Colpitts 7mhz oscillator

Nicely modelled. There really is no other way to kick-start the sim either lol.

Hello :) I built this circuit on hardware without the 2.5k parallel resistor, without the 100Ohm resistor from emmiter to common point of the capacitors and without the 330pF cap. It works perfectly. However I don't quite get how the feedback mechanism works and how the LC tank is formed if the L is not in parallel with the caps. I've read that Colpitts is based on the principle of feedback mechanism but I don't see any signal going back to the base of the bjt. Could someone please briefly explain these to me? Thanks.

hi gpapas1, Glad to hear that you actually built the circuit and it worked. I'll try to answer your questions as simply as I can. 1) The ideal DC pwr supply line is an AC (virtual) ground, therefore, the 1.16uh inductor is in parallel with the 330pf cap which in turn is in parallel with the series combination of the 100pf and 820pf caps. Just pretend the top of the inductor is at ground for AC analysis only. For DC it remains at +12volts. 2) The xstr is configured as common base by virtue of the .01uf cap from base to ground. It basically is a short to ground at the osc freq. Ignoring any parasitic capacirance, the feed back mechanism is thru the division of the xstr collector AC voltage via the 100pf and 820pf capacitors at their junction. I placed the 100 ohm resistor at the junction to provide for easy open loop gain adjustment. The other end of the resister feeds into a low impedance point at the emitter. Since the xstr is common base, the signal path is from emitter to the collector with no phase shift. Think of the emitter as being driven by a current source. The collector current is approximately the emitter current. It doesn't go back to the base of the bjt (it's at ground). Hope this helps you understand what's happening. BTW, all osc must have some sort of positive feedback to function.