# Draw the eqivalent circuit.

1. May 29, 2013

### animesh184

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2. May 29, 2013

### animesh184

3. May 29, 2013

### yungman

Is this a coupled line circuit? P and Q is the input/output ( interchangeable). If so, you don't have nearly enough info on the length and the line width. It's been a while I studied couplelines. This likely to be a band pass filter type. You really have to have the physical layout to know more, it cannot be just a schematic.

4. May 29, 2013

### animesh184

have to find eqivalent capacitance between p an q

5. May 29, 2013

### animesh184

have to find eqivalent capacitance between P anQ

6. May 29, 2013

### yungman

You need to draw out the dimension of the structure, specify the dielectric constant. You don't have nearly enough information to even talk here.

If the structure composes of parallel plates that the length and width is much larger than the d, then you can approximate by parallel plate capacitor. But if it's not, then it can get really complicated. Also it depends on the frequency you are working with also. You are asking a question that can potentially be very complicated with almost no information.

The only possibility this can be simpler is if you have parallel plates with large area and you don't need exact value. Then it's just P connect to Q with a parallel combination of a capacitor $C_{PQ}$ comprises of one plate from the P and the other from Q. Then $C_{PQ}$ is in parallel with a series combination of two capacitors $C_{PA}$ and $C_{AQ}$ that formed from plate of P to A, A to Q respectively.

And even at that, the largest dimension of the structure has to be less than $\frac 1 {20\lambda}$ of the highest frequency you use.

Last edited: May 29, 2013
7. May 30, 2013

### Staff: Mentor

Imagine you put a positive charge on P, what charge does that induce on the other plates? Once you have that you can figure out the total capacitance using a formula involving A and d.

8. May 31, 2013

### the_emi_guy

I would use FastCap

9. May 31, 2013

### the_emi_guy

I am curious where the OP got this, it sure does not look like homework. The eA/d formula (or any other closed form expression) will not work here due to the complex geometry, modeling with a field solver is necessary. As yungman pointed out, a lot of the necessary geometry is not shown, particularly what is happening in the third dimension.

10. Jun 3, 2013

### CWatters

My guess is that you are meant to ignore any field/capacitive effect of the end plates (eg the vertical lines in the drawing). Treat these as wires only.

Then don't you just have 4 capacitors connected in a series/parallel combination? Perhaps I'm too rusty.