Filters4. Low XL at low frequencies allows
current past inductor
5. High Xc at low frequencies prevents
current past capacitor
6. High R prevents current past resistor,
directing it instead to output
output
9. High XL at high frequencies, as
magnetic field builds and collapses,
prevents current from flowing
10. Because of low Xc at high frequencies,
current that passes inductor is shunted
to ground through capacitor
11. Low Pass Filter
a circuit that passes low frequencies
but blocks high frequencies
14. Low Xc at high frequencies allows
current past capacitor
15. High XL at high frequencies prevents
current past inductor
16. High R prevents current past resistor,
directing it instead to output
output
19. High Xc at low frequencies, as electric
field builds and drains slowly, prevents
current from flowing
+
+
+
20. Because of low XL at low frequencies,
current that passes capacitor is
shunted to ground through inductor
26. Result is that only a certain middle
range of frequencies can pass
output
R
high resistance of R results in most
current going to the output
30. at resonant frequency
10 mA from
L to C
10 V
1 kΩ 1 kΩ
10 mA from
C to L
Don’t use parallel resistance formula because
currents are out of phase with each other
32. When current reverses polarity,
energy from the magnetic field of inductor
fills up the electric field of capacitor
35. What if source voltage is removed?
Current continues to oscillate in tank
between electric field and magnetic field
37. Since R is ∞ at resonance,
all current flows past tank to output
output
Parallel Resonant Band Pass Filter
38. If tank is in series with output, current is
blocked by high impedance of tank
output
Band stop filter