colpitts oscillators

Posted on Feb 4, 2014

Colpitts oscillators are somewhat similar to the shunt fed Hartley circuit except the Colpitts oscillator, instead of having a tapped inductor, utilises two series capacitors in its LC circuit. With the Colpitts oscillator the connection between these two capacitors is used as the centre tap for the circuit. Perhaps the simplest Colpitts oscillato

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r to construct and get running is the "series tuned" version, more often referred to as the "Clapp Oscillator". Because there is no load on the inductor a high "Q" circuit results with a high L/C ratio and of course much less circulating current. This aids drift reduction. Because larger inductances are required, stray inductances do not have as much impact as perhaps in other circuits. Rather than present designs for specific frequencies for the Colpitts Oscillator we have submitted a schematic which may be "impedance" scaled to any frequency. Simply convert the suggested reactances back to the required inductor and capacitances at your band of interest. The Colpitts oscillator inductor should be around 250 - 300 ohms and the "NET" capacitive reactance should total around the same. Feedback capacitors Cfb, both "a" and "b" are each in the region of 45 ohms leading to very large values which is very helpful in swamping out the capacitive effects of the transistor used. The total capacitive reactance of the parallel combination of capacitors depicted as series tuning below the inductor in a series tuned Colpitts oscillator or "Clapp oscillator" should have a total reactance of around 200 ohms. Not all capacitors may be required in your particular application. Pay particular attention to our comments in Oscillator Basics. Perhaps the best approach to values used for tuning a Colpitts oscillator might be to give a practical...

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