Amateur radio 7 Mhz common base Colpitts oscillator.

181

01:59:40

This design used the return ratio method.The approximate value of the return ratio is: T=-gmRLC1/C2. C2 (the 22nF) was chosen by setting the parallel Q factor with re (the common base input resistance) at 10 with fo being 7 Mhz, where Q=re/XC2. Once you know C2 and you choose T (must be at least 1, I chose 3.6), you can rearrange the formula to find C1. 

re is given by re=0.026/Ic. In this circuit Ic is close to 2 mA.Therefore re is about 13 ohms.

If it does not oscillate make C2 smaller. If the sinewave is distorted make C2 bigger. Change by small increments. Since C2 is much greater than C1 the inductors value can be found from L=1/((2xpixfo)^2xC1).

Surprisingly the approximate equation for the value of the return ratio "T" is almost the same for both common emitter, common collector and common base configurations. In the common collector equation replace Rl with Rpi.

These equations have been developed by using a method put forward by Professor Ali Hajimiri. He has a full breakdown of the return ratio method used for a Colpitts oscillator on Youtube.

published 10 months ago

STIDEA

9 months ago

Your circuit should run at 6.7Mh if you change the left voltage divider 10k on a positive and 50k to ground change the value at 800p of the left cap and add a switch between the cap and the center tap of VD.

andypic

9 months ago

Thanks very much for your comment. I don't quite follow what you mean. What is the centre tap of VD? I would like to know what your thoughts are. The circuit is designed to operate above 7.2 Mhz since the tuning capacitance will bring the frequency down. The tuning capacitance is not included in the circuit. It is designed for the 7 Mhz amateur radio band. Thanks again for your comment!

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