The Colpitts circuit

Another way to give RIF feedback is to tap the capacitance instead of the inductance in tuned circuit. In the given figure NPN bipolar and N-channel JFET Colpitts oscillator circuits are drawn.

The amount of feedback can be controlled by ratio of capacitances. The coil, instead of the capacitors, is variable in this circuit. This is the matter of convenience. It is impossible to find a dual variable capacitor with the right capacitance  ratio between sections. Even if you find one, you cannot change the ratio of capacitances. It is easy to adjust the capacitance ratio by using a pair of fixed capacitors.

Unluckily, finding a good variable inductor might not be easier than getting hold of a appropriate dual-gang variable capacitor. A permeability-tuned coil is used, but ferromagnetic cores impair frequency stability of the RF oscillator. A roller inductor can be used, but these are expensive. An inductor having several switch-selectable taps can be used, but this would not allow for continuous frequency adjustment. The tradeoff is the Colpitts circuit provides exceptional stability and reliability when designed properly.
As with Hartley circuit, feedback should be kept to minimum necessary to sustain the oscillation.

In these types of circuits, the outputs are taken from emitter or source. Why don’t we take from the collector or drain? The answer is that output can be taken from collector or drain circuit, and an oscillator will work just fine. But gain is not significant in an oscillator; what matters is stability under the varying load conditions. Stability is improved when the output of an oscillator is taken from emitter or source portion of the circuit.

To avoid the output signal from being short circuited to ground, an RF choke (RFC) is connected in series with emitter or source lead in Colpitts circuit. The choke lets direct current pass while blocking alternating current (the opposite of a blocking capacitor). Characteristic values for RF chokes range from about 100 µH at high frequencies,  like 15 MHz, to 10 mH at the low frequencies, such as 150 kHz.