# Design of a 4MHZ Crystal Colpitts Oscilator Circuit

Thread Starter

#### Thandanani Mbanjwa

Joined May 17, 2017
2
Hi

Please check if my calculations are correct. I need to design a crystal colpitts Oscilator circuit. Please help me calculate the component values. I have done some draft work below, please assist. using a 2N3904 transistor.

I want to calculate the values of R1, R2, C1, C2, C3 and C4, see attachment.

I have selected C1 = 68pF and C2 = 220pF.
I saw a guide saying C1 should be a value between (39 – 82)pF and C2 between (39 - 220)pF for good performance

Ve = 10% of Vcc
Vcc = 12V
therefore Ve = 10% of 12V
=1.2V

Since Vbe for silicon is approximately 0.6V
Vbe = Ve + Vbe
= 1.2 + 0.6
= 1.8V

Also Ie = Ib + Ic
since Ic > Ib
there Ie = Ic = 1,2mA

Beta = Ic/Ib = (1,2mA/12µA)=100

therefore R2 = Ve/Ie = 1,2/1,2mA = 1K

Rc = 12/1,2mA = 10KΩ

R1 = (Vcc -Vb)/10Ib = (12-1.8)/(10X12µA) = 85KΩ
R2 = Vb/10Ib = 1,8/(10X12µA) = 15KΩ

#### bertus

Joined Apr 5, 2008
21,237
Hello,

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Please use the "Upload a File" button to post it here.

Bertus

Thread Starter

#### Thandanani Mbanjwa

Joined May 17, 2017
2
Hi,
I have the uploaded the .png file indicating the circuit i want to build

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