Differences between Colpitts oscillator circuit designs
What are the differences between this colpitts oscillator design?




The one that sticks out as being different to the other three is (3) (bottom left) because it does not use positive feedback to the BJT base. The other three do use positive feedback to the base and this is fine at low to middle frequencies (up to a few tens of MHz) but when you get to higher frequencies the BJT's miller capacitance messes with things and (3) is preferred. In fact, as frequency rises into high VHF and UHF this fifth colpitts oscillator is the turn-to choice: -



Does #5 exclusively only run at VHF/UHF - no it will work at low frequencies and, given that I'm happier with this version I always use this it for whatever frequency I need.

If you look closely at (2) and (4) there is no difference. (1) differs from (2) and (4) by replacing the RFC (radio frequency choke) with a resistor and this makes component selction choices simpler but will run out of steam at higher frequncies a little before (2) and (4).

Common collector Colpitts oscillator working principle