EE301 ELECTRONIC CIRCUITS

Transcription

1 EE30 ELECTONIC CICUITS CHAPTE 5 : FILTES LECTUE : Engr. Muhammad Muizz Electrical Engineering Department Politeknik Kota Kinabalu, Sabah.

2 5. INTODUCTION Is a device that removes or filters unwanted signal. Are used to block or pass a specific range of frequencies CATEGOIES OF FILTE PASSIVE FILTE ACTIVE FILTE

3 5. INTODUCTION (cont ) DIFFEENCE BETWEEN PASSIVE FILTE & ACTIVE FILTE PASSIVE FILTES Do not contain an amplifying device. Use resistors, capacitors and / or inductors. Always has insertion loss ACTIVE FILTES Contain some type of amplifying device. Use a transistor and / or operational amplifier in combination with resistors and capacitors to obtain the desired filtering effect. Have very low insertion loss

4 5. INTODUCTION (cont ) WHAT IS INSETION LOSS? Passive filters never have a power output equal to or greater than the input because the filter always has insertion loss. This loss represents the difference between the power received at the load before insertion (installation) of a filter and the power after insertion. Is stated as the log of a ratio of power output to power input and is the result of power loss because of resistance in the circuit.

5 5.2 PASSIVE FILTES Use only passive components such as inductors, capacitors and resistors. To minimize distortion in the filter characteristic, it is desirable to use inductors with high quality factors. However these are difficult to implement at frequencies below khz. Disadvantages: They are particularly non-ideal (lossy) They are bulky and expensive

6 5.2 PASSIVE FILTES (cont ) TYPES Low-pass filter High-pass filter Band-pass filter Band-stop filter

7 5.2 PASSIVE FILTES (cont ) Four types of filters - Ideal lowpass highpass bandpass bandstop

8 5.2 PASSIVE FILTES (cont ) ealistic Filters: lowpass highpass bandpass bandstop

9 5.2 PASSIVE FILTES (cont ) INTEACTIVE LEANING Ideal Filters ideal filter.swf

10 5.2. LOW-PASS FILTE Is a circuit that has a constant output voltage up to a cut-off or critical frequency, fc. The frequencies above fc are effectively shorted to ground and are described as being in the attenuation band, or stop-band. The frequencies below fc are said to be in the pass-band. Designed using combinations of resistors, inductors and capacitors.

11 5.2. LOW-PASS FILTE (cont ) Vin C Vout C LOW PASS FILTE At low frequencies, Xc is greater than, and most of the input signal will appear at the output. The cutoff frequency occurs when Xc= L LOW PASS FILTE Vin L Vout The inductor is shorted at low frequencies, and most of the output voltage of the circuit dropped across. At high frequencies the input acts as an open, and the majority of the input signal is dropped across L. L circuit passes low frequencies and blocks high frequencies

12 5.2. LOW-PASS FILTE (cont ) Cutoff frequency db fc 2 C fc L 2 esponse curve for low-pass filter fc Passes low frequencies Attenuates high frequencies When output voltage is of the input, the power output is 50% of the input power. This condition exists at fc (cutoff frequency) If Pout = ½ Pin, then a 3 db loss of signal has occurred.

13 5.2. LOW-PASS FILTE (cont ) Have a wide variety of applications in electronic circuits. One common application is in smoothing the output of a pulsating DC signal. The action of capacitor in an C circuit is to resist a change in voltage across the capacitor. If the direct current in the circuit has a ripple, the action of the C circuit is to suppress or absorb the variations and to delivered a filtered DC current and voltage to the load. AC INPUT ECTIFIE LOW PASS FILTE LOAD A block diagram of a low-pass filter and rectifier

14 5.2. LOW-PASS FILTE (cont ) Vin Vout Example 2.2kohm Determine the cutoff frequency of the circuit. C 0.0µF Answer : 7.23 khz Vin L 0mH Vout 33kohm Example 2 For the circuit, calculate the half-power frequency. Answer : khz

15 5.2.2 HIGH-PASS FILTE Allows frequencies above the cutoff frequency to appear at the output, but blocks or shorts to ground all those frequencies below the -3dB point on the curve.

