a high circulating current between the inductor and capacitor. Energy is being stored and transferred between the inductor and capacitor. Because such a circuit acts as a kind of storage vessel for electric energy, it is often referred to as a tank circuit and the circulating current is referred to as the tank current.

Tank circuit

 

Ques.44. The bandwidth of a series resonant circuit is the range of frequencies for which the circuit current is equal to or greater than ______of the circuit current at resonance.

  1. 100%
  2. 50%
  3. 70.7%
  4. 10%

Answer.3. 70.7%

Explanation

Bandwidth of a Series Resonant Circuit

Consider the current versus frequency graph of an R-L-C series circuit. It is clear from the graph that the current reaches maximum value (= Ir) at resonance. It is also clear that at frequencies close to resonance, the current level is only a little below its maximum value. Thus the resonant circuit is said to select a band (i.e., range) of frequencies rather than just one frequency fr.

We select frequency f1 below fr and frequency f2 above fr such that at f1 andf2, the circuit current 0.707 Ir where Ir is the circuit current at resonance.

Then, the Bandwidth of the series resonant circuit is

Bandwidth, BW = Δf = f2 —f1

Hence bandwidth of a series resonant circuit is the range of frequencies for which the circuit current is equal to or greater than 70.7% of the circuit current at resonance (i.e., Ir).

 

Ques.45. The high Q factor mean _______ and a low Q factor mean _____

  1. Low selectivity, low selectivity
  2. High selectivity, Low selectivity
  3. Low selectivity, High selectivity
  4. None of the above

Answer.2. High selectivity, Low selectivity

Explanation

Selectivity is the ability of the resonant circuit to extract the resonant frequency and attenuate other frequencies.

Selectivity is measured by a Q-factor, which, for a series-tuned circuit, is the ratio of the inductive (or capacitive reactance) to the total resistance in the circuit (this includes source and load resistances). It is a measure of how well the circuit extracts a band of frequencies with little attenuation but rejects other frequencies outside this band. Selectivity is also dependent on the inductor-capacitor ratio.

A high Q-factor value means high selectivity and a low Q-factor means low selectivity. For good series-tuned selectivity, the circuit must be fed from a voltage source because the source resistance feeding the circuit has minimum resistance and hence has minimum effect on the Q-factor (Ideal voltage source impedance is zero). Any series resistance added will reduce the overall selectivity.

A parallel-tuned circuit is fed from a current source because it has a high source resistance and hence minimum loading. Any external resistive loading reduces the overall selectivity of the parallel-tuned circuit.

 

Ques.46. The cut-off frequency of an RLC circuit is also called as

  1. Full power Frequency
  2. Low power frequency
  3. High power frequency
  4. Half power frequency

Answer.4. Half power frequency

Explanation

The cutoff frequencies are called half-power frequencies because, at those frequencies, the power is half of the power at the resonant frequency.

 

Ques.47. The 3 dB frequency of an RLC circuit is also called as

  1. Full power Frequency
  2. Low power frequency
  3. Cut-off frequency
  4. Half power frequency

Answer.3. Cut-off frequency

Explanation

The cutoff frequencies are called 3 dB frequencies because, at those frequencies, the signal is 3 dB below its maximum value.

 

Ques.48. The following property of a series RLC resonance circuit is listed below. Select the correct option

  1. Voltage and current are in phase
  2. Unity Power factor
  3. Maximum admittance
  4. All of the above

Answer.4. All of the above

Explanation

The properties of series resonance are following:

  1. The voltage and current are in phase. So the power factor is unity.
  2. The net reactance is zero because of XL = XC.
  3. So the impedance of the circuit at resonance is pure resistance and is the minimum.
  4. The admittance of the circuit at resonance is maximum.
  5. The current is maximum.
  6. VL = VC and the net voltage across LC combination is zero but each voltage is many times greater than V, i.e., voltage magnification takes place.

 

Ques.49. In an ideal tank circuit, the total current from the source at resonance is ______

  1. Minimum
  2. Maximum
  3. Twice
  4. Half

Answer.1. Minimum

Explanation

In the ideal tank circuit, the total current from the source at resonance is zero because the impedance is infinite. In the nonideal case when the winding resistance is considered, there is some total current at the resonant frequency, and it is determined by the impedance at resonance.

Itot = Vs/Zr

 

Ques.50. The phase angle of the parallel resonant circuit is______

  1. 90°
  2. 180°
  3. −90°

Answer.4.

Explanation

The phase angle of the parallel resonant circuit is 0° because the impedance is purely resistive at the resonant frequency.


 

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