2. at low frequencies

3. Current starts to flow

4. 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

7. at high frequencies

8. Current starts to flow

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

12. at high frequencies

13. Current starts to flow

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

17. at low frequencies

18. Current starts to flow

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

21. High Pass Filter
a circuit that passes high frequencies
but blocks low frequencies

23. Current starts to flow

24. Inductor blocks high frequencies but
passes low frequencies of current

25. Capacitor blocks low frequencies but
passes high frequencies of current

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

27. Combining low pass & high pass filters
results in Bandpass Filter

28. How about combining both
low pass and high pass filters?

29. Tank circuit
10 V
1 kΩ 1 kΩ

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

31. at resonant frequency
energy from the electric field of capacitor
fills up the magnetic field of inductor

32. When current reverses polarity,
energy from the magnetic field of inductor
fills up the electric field of capacitor

33. Like a flywheel,

34. All current entering tank
stays in tank, no current leaves

35. What if source voltage is removed?
Current continues to oscillate in tank
between electric field and magnetic field

36. If source voltage is 0, what is
resistance of tank?
R=V/I I = 0, so R = ∞

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