16 5.2.2 HIGH-PASS FILTE (cont ) Vin C Vout C HIGH-PASS FILTE Capacitive reactance is large at low frequencies and small at high frequencies. The capacitor acts as a coupling capacitor. L HIGH-PASS FILTE Vin L Vout There is a high value of inductive reactance at high frequencies. The high frequencies produce a large output voltage across the inductor. At low frequencies, X L will be small and the low frequencies will be effectively shorted to ground. It should be noted that L filter are not often used in active circuits because of their size, weight and losses.

17 5.2.2 HIGH-PASS FILTE (cont ) Cutoff frequency db fc 2 C fc L 2 fc esponse curve for high-pass filter The cutoff frequency on the highpass response curve is called the low cutoff frequency. Passes high frequencies Attenuates low frequencies

18 5.2.2 HIGH-PASS FILTE (cont ) Example 3 Design an L high-pass filter that will have a cutoff frequency of 500 Hz. Assume = kω Answer : 06. mh

19 5.2.3 BAND-PASS FILTE Allows a certain range or band of frequencies to pass through it relatively unattenuated. Designed to have a very sharp, defined frequency response. Equivalent to combining a low-pass filter and a high-pass filter. The impedance of the circuit increases as the frequencies increase and decrease from the resonant frequency. Only the frequencies close to resonance are passed. All others are blocked because of high circuit impedance.

20 5.2.3 BAND-PASS FILTE (cont ) IO C L IO3 L C IO2 IO4 Series resonant band-pass filter Parallel resonant band-pass filter esonant frequency fr 2 LC

21 5.2.3 BAND-PASS FILTE (cont ) db High pass filter f c Bandwidth f c Low pass filter f f r f 2 f esponse curve for band-pass filter The cutoff frequency of the high pass section becomes the lower frequency limit in the passband f. The upper frequency in the passband f 2, is the result of the cutoff frequency in the low pass section. The passband, or bandwidth, is the difference between f 2 and f. At resonant frequency fr, X L = X C, and the output voltage is equal to the input signal.

22 5.2.3 BAND-PASS FILTE (cont ) Depending on the design and purpose of the band-pass filter, the bandwidth may be very wide or very narrow. Some of common uses of band-pass filters are: i. Audio to equalize sound levels ii. iii. Communications to select a narrow band of radio frequencies. Audio to produce speaker crossover networks.

23 5.2.3 BAND-PASS FILTE (cont ) Example 4 For the circuit, calculate the resonant frequency and bandwidth. Solution fr Hz 2 LC 2 (00mH )(0.F ) C 0.uF L 00mH 00ohm For series resonant frequencies Q BW fr Q L C Hz 0 00mH 0.F 59.6Hz 0

24 5.2.3 BAND-PASS FILTE (cont ) Example 5 A parallel resonant band-pass filter has a lower cutoff frequency of 6.3 khz and an upper cutoff frequency of 8 khz. Determine the bandwidth and resonant frequency Solution BW f c f c2 8kHz 6.3kHz.7kHz fr f c f c2 (8kHz )(6.3kHz ) 7.kHz

25 5.2.4 BAND-STOP FILTE ejects signals at frequencies within a special band and passes signals at all other frequencies. Also referred to as a band-reject, notch, wave-trap, band-elimination, or band-suppression filter. Like the band-pass filter, the band-stop filter may also be formed by combining a low-pass and high-pass filter. Lowpass filter fc Band stop fc Highpass filter The band-stop filter is designed so that the cutoff frequency of the lowpass filter is below that of the highpass filter

26 5.2.4 BAND-STOP FILTE (cont ) L C L C SEIES ESONANT BAND-STOP FILTE When a series resonant LC circuit is connected in parallel with a load resistor, the LC network provides a low impedance shunt path. At resonance, the LC circuits acts as a short circuit and the signal does not pass through. At frequencies above or below resonance, the LC circuit offer high resistance and current flow through the load. PAALLEL ESONANT BAND-STOP FILTE When the parallel LC network is at resonance, the impedance is very high and virtually no current flows to the load. The impedance decrease above and below resonance, so all other signals are passed through the filter.

