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Q - QUALITY FACTOR

Introduction | Questions | Quiz

INTRODUCTION

Q is the quality factor, or a figure of merit, for a resonant circuit.

Q for unloaded resonant circuits:

    Q = X / R

    X = reactance in ohms (inductive or capacitive)
    R = series resistance in ohms

    In unloaded resonant circuits, the greatest power loss occurs in the resistance of the resonant circuit.

Q for loaded parallel resonant circuits:
    Q = R / X

    R = parallel load resistance
    X = reactance in ohms

    To calculate reactance:

      XL = 2piFL or XC = 1 / 2piFC

      2pi = 6.28
      F = frequency in hertz
      L = inductance in henrys
      C= capacitance in farads

Example: Calculate Q for a 14.128 MHz loaded parallel resonant circuit where L = 2.7 microhenrys and R is 18 kilohms.
    Q = 18000 / (6.28 * 14128000 * .0000027)

    Q= 75.1

Adding resistance to a parallel resonant circuit will decrease Q and increase bandwidth.

In loaded resonant circuits, the circuit delivers power to a load. The power dissipated by the resistance of the resonant circuit is negligible in comparison.

A-001-005-001.... What is the Q of a parallel RLC circuit, if it is resonant at 14.128 MHz, L is 2.7 microhenrys and R is 18 kilohms?
  1. 7.51
  2. 0.013
  3. 71.5
  4. 75.1
A-001-005-002.... What is the Q of a parallel RLC circuit, if it is resonant at 14.128 MHz, L is 4.7 microhenrys and R is 18 kilohms?
  1. 13.3
  2. 43.1
  3. 0.023
  4. 4.31
A-001-005-003.... What is the Q of a parallel RLC circuit, if it is resonant at 4.468 MHz, L is 47 microhenrys and R is 180 ohms?
  1. 0.136
  2. 7.35
  3. 0.00735
  4. 13.3
A-001-005-004.... What is the Q of a parallel RLC circuit, if it is resonant at 14.225 MHz, L is 3.5 microhenrys and R is 10 kilohms?
  1. 7.35
  2. 31.9
  3. 0.0319
  4. 71.5
A-001-005-005.... What is the Q of a parallel RLC circuit, if it is resonant at 7.125 MHz, L is 8.2 microhenrys and R is 1 kilohm?
  1. 2.73
  2. 36.8
  3. 0.368
  4. 0.273
A-001-005-006.... What is the Q of a parallel RLC circuit, if it is resonant at 7.125 MHz, L is 10.1 microhenrys and R is 100 ohms?
  1. 22.1
  2. 0.00452
  3. 0.221
  4. 4.52
A-001-005-007.... What is the Q of a parallel RLC circuit, if it is resonant at 7.125 MHz, L is 12.6 microhenrys and R is 22 kilohms?
  1. 39
  2. 22.1
  3. 0.0256
  4. 25.6
A-001-005-008.... What is the Q of a parallel RLC circuit, if it is resonant at 3.625 MHz, L is 3 microhenrys and R is 2.2 kilohms?
  1. 25.6
  2. 31.1
  3. 32.2
  4. 0.031
A-001-005-009.... What is the Q of a parallel RLC circuit, if it is resonant at 3.625 MHz, L is 42 microhenrys and R is 220 ohms?
  1. 2.3
  2. 4.35
  3. 0.23
  4. 0.00435
A-001-005-010.... What is the Q of a parallel RLC circuit, if it is resonant at 3.625 MHz, L is 43 microhenrys and R is 1.8 kilohms?
  1. 0.543
  2. 54.3
  3. 23
  4. 1.84
A-001-005-011.... Why is a resistor often included in a parallel resonant circuit?
  1. to increase the Q and decrease the skin effect
  2. to decrease the Q and increase the resonant frequency
  3. to increase the Q and decrease bandwidth
  4. to decrease the Q and increase the bandwidth
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