1. TOPIC 3: FREQUENCY SELECTIVE CIRCUITS

2. Transfer Function Frequency Selective Circuits
INTRODUCTION
Transfer Function
Frequency Selective Circuits

3. TRANSFER FUNCTION
The s-domain ratio of the Laplace transform of the output (response) to the Laplace transform of the input (source) when all initial conditions are zero.
The transfer function depends on what is defined as the output signal.

4. DEFINITION

5. POLES AND ZEROS
The roots of the denominator polynomial are called the poles of H(s): the values of s at which H(s) becomes infinitely large.
The roots of the numerator polynomial are called the zeros of H(s): the values of s at which H(s) becomes zero.

6. FREQUENCY RESPONSE
The transfer function is a useful tool to compute the frequency response of a circuit (i.e. the steady state response to a varying-frequency sinusoidal source).
The magnitude and phase of the output signal depend only on the magnitude and phase of the transfer function, H(j).

7. FREQUENCY RESPONSE
Frequency response analysis is used to analyze the effect of varying source frequency on circuit voltages and currents.
The circuit’s response depends on:
the types of elements
the way the elements are connected
the impedance of the elements

8. FREQUENCY SELECTIVE CIRCUITS
Frequency selective circuits is a circuits that pass to the output only those input signals that reside in a desired range of frequencies.
Can be constructed with the careful choice of circuit elements, their values, and their connections.

9. Passband & Stopband Cutoff Frequency Bode plot
PASSIVE FILTERS
Passband & Stopband
Cutoff Frequency
Bode plot

10. PASSIVE FILTERS Frequency selective circuits are also called filters.
Filters attenuate, that is weaken or lessen the effect of any input signals with frequencies outside a particular frequency.
Called passive filters because their filtering capabilities depend only on the passive elements (i.e. R,L,C).

11. PASSBAND & STOPBAND
The signal passed from the input to the output fall within a band of frequencies called Passband.
Frequencies not in a circuit’s passband are in its Stopband.
Filters are categorized by the location of the passband.

12. FREQUENCY RESPONSE PLOT
One way of identifying the type of filter circuit is to examine a frequency response plot.
Two parts: one is a graph of H(j)  vs frequency. Called magnitude plot.
The other part is a graph of (j) vs frequency. Called phase plot.

13. TYPE OF FILTERS Low Pass Filter High Pass Filter Band Pass Filter
Band Reject Filter

14. FILTER’S FREQUENCY RESPONSE
lowpass
highpass c c
bandpass
bandreject
c1
c2
c2
c1

15. MAGNITUDE TYPE H(0) H(∞) H(ωC)@H(ωo) LOWPASS 1 1/√2 HIGHPASS BANDPASS
1/√2
HIGHPASS
BANDPASS
BANDREJECT

16. CUT OFF FREQUENCY
LPF and HPF have one passband and one stopband, which are defined by the cut off frequency that separates them.
BPF passes a input signal to the output when the input frequency is within the band defined by the two cut off frequencies.
BRF passes a input signal to the output when the input frequency is outside the band defined by the two cut off frequencies.

17. CUT OFF FREQUENCY
The cutoff frequency (fc) is the frequency either above which or below which the power output of a circuit, such as a line, amplifier, or filter, is reduced to 1/2 of the passband power; the half-power point.
This is equivalent to a voltage (or amplitude) reduction to 70.7% of the passband, because voltage, V2 is proportional to power, P.

18. CUT OFF FREQUENCY
This happens to be close to −3 decibels, and the cutoff frequency is frequently referred to as the −3 dB point.
Also called the knee frequency, due to a frequency response curve's physical appearance.

19. CUT OFF FREQUENCY 6

20. BODE PLOT
The most common way to describe the frequency response is by so called Bode plot.
Bode lot is a log-log plot for amplitude vs frequency and a linear-log plot for phase vs frequency.
Many circuits (e.g. amplifiers, filters, resonators, etc.) uses Bode plot to specify their performance and characteristics.

21. FILTER’S RESPONSE BODE PLOT

22. LOW PASS FILTER

23. LOW PASS FILTER (LPF)
The filter preserves low frequencies while attenuating the frequencies above the cut off frequencies.
There are two basic kinds of circuits that behave as LPFs:
Series RL
Series RC.

