CHAPTER 12: Filter Circuits ppsx

CHAPTER 12 Filter Circuits Objectives Describe and Analyze: • Filter types: LPF, HPF, BPF, BSF • Passive filters • Active filters • LC tuned amplifiers • Other filter topics • Troubleshooting Introduction A filter is a circuit designed to separate signals from each other based on their frequency. There are four basic types: • Low-Pass Filter (LPF): passes signals below some frequency • High-Pass Filter (HPF): passes signals above some frequency • Bandpass Filter (BPF): passes signals between two frequencies • Bandstop Filter (BSF): blocks signals between two frequencies Frequency Response Bands are measured to the 3dB points. Passive RC Filters • All four types of filter can be made with just resistors and capacitors. They are not “high performance”, but they work. RC Filters The frequency f = 1/(2RC) has several names: • Break frequency • Corner frequency • Cutoff frequency • Roll-off frequency • 3dB frequency (or -3dB frequency) • 0.707 frequency • f 0 (“eff-zero”) • f B (“eff-bee”) RC Filters Bode plot for a”single-stage” LPF (one R and one C) RC LPF Example What is the cutoff frequency for a LPF with R = 1.59 k and C = 0.01 F ? Time constant  = RC = (1.59 k)  (0.01 F) = 15.9 s f 0 = 1 / (2 ) = 10 kHz Bode Plots Note that 6 dB /octave is equal to 20 dB /decade. Order • The term order used to describe filters tells us how fast the Bode plot rolls off. A first-order filter, such as an RC filter made with one capacitor, has a roll-off of 20 dB/decade, a second-order filter has a roll-off of 40 dB/decade, and so on. • The roll-off is N  20 dB/decade where N is the order of the filter. • You will see the word pole used to mean the same thing: a 1-pole filter is a first-order filter, a 2-pole filter is a second-order filter, and so on. [...]... feedback loop allows you to control the shape of the Bode plot • Feedback RC filters are often called “Sallen & Key” filters after the two men who first described them in the 1950s Of course, they used vacuum tubes! Active Filters (a) is a LPF, (b) is a HPF Active Filters Active Filters Switched Capacitor Filters Break-point is controlled by clock rate LC Tuned Amplifier fo =... ceramics act like tuned circuits Troubleshooting • Determine the filter type you are working on • Use an oscilloscope to look at inputs and outputs to check for correct filter response • If necessary, inject a sine wave and vary the frequency to test the filter response • Tuned -circuits sometimes “drift”, and may have a small trimmer capacitor to make adjustments • Active filters either work or the... The Bode plot says it all Active Filters • When you make a filter, you want its Bode plot to have a shape appropriate to the application While a second-order filter can be made with two resistors and two capacitors, its Bode plot will not have a “clean” break-point That’s where active filters come in • Active filters use an op-amp together with Rs and Cs The opamp’s feedback... bandpass filter with a very narrow pass band The parameter Q (stands for “Quality”) measures the narrowness of the pass band How high is high depends on the application, but usually Q = 10 or more is high Q The width of the pass band is the center frequency fo = 1/(2 π LC ) divided by Q • Q = REQ /(2 fo L) where REQ is the equivalent resistance across the parallel LC circuit Piezoelectric Filters . CHAPTER 12 Filter Circuits Objectives Describe and Analyze: • Filter types: LPF, HPF, BPF, BSF • Passive filters • Active filters • LC tuned amplifiers • Other filter topics. N is the order of the filter. • You will see the word pole used to mean the same thing: a 1-pole filter is a first-order filter, a 2-pole filter is a second-order filter, and so on. Order. to describe filters tells us how fast the Bode plot rolls off. A first-order filter, such as an RC filter made with one capacitor, has a roll-off of 20 dB/decade, a second-order filter has