# HELP: how to determaine the capacitance?

#### Sumychia

Joined Sep 3, 2005
2
can someone helps me with this question:

suggest with explaination the criteria to determine the values of C1, C2 and Cs. (do not need to determine the actual value of C1,C2, and Cs for the circuit.) thanks... >.<

#### n9352527

Joined Oct 14, 2005
1,198
Originally posted by Sumychia@Nov 5 2005, 08:31 PM
can someone helps me with this question:

suggest with explaination the criteria to determine the values of C1, C2 and Cs. (do not need to determine the actual value of C1,C2, and Cs for the circuit.) thanks... >.<
[post=11470]Quoted post[/post]​
C1 and Ri act as potential divider and filter. Similarly for Rl, Ro and C2.
Cs is used to compensate gain at high frequency.
Have a look at the tutorials on this site for detailed explanation.

#### mozikluv

Joined Jan 22, 2004
1,437
hi,

the value of C1 will depend on what cut-off frequency you desire and also serves as a dc blockade so that the gate bias will not be disturb likewise with the signal source. Ditto with C2.

moz

#### n9xv

Joined Jan 18, 2005
329
All the above and;

As a general rule you can calculate the following;

C1 = 1 / (6.28 * Ri * Fc)

C2 = 1 / (6.28 * Ro * Fc)

Cs is chosen to have .1 * Xc of Rs. That is, if Rs was 10-Kohm then Cs should have a Xc (capacitive reactance) of 1/10th the value of Rs @ Fc (cuttoff frequency). So, 1/10th of 10-Kohm (Rs) would be 1-Kohm. The Xc of Cs is to be 1-Kohm. Now find Cs (shunt capacitance) as;

Cs = 1 / (6.28 * Fc * Xc) or 1 / (6.28 * Fc * 1000)

Some "fudging" with the value of C1, Cs & C2 are necessary to find a standard value of C that will "fit".