24. (a) LPF RL CIRCUIT
OUTPUT

25. Transfer Function

26. The voltage transfer function
To study the frequency response, substitute s=j:

27. Magnitude and Phase

28. When =0 and =

29. Qualitative Analysis At low frequencies (L<< R):
jL is very small compared to R, and inductor functions as a short circuit.

30. Qualitative Analysis At high frequencies (L>> R):
jL is very large compared to R, and inductor functions as a open circuit.

31. LPF Frequency Response
H(j)
1.0
0 
-90  c 

33. Cutoff Frequency
At the cutoff frequency, voltage magnitude is equal to (1/2)Hmax :

34. Ex.
Electrocardiograph is an instrument that is used to measure the heart’s rhythmic beat. This instrument must be capable of detecting periodic signals whose frequency is about 1 Hz (the normal heart rate is 72 beats per minute).
The instrument must operate in the presence of sinusoidal noise consisting of signals from the surrounding electrical environment, whose fundamental frequency is 50 Hz- the frequency at which electric power is supplied.

35. Ex.
Choose values for R and L in the series RL circuit such that the resulting circuit could be used in an electrocardiograph to filter out any noise above 10 Hz and pass the electric signals from the heart at or near 1 Hz. (choose L=100 mH)
Then compute the magnitude of Vo at 1 Hz, 10 Hz, and 50 Hz to see how well the filter performs.

36. Known quantities Inductor, L = 100 mH Cut off frequency, fc = 10 Hz
therefore, c = 2fc = 20 rad/s

37. Find R

38. Find the magnitude of Vo
Using the transfer function, the output voltage can be computed:

39. f(Hz)
Vi 
Vo  1
1.0
0.995 10
0.707 50
0.196

40. (b) LPF RC CIRCUIT
OUTPUT

41. Transfer Function

42. Magnitude and Phase of H(j)

43. Frequency increasing from zero:
Zero frequency (=0):
the impedance of the capacitor is infinite, and the capacitor acts as an open circuit.
Vo and Vi are the same.
Frequency increasing from zero:
the impedance of the capacitor decreases relative to the impedance of the resistor
the source voltage divides between the resistive impedance and the capacitive impedance.
Vo is smaller than Vi.
Infinite frequency (=):
the impedance of the capacitor is infinite, and the capacitor acts as an open circuit.
Vo is zero.

44. Cutoff Frequency
The voltage magnitude is equal to (1/2) Hmax at the cutoff frequency:

45. GENERAL LPF CIRCUITS
OUTPUT
OUTPUT

46. Ex. For the series RC circuit of LPF:
Find the transfer function between the source voltage and the output voltage
Choose values for R and C that will yield a LPF with cutoff frequency of 3 kHz.

47. a) Find the transfer function
The magnitude of H(j):

48. b) Find R & C
R and C cannot be computed independently, so let’s choose C=1F.
Convert the specified cutoff frequency from 3 kHz to c=2(3x10-3) rad/s.

49. Calculate R

50. HIGH PASS FILTER

51. HIGH PASS FILTER
HPF offer easy passage of a high frequency signal and difficult passage to a low frequency signal.
Two types of HPF:
RC circuit
RL circuit

52. a) CAPACITIVE HPF
OUTPUT

53. s-Domain Circuit

54. When =0 and =

55. Frequency increasing from zero:
Zero frequency (=0):
the capacitor acts as an open circuit, so there is no current flowing in R.
Vo is zero.
Frequency increasing from zero:
the impedance of the capacitor decreases relative to the impedance of the resistor
the source voltage divides between the resistive impedance and the capacitive impedance.
Vo begins to increase.
Infinite frequency (=):
the capacitor acts as an short circuit and thus there is no voltage across the capacitor.
Vo is equal to Vi.

56. HPF Frequency Response

57. Transfer Function

58. Magnitude & Phase

59. Ex: INDUCTIVE HPF
Show that the series RL circuit below also acts as a HPF.

60. Ex. Derive an expression for the circuit’s transfer function
Use the result from (a) to determine an equation for the cutoff frequency
Choose values for R and L that will yield HPF with fc = 15 kHz.

61. s-Domain circuit

62. Transfer Function

63. Magnitude & Phase
H()=1 andH(0)=0  HPF

64. Cutoff Frequency

65. R and L
Choose R=500 , and convert fc to c:

66. GENERAL HPF CIRCUITS
OUTPUT
OUTPUT

67. REMARKS
The components and connections for LPF and HPF are identical but, the choice of output is different.
The filtering characteristics of a circuit depend on the definition of the output as well as circuit components, values, and connections.
The cutoff frequency is similar whether the circuit is configured as LPF or HPF.

