# Why should capacitive reactance be small?

Discussion in 'Homework Help' started by Heidi L, Feb 24, 2013.

1. ### Heidi L Thread Starter New Member

Nov 12, 2012
14
0
It says on my text that the capacitive reactance should be small and the inductive reactance should be large. Whould you please tell me the reason with a simple illustration? Thank you!

2. ### #12 Expert

Nov 30, 2010
16,665
7,314
Depends on the circuit, but...a capacitor with small reactance would indicate that it has enough microfarads compared to the frequency it has to deal with. Same with the inductor. More reactance indicates a larger inductor.

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3. ### blah2222 Well-Known Member

May 3, 2010
565
33
Is there an example of what they are talking about? A number of different variables could come into play to make that statement void.

$Capacitive \ reactance = -j\frac{1}{2\pi fC}$

$Inductive \ reactance = j2\pi fL$

Each can be large or small depending on frequency, capacitance, or inductance.

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4. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
It's a picky point, but you drop the j when talking about reactance. Reactance is a real number measured in ohms. Impedance is the complex quantity whose magnitude is also measured in ohms.

5. ### Heidi L Thread Starter New Member

Nov 12, 2012
14
0
The original problem is:

A sinusoidal signal, v1(t)=2.5cos(wt),when added to a dc level of V2=2.5 V, provides a 0-to 5-V clock signal used to control a microprocessor. If the oscillation frequency of the signal is to be 1GHz, let us design the appropriate circuit by using a single capacitor and inductor.

The solution suggests that the capacitive reactance should be small, and the inductive reactance should be large while choosing C an L. I don't know why.

6. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
I'd guess that very small value inductors are hard to make.

If this is for simulation only, maybe it is a hint. I'm not sure how to get a square wave from a sine wave with ONLY 1 inductor and 1 capacitor.

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7. ### blah2222 Well-Known Member

May 3, 2010
565
33
Yeah, I meant impedance. Thanks for being so picky.

8. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
That adds a lot more context, but it is still pretty vague. Are you designing an oscillator circuit to produce the sinusoid, or taking an existing biased-sinusoid and filtering it to get a squarewave for the clock input?

Are you dealing with Q factor at all?