Here it is illustrated drain feedback biased enhancement MOSFET based Colpitts oscillator. The drain feedback biased E-MOSFET amplifier is used as the amplifying element of the oscillator. It is shown how to use online drain feedback biased E-MOSFET amplifier and online Colpitts oscillator calculators to design the amplifier and the calculate the LC network component values. Thus this tutorial helps to easily and quickly design Colpitts oscillator based on drain feedback biased E-MOSFET.
Below is a picture of the Drain Feedback biased E-MOSFET Colpitts Oscillator which was build on a breadboard.
The following shows the signal waveform of the oscillator on oscillscope.
The following video demonstrates how the drain feedback biased E-MOSFET amplifier based Colpitts oscillator works and the signal waveform and frequency on oscilloscope.
The circuit diagram of the above designed drain feedback biased E-MOSFET amplifier based Colpitts oscillator is shown below.
Now it is explained how the drain feedback biased enhancement based Colpitts oscillator was designed.
To build the oscillator we will use frequency of 50KHz. We will use the online Colpitts oscillator calculator to calculate the inductor and capacitors value of the feedback LC network. As shown below for frequency of 50KHz, and using C1=C2=0.01uF we obtain inductor value of 2mH.
Next we will use the E-MOSFET amplifier calculator to calculate the biasing component values and the coupling capacitor values. This is shown below.
The inputs are the DC power supply which is +5V, the signal input frequency which in this case is 50KHz. The gate to source threshold voltage is 2.1V which you have to obtain from the MOSFET datasheet. The drain voltage is set to 3V and for drain feedback biased circuit it means that the gate to source voltage is also 3V. The drain current is 23mA at gate to source voltage of 3V and drain to source voltage of 3V. This can be obtained from the datasheet or the drain curve of the E-MOSFET.The load resistance value here is 87Ohm which is the input impedance of the LC tank circuit. We can use circuit simulator like proteus to determine the input impedance of the LC tank circuit. The tutorial How to calculate Input and Output Impedance using Proteus explains in details how to calculate the input impedance of a circuit. Here the input impedance of the LC tank circuit of the colpitts oscillator is calculated by the following circuit diagram with voltage input and current probe.
Using Frequency response analysis we can get the input impedance value from the frequency response graph as shown below.
From the graph at frequency of 50KHz the input impedance of the LC tank circuit is 87.6 Ohm. This value is used as the load resistance value in the above amplifier design calculator.
The process of the designing drain feedback biased E-MOSFET is explained in details in the tutorial Drain Feedback Biased Enhancement MOSFET.
Practical Aspects
The designed circuit has impractical values so practically available component values should be used. Here the calculated drain resistor value is 79.05Ohm, the calculated input and output coupling capacitor were 1.06nF and 766.42nF. For the drain resistor a practical value of 82Ohm was used. The feedback resistor RG value used was 100KOhm. Values for the coupling capacitor should be more rather than less obtained from the calculator. The values of the input and output coupling capacitor used were 2.2nF ceramic and 1uF electrolytic capacitor respectively. Also two 1mH inductors in series was used for the 2mH inductance as calculated.
Video demonstration
The following video demonstrates how to use the online E-MOSFET amplifier design calculator and the online oscillator calculator to design the drain feedback biased enhancement MOSFET Colpitts oscillator. It also shows how to obtain the input impedance of the LC feedback network of the Colpitts oscillator.
