A planar capacitor analysis by EM simulator 2
Issued on August 21, 2009

Next I analyzed a planar capacitor of which size of electrodes is not same to the other. It is like this.
Fig. 2.0
The bottom electrode has the same area to the previous analysis, 10mm*10mm. Here I changed the size of the upper electrode.
According to the previous analysis, we can easily suppose that the capacitance here would be the same capacitance to a capacitor which has the small electrodes and that the area of the bottom electrodes which is open would be negligible.
Hereafter, let's see whether it is true or not.
The base point in this analysis is the same to the previous one. The capacitance is 9.18pF.
Here I put the summary table.
Table 2.1
Overlapping ratio of electrodes 
100% 
72.25% 
49% 
36% 
25% 
16% 
9% 
Size of the upper electrode 
10mm*10mm 
8.5mm*8.5mm 
7mm*7mm 
6mm*6mm 
5mm*5mm 
4mm*4mm 
3mm*3mm 
Figure 







Estimated capacitance 
9.18pF 
6.77pF 
4.65pF 
3.45pF 
2.42pF 
1.57pF 
0.904pF 
Ratio of capacitance 
100% 
74% 
51% 
38% 
26% 
17% 
10% 
The ratio of capacitance is well fit to the overlapping ratio. The error is just less than 2%.
Then we understand that the capacitance of the capacitor here depends on the area of the small electrode. Here we have verified the expectation at the first of this section.
As well as the previous analysis, we have electric field which flows out from the perimeter of electrodes. So we have some error.
Capacitance of a planar capacitor depends on the area of the small electrode.
(C)KANAYA Hidenori
Hide's Radio Shack