3.3 Determination of Capacitances by Quasi Static Approximation
 
The capacitances shown in  Figure 3.1 are considered to be frequency independent. Thus, they can be determined by a quasi static approximation. The voltages on the device are defined in  Figure 3.3.
 

The charge on the gate is a function of two voltages, therefore the capacitance can be written in the general form
 

.

(13)

 
If the transfer characteristics of a FET is considered V_DS is kept constant and the first part in (13) is zero. In this case V_GS and therefore also V_GD is changed. This way the total gate capacitance C_G = (C_GS+C_GD) is determined by
 

.

(14)

 
Considering the output characteristics of a FET V_DS is changed and V_GS is kept constant, therefore the second part in (13) is zero and only C_GD is given by
 

.

(15)

 
Thus, C_GS is determined by C_GS = C_G - C_GD.
 

   
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