27 5.2.4 BAND-STOP FILTE (cont ) Popular in variety of applications, particularly audio electronics. These filters can be used in groups to provide equalization of the entire audio spectrum. For example, a typical one-third octave equalizer will contain 28 band-rejection filters. Each filter section is designed to remove a specific band of frequencies and provides up to 5 db attenuation at its resonant frequency.

28 5.2.4 BAND-STOP FILTE (cont ) Example 6 For the circuit, calculate the resonant frequency. Given L = 200 mh and C = 0.0 µf. Solution fr 3. 56kHz 2 LC 2 (200mH )(0.0F ) L C

29 5.3 ACTIVE FILTES Use active components or devices such as transistors and op-amps, that can amplify. First order filter use one resistor and one reactive component, which is either an inductor or capacitor. Second order filter use two resistors and two reactive components.

30 5.3 ACTIVE FILTES (cont ) TYPES Low-pass filter High-pass filter Band-pass filter Band-stop filter

31 5.3 ACTIVE FILTES (cont ) Advantages of active filters include: reduced size and weight, and therefore parasitic increased reliability and improved performance simpler design than for passive filters and can realize a wider range of functions as well as providing voltage gain in large quantities, the cost of an IC is less than its passive counterpart

32 5.3 ACTIVE FILTES (cont ) Active filters also have some disadvantages: limited bandwidth of active devices limits the highest attainable pole frequency and therefore applications above 00 khz (passive LC filters can be used up to 500 MHz) the achievable quality factor is also limited require power supplies (unlike passive filters) increased sensitivity to variations in circuit parameters caused by environmental changes compared to passive filters For many applications, particularly in voice and data communications, the economic and performance advantages of active filters far outweigh their disadvantages.

33 5.3. ACTIVE LOW-PASS FILTE Cf At low frequencies, the capacitor has an extremely high capacitive reactance. Vin 2 f 3 Vo Therefore, the circuit acts as an inverting amplifier with a voltage gain of f /. At higher frequencies, the capacitive reactance of C f decreases. This causes the voltage gain to decrease due to the increased amount of negative feedback. 20 db/decade When Xc f = f, the voltage gain equals 70.7% of its midband value. This is the cutoff frequency fc: fc fc 2 f C f

34 5.3. ACTIVE LOW-PASS FILTE (cont ) The voltage gain for any frequency is calculated as Av where The voltage gain in decibels Z f Z f Xc 2 f f f Xc f 2 Av ( db) 20log Z f

35 5.3. ACTIVE LOW-PASS FILTE (cont ) Example 7 efer to figure: i) Calculate the cutoff frequency, fc ii) Calculate the voltage gain at (a) 0 Hz and (b) MHz iii) Calculate the db voltage gain at (a) 0 Hz and (b).59 khz. 0.0 µf Cf 0 kω Vin kω f 3 Vo 2

36 5.3. ACTIVE LOW-PASS FILTE (cont ) Solution i) fc 2 f C f 2 (0k)(0.0 F).59kHz ii) (a) (b) At 0 Hz, Xc f Ω. Therefore, the voltage gain is Av At MHz, f 0k 0 k Xc f fC 2 (MHz )(0.0F ) f Z f Xc f f (5.9)(0k) 59k Xc 0k 5.9 k f f Av f 5.9 k 0.059

37 Solution iii) (a) 5.3. ACTIVE LOW-PASS FILTE (cont ) At 0 Hz, Xc f Ω. Therefore, Z f = f Av (b) The frequency.59 khz is the cutoff frequency, f c. At this frequency, Av 0k 20log k f ( db) 20log 20 Z f = f = (0 kω) = 7.07 kω. Z 7.07k 20log k Notice that Av is down 3 db from its pass-band value of 20 db db f ( db) 20log 7 db

