3.3 Passive RC Filters

Chapter 3.3 Passive RC Filters

Physics Lecture Notes – Phys 395 Electronics Book
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Physics Lecture Notes – Phys 395 Electronics Book

  • CHAPTER 3. FILTER CIRCUITS46dB/decade = 20n log10(10) = 20n.(3.10)3.3Passive RC FiltersWe will now use our passive circuit elements to design some filter circuits. Inductors are notvery good devices and hence we will concentrate on the use of resistors and capacitors.3.3.1Low-Pass FilterFigure 3.2 shows one possible low-pass filter. The circuit is essentially a frequency-sensitivevoltage divider.At high frequencies the output behaves as if it is shorted while at lowfrequencies the output appears as an open circuit.RCBADCH1omega comegaomega -1cornerslope = -1Figure 3.2: RC low-pass filterMathematically we havevout =1/(jωC)R +1/(jωC)vin.(3.11)H(jω)≡voutvin=11+ jωRC.(3.12)The approximations areω→ 0⇒ H(jω)→ Hlow =1.(3.13)ω→∞⇒ H(jω)→ Hhigh =1jωRC,(3.14)|Hhigh| =1RCω−1.(3.15)At the corner