68. BANDPASS FILTER

69. Bandpass Filter
BPF is essential for applications where a particular band or frequencies need to be filtered from a wider range of mixed signals.
There are 3 important parameters that characterize a BPF, only two of them can be specified independently:
Center frequency (and two cutoff frequencies)
Bandwidth
Quality factor

70. Cutoff freq. & Center freq.
Ideal bandpass filters have two cutoff frequencies, c1 and c2, which identify the passband.
c1 and c2 are the frequencies for which the magnitude of H(j) equal (1/2).
The center frequency, o is defined as the frequency for which a circuit’s transfer function is purely real.
Also called as the resonant frequency.

71. Bandwidth,  and Quality Factor, Q
The bandwidth,  tells the width of the passband.
The quality factor, Q is the ratio of the center frequency to the bandwidth.
The quality factor describes the shape of the magnitude plot, independent of frequency.

72. a) BPF: Series RLC

73. At =0 and =

74. At =0 and = Zero frequency (=0): Infinite frequency (=):
the capacitor acts as an open circuit and the inductor behaves like a short circuit, so there is no current flowing in R.
Vo is zero.
Infinite frequency (=):
the capacitor acts as an short circuit and the inductor behaves like an open circuit, so again there is no current flowing in R.
Vo is zero

75. Between =0 and =
Both capacitor and inductor have finite impedances.
Voltage supplied by the source will drop across both L and C, but some voltage will reach R.
Note that the impedance of C is negative, whereas the impedance of L is positive.

76. At some frequency, the impedance of C and the impedance of L have equal magnitudes and opposite sign cancel out!
Causing Vo to equal Vi
This is happen at a special frequency, called the center frequency, o.

77. BPF Frequency Response

78. Center Frequency

79. s-Domain Circuit

80. Transfer Function

81. Magnitude & Phase

82. Center Frequency
For circuit’s transfer function is purely real:

83. Cutoff Frequencies

84. Relationship Between Center Frequency and Cutoff Frequencies

85. Bandwidth, 

86. Quality Factor, Q

87. Cutoff Frequencies in terms of 

88. b) BPF: Parallel RLC

89. GENERAL BPF CIRCUITS
BPF series RLC:
BPF parallel RLC:

90. Remarks
The general circuit transfer functions for both series and parallel BPF:

91. BANDREJECT FILTER

92. Bandreject Filter
A bandreject attenuates voltages at frequencies within the stopband, which is between c1 and c2. It passes frequencies outside the stopband.
BRF are characterized by the same parameters as BPF:
Center freq. (and two cutoff frequencies)
Bandwidth
Quality Factor

93. BRF: Series RLC
OUTPUT

94. At =0 and =

95. At =0 and = Zero frequency (=0): Infinite frequency (=):
the capacitor acts as an open circuit and the inductor behaves like a short circuit.
the output voltage is defined over an effective open circuit.
Magnitude of Vo and Vi are similar
Infinite frequency (=):
the capacitor acts as an short circuit and the inductor behaves like an open circuit,.
the output voltage is defined over an effective open circuit.
Magnitude of Vo and Vi are similar

96. Between =0 and =
Both capacitor and inductor have finite impedances of opposite signs.
As the frequency is increased from zero, the impedance of the inductor increases and that for the capacitor decreases.
At some frequency between the two passbands, the impedance of C and L are equal but opposite sign.
The series combination of L and C is that short circuit so the magnitude of Vo must be zero. This is happen at a special frequency, called the center frequency, o.

97. BRF Frequency Response

98. Center Frequency
The center freq. is still defined as the frequency for which the sum of the impedances of L and C is zero.
Only, the magnitude at the center freq. is minimum.

99. s-Domain Circuit

100. Transfer Function

101. Magnitude & Phase

102. Center Frequency
For circuit’s transfer function is purely real:

103. Cutoff Frequencies

104. Relationship Between Center Frequency and Cutoff Frequencies

105. Bandwidth, 

106. Quality Factor, Q

107. Cutoff Frequencies in terms of 

108. Remarks

109. GENERAL BRF CIRCUIT
The general circuit transfer functions for both series and parallel BRF: