Amateur radio 7 Mhz common collector Colpitts oscillator using the return ratio design method.

This oscillator was designed using the return ratio method. The 47 pf capacitor represents the tuning capacitance. It can be tuned over the 7 Mhz amateur radio band. The approximate value of the return ratio is: T=-gmRpiC2/C1. C2 (the 470 pf) was chosen by setting the parallel Q factor with RE (the 470R resistor) 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 2), you can rearrange the formula to find C1. Rpi can be found from the following equation, Rpi=hfe/gm. If it does not oscillate make C1 smaller. If the sinewave is distorted make C1 bigger. Change by small increments. Since C1 is much greater than C2 the inductors value can be found from L=1/((2xpixfo)^2xC2. Surprisingly the approximate equation for the value of the return ratio "T" is almost the same for both common emitter and common collector configurations. RL is replaced by Rpi in the common collector equation and the ratio becomes C2/C1. It is worth noting that gmxRpi=hfe. Therefore the return ratio value can be found from, T=-hfexC2/C1. 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