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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.
The basic Colpitts oscillator circuit look like this and you will see some similarities with the Hartley Oscillator..
Figure 1 - schematic of a collpitts oscillator
Perhaps the simplest Colpitts oscillator 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.
Figure 2 - schematic of a series tuned colpitts or "clapp" oscillator
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 example. Consider constructing an oscillator which tunes part of the 40M amateur radio band, 7.0 - 7.2 Mhz. Now that is a frquency ratio of 1.02857 requiring a modest net capacitance variation of 1.058. If that is not understandable go back to our basics.
Using an inductor of 300 ohms at 7 Mhz for our Colpitts or Clapp oscillator yields a value of about 6.8 uH. Each Cfb at 45 ohms works out at 500 pF so we will try 470 pF. Using an inductor of 6.8 uH requires a total capacitance of 76 pf to resonate at 7.0 Mhz. At 7.2 Mhz this value has dropped down to 71.86 pF a small variation.
Effectively all the capacitors are in series in a Colpitts oscillator. That is Cfb-a and Cfb-b are each in series with the total parallel combination below the inductor. Given Cfb are each 470 pF what values are the parallel combination to achieve outcomes of net 76 pF and 71.86 pF? Had you done basics and in particular capacitance you would know the answer. We're not being smart here just pointing out there is no such thing as a "free" lunch, you have to know "the basics". Having done these lectures on the internet for several years, yes I'm Ian Purdie VK2TIP, I don't appreciate email questions from lazy students expecting me to complete or provide their assignments for them. Do some work for yourself!.
Back to our Colpitts oscillator, I'll give you a "FREE" clue to the answer with these colourful formulas for the answer in figure 3. I know a shorter method. See this example in capacitance.
Figure 3 - calculations for a series tuned colpitts or "clapp" oscillator
Here in our Colpitts oscillator Ctotal-max is the maximum of the parallel combination including the variable capacitor Cv, set at maximum while Ctotal-min is the same combination with Cv set at minimum. Clear on that?
Note that the other series capacitor depicted in line with Cv in figure 2 may or may not be required in your particular application. Anyhoo! your calculations should have yielded a Cmax of 112.3 and Cmin of 103.51.
Now that is a pretty tiny swing, so you can see the possible need for a series capacitor with Cv if all you have available is a fairly high value Cv. Let's assume Cv is 5 - 25 pf (a variation of 20 pF) and Ct is a 10 pF trimmer. Where do we stand now with our Colpitts oscillator calculations?
Allowing for taking strays into account we will take the full value of Ct of 10 pF, therefore the balance of the parallel combination accounts for Cmax of 102.3 and Cmin of 93.51. The net variation of C still remains 102.3 - 93.51 = 8.79 pF so Cv needs to be seriously reduced. How is largely determined by the same method below (suck and see approach) but I come up with a "starting point" of about 12 pF in series with Cv.
Therefore nearing completion of calculations for our Colpitts oscillator, we find this new series combination with Cv produces a net Cv-max 8.1 pF and Cv-min 3.53 pF. This is obviously far too much reduction using the 12 pF capacitor, so try using some higher values like 27 pF!
Using 27 pF in our selection gives Cv-max 12.98 pF and Cv-min 4.22 pF and a net variation of 8.76 pF. Kicked a goal!
New Cv is (at max) 13 pF and Ct assumed at 10 pF a total of 23 pf from a required maximum of 112.3 leaving about 89 pf to be made up of fixed capacitors. I'd use at least three fixed capacitors, say 33 pF plus 2 X 27 pF.
Ideally your frequency determining components L1 and the parallel capacitors should be sceened in an earthed shield.
All those sums you did above are just the starting point for your Colpitts oscillator. Lifes realities are that all things "won't go according to plan". Stray inductance and stray capacitance will play havoc with your calculations, but you're right in the "ball park" and you should understand what needs to be done to adjust frequency. Remember no inductor you wind is going to be a perfect 6.8 uH inductor for L1 in figure 2 above.
The output from the Colpitts oscillator is through output capacitor Co, this should be the smallest of values possible, consistent with continued reliable operation into the next buffer amplifier stage. That statement is true for all oscillators. Typical values for Co in a Colpitts oscillator might be 47 pF.
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