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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.

Marcus Müller
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dumbedkid
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    Removed all of the unnecessary "fluff" from your question, so that it's easier to read. No need to thank us before we did anything. This is a place where people come to ask and get help, so: welcome! – Marcus Müller Nov 25 '18 at 10:13
  • What would be helpful to know would be a) why Colpitts? b) what frequency? c) where exactly do you extract the energy for your wireless energy transfer from said circuits? – Marcus Müller Nov 25 '18 at 10:14
  • d) school: what level are we talking about? Essentially, do they expect you to have in-depth knowledge of power electronics, or is this an electronic beginner's project? – Marcus Müller Nov 25 '18 at 10:17
  • just only electronic beginners project – dumbedkid Nov 25 '18 at 10:36
  • i just saw someone from the net using colpitts oscillator for his wireless energy project so i decided to use one..for the frequency im going to use between 100khz to 120khz and for the power supply for my circuit im going to use 12 volt battery – dumbedkid Nov 25 '18 at 10:40

2 Answers2

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What are the differences between this colpitts oscillator design?

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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: -

enter image description here

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

Andy aka
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  • thanks, now i can go and build this for the transmitter of my wireless energy – dumbedkid Nov 25 '18 at 11:12
  • Don't forget that the output from the Colpitts oscillator is "weak" and needs to be buffered by another transistor if you are trying to create a significant magnetic field for use in wireless energy transfer (on of my pet subjects!). – Andy aka Nov 25 '18 at 11:17
  • can u give me schematic sir i dont understand what u mean by buffered by another transistor im just new to this things – dumbedkid Nov 25 '18 at 11:21
  • I would also recommend that you consider operating at a higher frequency such as 500 kHz (should this be allowed for your project). The coils will be easier to make and you will tend to get a better energy transfer and, the frequency isn't so high as to cause real problems in making a power amplifier to deliver that frequency. Of course, if you were able to work at it over a longer time period I'd go for the industry standard 13 MHz. – Andy aka Nov 25 '18 at 11:21
  • its ok to use higher frequency..i just randomly pick 120 khz for starter – dumbedkid Nov 25 '18 at 11:23
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    I can't produce a circuit as easily as that because it all depends on what components you have to hand and what your coil design is and how you are resonant tuning it. I would say that you should google solutions and raise a new question on the subject because it is fundamentally different to what you have asked here and more people will likely be able to contribute. – Andy aka Nov 25 '18 at 11:23
  • Here is a useful link: https://www.eleceng.adelaide.edu.au/students/wiki/projects/index.php/Projects:2015s1-71_Inductive_Power_Transfers - you should read more about the subject and find solutions on-line that already have solved a lot of the problems. Another link: https://www.powerelectronics.com/pmics/wireless-power-receiver-supports-contactless-battery-charging – Andy aka Nov 25 '18 at 11:35
  • when you say "Don't forget that the output from the Colpitts oscillator is "weak" and needs to be buffered by another transistor " did you mean to amplify the output of the oscillator? im sorry if my questions are bit dumbed. – dumbedkid Nov 25 '18 at 12:48
  • A buffer amplifier doesn’t have to amplify the voltage, it just has to provide an output that can drive a few hundred mA to your coil and has a reasonably high input impedance so that the oscillator output is not excessively loaded. – Andy aka Nov 25 '18 at 12:52
  • so i need a current buffer instead of voltage buffer – dumbedkid Nov 25 '18 at 12:56
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Three circuit you've shown have this KEY signal inversion:

schematic

simulate this circuit – Schematic created using CircuitLab

Along with the polarity inversion of the grounded-emitters in those 3 circuits, the net loop polarity is INPHASE, required for such an oscillator.

To remain on frequency, you should use one of these as your master oscillator. Then buffer this oscillator into a class "C" power amplifier, and use a PI filter to attenuate harmonic distortion and transfer the desired energy into your inductive antenna.

analogsystemsrf
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