# How the two Colpitts oscillators work?

I have just seen two Colpitts oscillators from the page as below.

However, the page is not written in English. I tried to translate it into English but I could not understand it.

Could anyone explain why there is a resistor in the first case and a capacitor in series with inductor in the second case?

In addition, if you have a link explaining how each circuit works, that would be great. Thank you. • The schematic on the right is incomplete, the gate of the FET has no DC bias, there's no DC path through the FET so it cannot be biased. – Bimpelrekkie Jul 5 '16 at 6:27
• There are lots of resources out there in English. Why are you worried about this particular page instead of finding one you can read? – Austin Jul 5 '16 at 19:13
• Because I don't know the keyword for that special type of Colpitts. I wouldn't ask if I can find it. Should I??? – anhnha Jul 5 '16 at 19:22

First case: The Colpitts oscillator is sort of a reverse on the Hartley oscillator. Instead of a tapped coil, it uses two capacitors in series to provide the feedback point.

Because feedback is through the capacitive leg of the LC tank circuit, the total capacitance of this leg is the series combination of C1 and C2, so that C = (C1*C2)/(C1+C2). The operating frequency is controlled by the tank circuit: ω = 2πf = 1/√(LC)

Because the transistor cannot be biased through the capacitors, we need a separate DC biasing circuit for the transistor itself. That is the purpose of the source resistor in this circuit. Of course, parasitic capacitance in the resistor and the transistor will have a small effect on operating frequency. However, this can be balanced out by careful adjustment of C in the tank circuit.

Second case:(Source is here)

Clapp oscillator is just a modification of Colpitts oscillator. The only difference is that there is one additional capacitor connected in series to the inductor in the tank circuit. The main purpose of adding this additional capacitor C3 is to improve the frequency stability. The insertion of this additional capacitor C3 prevents the stray capacitances and other parameters of the transistor from affecting C1 and C2. In variable frequency applications using Clapp oscillator, the common practice is to make the C1, C2 fixed and C3 is made variable. While deriving the frequency equation the additional capacitor must be also taken into consideration and the equation is: The value of C3 is usually selected much small and so the value of C1 and C2 has less effect on the net effective capacitance. As a result the equation for frequency can be simplified as 