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1 Lecture #23 EGR 272 – Circuit Theory II Bode Plots - Recall that there are 5 types of terms in H(s). The first four have already been covered. Read: Chapter 14 in Electric Circuits, 6 th Edition by Nilsson 5.Complex terms in H(s) A second order term in H(s) with complex roots has the general form s 2 + 2 s + w o 2. After factoring out the constant has the w o 2, the corresponding term in H(jw) is: Key point: The complex term above is identical to a double real pole or a double real zero in the Bode straight-line approximation, but they differ in the exact curves.

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2 Lecture #23 EGR 272 – Circuit Theory II Complex peaks and damping ratio Exercise: Complete the table below showing the value of the complex peak (for a complex zero) * = 1 is not complex. It corresponds to a double-zero.

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3 Lecture #23 EGR 272 – Circuit Theory II The following graphs from the text (Electric Circuits, 6 th Edition, by Nilsson) below show the Bode straight-line approximation (Figure 14.41) and the exact curve for different values of (Figure 14.42).

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4 Lecture #23 EGR 272 – Circuit Theory II Example: Sketch the LM responses for H 1 (s), which has a double zero, and H 2 (s), which has a complex zero. A) Find H 1 (jw) and sketch the LM response

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5 Lecture #23 EGR 272 – Circuit Theory II Example: Sketch the LM responses for H 1 (s), which has a double zero, and H 2 (s), which has a complex zero. B) Find H 2 (jw) and sketch the LM response

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6 Lecture #23 EGR 272 – Circuit Theory II Filters A filter is a circuit designed to have a particular frequency response, perhaps to alter the frequency characteristics of some signal. It is often used to filter out, or block, frequencies in certain ranges, much like a mechanical filter might be used to filter out sediment in a water line. Basic Filter Types Low-pass filter (LPF) - passes frequencies below some cutoff frequency, w C High-pass filter (HPF) - passes frequencies above some cutoff frequency, w C Band-pass filter (BPF) - passes frequencies between two cutoff frequency, w C1 and w C2 Band-stop filter (BSF) or band-reject filter (BRF) - blocks frequencies between two cutoff frequency, w C1 and w C2

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7 Lecture #23 EGR 272 – Circuit Theory II Ideal filters An ideal filter will completely block signals with certain frequencies and pass (with no attenuation) other frequencies. (To attenuate a signal means to decrease the signal strength. Attenuation is the opposite of amplification.) LM w wCwC Ideal LPF LM w wCwC Ideal HPF LM w w C1 Ideal BPF w C2 LM w w C1 Ideal BSF w C2

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8 Lecture #23 EGR 272 – Circuit Theory II Filter order Unfortunately, we can’t build ideal filters. However, the higher the order of a filter, the more closely it will approximate an ideal filter. The order of a filter is equal to the degree of the denominator of H(s). (Of course, H(s) must also have the correct form.) LM w wCwC Ideal LPF slope = 3rd-order LPF slope = -60dB/dec 2nd-order LPF slope = -40dB/dec 1st-order LPF slope = -20dB/dec 4th-order LPF slope = -80dB/dec

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9 Lecture #23 EGR 272 – Circuit Theory II Low-pass filter (LPF) - Discuss the form of LM, H(jw), and H(s). High-pass filter (HPF) - Discuss the form of LM, H(jw), and H(s).

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10 Lecture #23 EGR 272 – Circuit Theory II Band-pass filter (BPF) - Discuss the form of LM, H(jw), and H(s). Band-stop filter (BSF) - Discuss the form of LM, H(jw), and H(s). Also define a “notch” filter.

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11 Lecture #23 EGR 272 – Circuit Theory II Practical Filter Examples: Discuss a stereo tuner, 5-band equalizer, FSK demodulator, 60 Hz interference block.

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12 Lecture #23 EGR 272 – Circuit Theory II Active and passive filters Passive filter - a filter constructed using passive circuit elements (R’s, L’s, and C’s) Active filter - a filter constructed using active circuit elements (primarily op amps) Analysis of active filters Active filters are fairly easy to analyze. Recall that a resistive inverting amplifier circuit has the following relationship: Replacing the resistors with impedances yields a transfer function, H(s):

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13 Lecture #23 EGR 272 – Circuit Theory II Example: Find H(s) =V o (s)/V in (s) for the circuit below and sketch the LM response.

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14 Lecture #23 EGR 272 – Circuit Theory II Impedance and Frequency Scaling Filter tables are commonly available that list component values for various types of filters. However, the tables could not possibly list component values for all possible cutoff frequencies. Typically, filter tables will list cutoff frequencies of perhaps 1 Hz or 1 kHz. The user can then scale the cutoff frequencies to the desired values using frequency scaling. Similarly, component values may be too large or too small (for example, a filter might be shown using 1 ohm resistors or 1F capacitors) so the components need to be scaled using impedance scaling.

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15 Lecture #23 EGR 272 – Circuit Theory II Impedance Scaling – a procedure used to change the impedance of a circuit without affecting its frequency response. Consider the series RLC circuit shown below. R sL Z(s) Show that to scale the impedance by a factor K Z :

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16 Lecture #23 EGR 272 – Circuit Theory II Frequency Scaling – a procedure used to scale all break frequencies in the frequency response of a circuit. The frequency response is scaled without affecting the circuit impedance (the impedance is the same at the original and new break frequencies). Consider the series RLC circuit shown below. Note that H(s) described above is the transfer function of a band-pass filter. The LM is shown on the following page. V o (s) R sL+ _ V i (s) + _ As seen in an earlier class, H(s) is described below.

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17 Lecture #23 EGR 272 – Circuit Theory II LM of the frequency-scaled band-pass filter (center frequency = K F w o ): LM of the original band-pass filter (center frequency = w o ):

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18 Lecture #23 EGR 272 – Circuit Theory II Show that to scale the frequency by a factor K F :

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19 Lecture #23 EGR 272 – Circuit Theory II Example: Impedance and frequency scaling. A)Determine H(s) = V o (s)/V i (s) B)Sketch the LM response

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20 Lecture #23 EGR 272 – Circuit Theory II Example: (continued) C)A cutoff frequency of 1 rad/s and component values of 1 ohm and 1 Farad are not very useful. Scale the frequency so that the cutoff frequency is 500 rad/s. Also increase the impedance of the circuit by a factor of 1000.

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21 Lecture #23 EGR 272 – Circuit Theory II Example: (continued) D)Draw the new circuit. Determine the new H(s) and verify that the LM response has been shifted as planned. E)Calculate the circuit impedance at the break frequency for the new circuit and compare it to the circuit impedance at the break frequency for the original circuit.

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Fundamentals of Electric Circuits Chapter 14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Fundamentals of Electric Circuits Chapter 14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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