38 5.3.2 ACTIVE HIGH-PASS FILTE Vin C 2 f 3 Vo At high frequencies, Xc 0 Ω and the voltage gain Av, equals f /. At very low frequencies, Xc Ω and the voltage gain Av approaches zero. The cutoff frequency fc, is given by: fc 2 C -3 db 20 db/decade fc At fc, the voltage gain Av, is at 70.7% of its midband value. The voltage gain for any frequency is calculated as f 2 2 Av where Z Xc Z Voltage gain in decibels Av ( db) 20log Z f

39 5.3.2 ACTIVE HIGH-PASS FILTE (cont ) Example 8 efer to figure, calculate the cutoff frequency, f c. Given C = 0. µf and = kω Solution fc. khz 2 C 2 (k )(0.F ) 59 Vin C 2 f 3 Vo

40 5.3.3 ACTIVE BAND-PASS FILTE Besides low-pass and high-pass filters, other common types are band-pass filter (rejects high and low frequencies, passing only signal around some intermediate frequency). The simplest band-pass filter can be made by combining the first order low pass and high pass filters that we just looked at.

41 5.3.3 ACTIVE BAND-PASS FILTE (cont ) This circuit will attenuate low frequencies (ω<</ 2 C 2 ) and high frequencies (ω >>/ C ), but will pass intermediate frequencies with a gain of - / 2. However, this circuit cannot be used to make a filter with a very narrow band. To do that requires a more complex filter.

42 5.3.3 ACTIVE BAND-PASS FILTE (cont ) Figure below shows a band-pass filter using two stages, the first a high-pass filter and the second a low-pass filter, the combined operation being the desired band-pass response.

43 5.3.3 ACTIVE BAND-PASS FILTE (cont ) Example 9 Calculate the cutoff frequencies of the band-pass filter circuit of figure in the slide before with =2=0 kω, C=0. µf, and C2=0.002 μf. Solution f 59. Hz LP 2 C 2 (0k)(0.F ) 5 f 7. khz HP 2 C 2 (0k)(0.002F)

44 5.4 DECIBELS & FEQUENCY ESPONSE CUVE In analyzing filters, the decibel (db) unit is often used to describe the amount of attenuation offered by the filter. In basic terms, the decibel is a logarithmic expression that compares two power levels. The power gain or loss in decibels can also be computed from a voltage ratio if the measurement are made across equal resistance. N db V 20log V out in Where Ndb = gain or loss in decibels Vin = input voltage Vout = output voltage

45 5.4 DECIBELS & FEQUENCY ESPONSE CUVE (cont ) Example 9 In figure below, calculate the attenuation, in decibels, at the following frequencies: (a) 0 Hz, (b).592 khz, (c) 5.92 khz. Assume that Vin = 0 V pp at all frequencies. Vin Vout Solution: a) At 0 Hz, V out =V in =0 V pp, since the capacitor C appears as an open. Therefore, N P 0V out pp 20log 20log 20 log P 0V db 0 in pp C db

46 5.4 DECIBELS & FEQUENCY ESPONSE CUVE (cont ) Solution: b) Since.592 khz is the cutoff frequency f c, V out will be (V in ) or V pp. Therefore P 7.07V out pp NdB 20log 20log 20log dB P 0V in c) To calculate N db at 5.92 khz, X c and Z T must first determined. X k c 2fC 2 (5.92kHz )(0.0F ) 2 2 ZT X c 0 k k 0. 05k N 2 X c k 20log 20log 20 log Z 0.05k db 20 T 2 pp db

47 5.4 DECIBELS & FEQUENCY ESPONSE CUVE (cont ) In Example 9, notice that N db is 0 db at a frequency of 0 Hz, which is in the filter s pass-band. This may seem unusual, but the 0 db value simply indicates that there is no attenuation at this frequency. For an ideal passive filter, N db = 0 db in the pass-band. As another point of interest, N db is -3 db at the cutoff frequency of.592 khz. Since V out = V in at fc for any passive filter, N db is always -3 db at the cutoff frequency of a passive filter The N db value of loss can be determined for any filter if the values of Vin and Vout are known

48