Self Biased JFET Colpitts Oscillator

 Here it is illustrated how to design a Colpitts oscillator with self biased JFET as the amplifying element. Here it is illustrated how one can utilize online calculators for calculating component values for self biased JFET amplifier and for calculating values for Colpitts oscillator.

Shown below is the Self Biased JFET amplifier based Colpitts oscillator circuit diagram.

Self Biased JFET Colpitts Oscillator circuit diagram
To make the oscillator first we will design the Colpitts oscillator. This will give us the frequency of oscillation which we will need when building the JFET amplifier part. For making the Colpitts oscillator we will use the online Colpitts oscillator design calculator

Lets say we want the frequency of the oscillation to be 100KHz. Then we choose the capacitor values like 0.01uF for both the capacitors. Then using the online Colpitts oscillator calculator we can determine the value for the inductor. This is shown below.

colpitts oscillator calculator

The calculated value for the inductor is 506.61uH. 

 The next task is to build the self biased JFET amplifier. Here we will use the JFET Biasing and Amplifier Design Calculator so that we can quickly find the biasing resistors values, the coupling capacitors values and also the bypass capacitor value. The following shows the calculated values for the amplifier using the online calculator.

self biased JFET amplifier calculator

 In the calculator we have to specify the frequency which is 100KHz in this case. We have to specify gate to source cutoff voltage(\(V_{GS}\)) which is -1.02V and the zero gate voltage drain current (\(I_{DSS}\)) which is 10mA for the 2N5459 JFET. These values can be obtained from the JFET datasheet. Next we need also to specify the the source voltage which is 5V. We also specify the drain voltage(\(V_D\)) which is set to 2.5V. The load resistor value is not important but it is included as 136Ohm for the oscillator feedback network input impedance. This can be calculated as described in the tutorial How to calculate Input and Output Impedance using Proteus. The gate resistor value can be anything high value and here we have used 100KOhm.

The calculated outputs are as shown below. Mainly we need the resistors and the capacitors values: the source resistor\(R_S\) value is 60Ohm and the drain resistor\(R_D\) value is 500Ohm. The coupling capacitors values are CC1=159.24 pF and CC2=31.85 nF. The source resistor bypass capacitor is CB=265.39 nF. The values of the coupling capacitors and the bypass capacitor should be equal or more than the calculated values but here we will use the same calculated values. Also the output coupling capacitor CC2 is not actually required for the Colpitts oscillator to work but here we will included this capacitor. How to components values are calculated by the online calculator is described in details in the tutorial self biased JFET calculation.

The completed circuit was shown above. So in this way we can quickly build self biased JFET based Colpitts oscillator with the help of the JFET amplifier design calculator and the oscillator calculator. The Colpitts oscillator can similarly be designed using voltage divider biased JFET using the amplifier design calculator. Similarly one can calculate JFET Hartley Oscillator using the two calculators.

The following video shows how to use the online calculators to build the self biased JFET colpitts oscillator:

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