What are the differences between this colpitts oscillator design?
I need to build one for the transmitter of my wireless energy circuit for my project in our school, but I don't know what to use.
Electronic – Differences between Colpitts oscillator circuit designs
oscillator
Related Solutions
the L and C form an LC tank. Frequency is given by:
If the active device is a transistor (BJT or FET) there are, in general, three different amplifier configuratios possible: Common emitter, common collector, common base.
The first two references show a common base Colpitt oscillator (base on signal ground). The last ref. contains a Colpitt oscillator in common emitter configuration.
Feedback path:
(a) Common base: The transistor output is connected to an LC resonant block which is high-resistive for the desired frequency. The feedback signal is sampled between both capacitors (cap. divider) and is in phase with the output. This is necessary for positive feedback because in common-base configuration there is no phase shift between emitter and collector. The ratio between both capacitors determines the amount of feedback.
(b) Common emitter: The working principle is somewhat different. The collector is connected to a grounded third order lowpass (r,out-C1 and L-C2). The output of this lowpass has a phase shift of -180deg at the desired frequency. This is necessary because there is a phase shift of another 180deg between base and (correction): collector (in common-emitter configuration). Hence, again we have positive feedback.
(Note that r,out is the transistor`s output resistance at the collector node).
Best Answer
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