Swapping the cap and the resistor in the low-pass circuit creates another type of circuit called a high-pass filter. Using your now supreme powers of deduction and intuition, you are thinking to yourself, “I’ll bet that means the circuit passes high frequencies while blocking low ones.
” You are correct, and the circuit looks like the one in Figure 2.34 .Hopefully, after our discussion on the low-pass circuit, the operation of this one is clear.
The cap acts like a larger resistor at low frequencies, making the voltage divider knock down the output. At higher frequencies the cap passes more current as it becomes a short, causing a higher voltage at the output.
The inductor version of this circuit looks like Figure 2.35
As you might have suspected, this fi lter is the inverse, circuit-wise, of the RC high-pass filter. Another little bit of serendipity is the fact that the half-voltage output point 29 is also at 1/tau ( tau means time constant generically, whether referring to an RC or an RL circuit), just like the low-pass filters.
To sum up, the high-pass and low-pass fi lters take advantage of the frequency response of either a capacitor or an inductor. This is done by combining them with a resistor to create a voltage divider that attenuates the unwanted frequencies while allowing the desired ones to pass.
Some cool things happen when we put the two reactive elements together. You can create notch and band pass filters where a specifi c band of frequencies is knocked out, or a specific band is passed while all others are blocked.
The phenomenon of resonance also occurs in what is called a tank circuit, where you have a capacitor combined with an inductor. The tank circuit will oscillate current back and forth from one component to the other.
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