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Active Filters

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Active Filters Active Filters • An active filters means using amplifiers to improve the filter. • An acive second-order RC low-pass filter still has two RC components in series. R R − vout vin + C C 1 2 1 Hj()ω ==⎛⎞--------------------------- ----------------------------------------------------------- ⎝⎠ωω⁄ 2 1 + j B ωω⁄ωω()⁄ 12+ j B – B 1 1 G()ω ==--------------------------------------------------------------------- -------------------------------- 2 ()ωω⁄ 2 ()ωω⁄ 4 1 + ()ωω⁄ 12++B B B • The amplifier buffer changes the impedance. • This filter is sharper than the non-buffered filter. LABORATORY ELECTRONICS II 1 of 12 Gyrator • Active feedback alters the effective impedance of an element. • The gyrator alters the apparent impedance of the load element. vin Zload R iin − vout + iin v in R • The op-amp rules are used to find the effective input impedance. On the non-inverting input: vin = vout + iinR On the inverting input: Z Z load load () vin ==----------------------- vout ----------------------- vin – iinR Zload + R Zload + R 1 Zload + R 1 iin ==--- 1–----------------------- vin –------------- vin R Zload Zload vin So and the impedance is the negative of the load. Zin ==------- –Zload iin LABORATORY ELECTRONICS II 2 of 12 Artificial Inductor • If the load on a gyrator is a capacitor the phase behaves like an inductor. 1 j Z = –---------- = -------- in jωC ωC • This would not have the frequency dependence of an inductor. • With two gyrators a capacitor can act as an inductor in phase and frequency. Zin GYR R R C 2 GYR LR= C R –RR()+ 1 ⁄ jωC 2 Z ==– R + --------------------------------------- jωR C in 1 ⁄ jωC • Inductors are difficult to miniaturize and have intrinsic resistance that may be undesirable. • The inductor can be replaced by a gyrator circuit. LABORATORY ELECTRONICS II 3 of 12 Active Bandpass • A single pole filter uses a parallel RLC circuit as the feedback network. C L R vin 2 − vout + R1 • Use a notch filter in a inverting amplifier also makes a bandpass filter. Twin-T vin R2 − vout + R1 At the notch frequency the Twin-T has infinite impedance, A = -R2/R1. If R1 >> ZTT, for other frequencies A = -RTT/R1 = 0. LABORATORY ELECTRONICS II 4 of 12 Bootstrapping • Bootstrapping refers to the use of feedback to make a very large input impedance by setting iin = 0 through a coupling capacitor for AC frequencies of interest. vin vin R2 C − vout = vin R1 + C vin vin iin • The circuit is set for an AC input signal with capacitive coupling to the non-inverting input. • The feedback circuit will give unity gain of the AC part of the signal. • If the AC part of the signal can pass through the coupling capacitor, it can also pass through the capacitor at the inverting input. • The signal is the same on both sides of resristor R1. • Since the voltage drop across R1 = 0 and no current enters the opamp, iin = 0. • The input impedance Zin = vin/iin, and should be very large. LABORATORY ELECTRONICS II 5 of 12 Sallen-Key Filter • The Sallen-Key filter looks like two RC filters (two pole) and a x1 amplifier (buffer). • There is a bootstrap to create a large input impedance. • For example, a high-pass Sallen-Key filter uses a resistor as the bootstrap. R1 vin − C1 C2 vout = vin + vin vin vin R2 iin • The breakpoint frequency is ω 1 b = ------------------------------- R1C1R2C2 • The roll-off for very low frequencies is 40 dB/decade or 12 dB/octave. LABORATORY ELECTRONICS II 6 of 12 Low Pass Sallen-Key • The low-pass Sallen-Key filter swaps the resistors and capacitors. C1 − R1 R2 vout vin + C2 • The circuit behavior is equivalent to a damped driven mechanical oscillator. • The driving force is vin. • The damping factor is 1 d ==---- ()R + R C ω 0 Q 1 2 2 b • As a passive network, the oscillator can be overdamped, underdamped or critically damped: 2 overdamped (d0 > 2), 2 underdamped (d0 < 2), 2 critically damped (d0 = 2). LABORATORY ELECTRONICS II 7 of 12 Voltage-Controlled Voltage Source • A voltage-controlled voltage source (VCVS) replaces the Sallen-Key unity gain buffer with a non- inverting amplifier. • The high-pass VCVS has two additional resistors to create a gain A. R4 R3 R1 C1 − vout + vin C2 R2 A = 1 + R3/R4 • The gain is usually expressed as a factor K. R KA==1 + -----3- R4 LABORATORY ELECTRONICS II 8 of 12 VCVS Damping • As with the Sallen-Key, the low-pass VCVS swaps pairs of resistors and capacitors. R4 R3 C1 R1 − vout + vin R2 C2 A = 1 + R3/R4 • With a variable gain for negative feedback and matching R1 = R2 = R, C1 = C2 = C, the gain and 1 damping are independent of the break frequency ω = -------- . B RC R ------3 • The gain remains A0 = 1 + . R4 R -----3- • The damping is d0 ==3 –2A0 – R4 LABORATORY ELECTRONICS II 9 of 12 Butterworth Low-Pass • The VCVS can be used to make an active filter version of the Butterworth filter. R R C 4 3 R − vout + v in R C K = 1 + R3/R4 − • The desired damping is d0 = 1.414, so K = 3 d0 = 1.586. − The ratio R3 / R4 = 2 d0 = 0.586. ω 3 -1 • For a cutoff frequency at 1 KHz, B = 6.28 x 10 s . Resistors are best in range from 1 KΩ to 10 KΩ ω μ Ω So typical C = 1/R B; about 0.1 F at 1.5 K . • High pass just inverts R and C in the circuit. • Higher order filters requires multiple stages, with K-values set for each stage. LABORATORY ELECTRONICS II 10 of 12 Bessel and Chebyshev VCVS • The VCVS can also be used to make other special purpose active filters. ω • The Bessel filter needs a normalizing factor fn for the frequency: RC = 1/fn B. The gain for a Bessel filter requires K = 1.268, and the frequency factor is fn= 1.272. • The Chebyshev filter has an additional variable that controls the ripple band size. For a ripple band of 0.5 dB: K = 1.842, fn= 1.231. For a ripple band of 2.0 dB: K = 2.114, fn= 0.907. • Users of specific values and higher order filters rely on tanles of K and fn values for circuit design. LABORATORY ELECTRONICS II 11 of 12 Bandpass VCVS Filter • The VCVS circuit can also be used to create a bandpass filter. • Use one-pole for each cutoff frequency. R R R1 4 3 R5 − v vin + out C2 R2 C1 • The cutoff frequencies are at 1/R1C1 and 1/R2C2. LABORATORY ELECTRONICS II 12 of 12.
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