Good day, everyone!
I'm working on a design that starts with a standard Colpitts oscillator. As an example, consider the following:
It works, but I'm not sure how to determine the values of the individual components. I've run several simulations and have a paper with numbers and results, but I'm still stumped about which two Cs to use for the LC tank. If the values are not properly matched to those of the coil, the oscillation does not start or is distorted (even if the relationship between Cs is roughly 10). I've been looking for a direct solution to this question, but despite a few discussions on the issue, there aren't any conclusive responses in the forum.
I know F=1/(2pi sqrt(LC)), but I can't see making a 5 Mhz oscillator with an mH coil and pF coils, or a 5Khz with a 1uH coil and 1000uF capacitors (approximate values).
I'm not sure, but I believe it all has to do with the components' impedances. In any manner possible, the impedance should be matched to the amplifiers in and out resistances. It could be an issue with common-base amplifiers.
I've read some tutorials like https://www.apogeeweb.net/electron/the- ... lator.html, but I'm still stumped.
Any and all suggestions would be greatly welcomed.
Colpitts Oscillator
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Nigella
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MN785
Re: Colpitts Oscillator
I am by no means an expert with electronics and I am sure there are others here better suited to answer this, but from the little experimentation I have done, I can make a couple general suggestions. In the link you provide, lets take the schematic in section III. Cin should be just enough to maintain oscillation. Too little capacitance and it don't go, too much and it's swamping the transistor making trash and degrading the Q of the tank. The few times I've made a colpitts oscillator, I've started with a few pF and increased it until it started oscillating good and left it alone. As for the tank circuit components, it is true that there is an infinite number of combinations that mathematically resonate at a particular frequency, but given the fact capacitors are relatively high Q compared to inductors, I would recommend using the heaviest wire you can with the fewest turns practical and base the capacitor value on that inductance. And be sure to use tank capacitors with decent stability, perhaps polycarbonates, with a parallel air variable for adjustment.
Post Merge Complete
Added 17 minutes 30 seconds after previous.
I should also add that the majority of my trouble learning RF stuff came from two sources not generally covered in the literature. #1-these new transistors, even 2N3904's and ones made for audio applications can oscillate into the UHF range and avoiding parasitics/dampening unwanted oscillations must be considered even when working with relatively low HF or audio frequencies. Give that transistor a long enough trace and BOOM! that bugger is going nuts! #2-don't buy a single active component off amazon, not a single one, and be mighty suspicious of anything you find on ebay. If, for some reason you do not heed that advice, at least get a simple transistor tester that measures the beta (Hfe) because 9 times out of 10, something like a 2N3904 purchased on amazon will have a Hfe in the 400 range when an authentic 3904 has a beta around 142. That number alone can usually tell you that someone laser-etched a fake number on a floor-swept out-of-spec piece of industrial trash (save 'em for switches/TTL logics lol).
Post Merge Complete
Added 54 minutes 10 seconds after previous.
One more thought. I guess I misread the original question and now see it is regarding the ratio of C1 and C2 in the tank. You can think of these as a voltage divider. If the entire tank voltage was sent back to the input, well... it needs to be smaller because the transistor circuit has gain. If it amplifies what goes in, we must cut down the amount of it returned. For example, if the emitter resistor is bypassed, the gain of a common emitter amplifier is roughly the voltage across the collector resistor at its quiescent point divided by .026 (at room temperature at least). If you know then gain of the amplifier, you can determine the amount of feedback you need, and thus the ratio of C1 to C2 to obtain the required amount of feedback. If you go on youtube and watch W2AEW's video # 273, he covers common emitter amplifier gain. An oscillator simply returns just enough of the output back to the input to sustain the oscillation.
Post Merge Complete
Added 17 minutes 30 seconds after previous.
I should also add that the majority of my trouble learning RF stuff came from two sources not generally covered in the literature. #1-these new transistors, even 2N3904's and ones made for audio applications can oscillate into the UHF range and avoiding parasitics/dampening unwanted oscillations must be considered even when working with relatively low HF or audio frequencies. Give that transistor a long enough trace and BOOM! that bugger is going nuts! #2-don't buy a single active component off amazon, not a single one, and be mighty suspicious of anything you find on ebay. If, for some reason you do not heed that advice, at least get a simple transistor tester that measures the beta (Hfe) because 9 times out of 10, something like a 2N3904 purchased on amazon will have a Hfe in the 400 range when an authentic 3904 has a beta around 142. That number alone can usually tell you that someone laser-etched a fake number on a floor-swept out-of-spec piece of industrial trash (save 'em for switches/TTL logics lol).
Post Merge Complete
Added 54 minutes 10 seconds after previous.
One more thought. I guess I misread the original question and now see it is regarding the ratio of C1 and C2 in the tank. You can think of these as a voltage divider. If the entire tank voltage was sent back to the input, well... it needs to be smaller because the transistor circuit has gain. If it amplifies what goes in, we must cut down the amount of it returned. For example, if the emitter resistor is bypassed, the gain of a common emitter amplifier is roughly the voltage across the collector resistor at its quiescent point divided by .026 (at room temperature at least). If you know then gain of the amplifier, you can determine the amount of feedback you need, and thus the ratio of C1 to C2 to obtain the required amount of feedback. If you go on youtube and watch W2AEW's video # 273, he covers common emitter amplifier gain. An oscillator simply returns just enough of the output back to the input to sustain the oscillation.
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MDYoungblood
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Re: Colpitts Oscillator
Boy this takes me back to my high school electronics class days. If you read up on Colpitts, it took him several years to get his working so it is trail and error. Luckily in todays world, everything needed to build his oscillator can be found in an adjustable form. Look at some plans for an amateur spark gap radio, the values of the components are close to what you need.
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Greg
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MN785
Re: Colpitts Oscillator
I just stumbled on this and he does a great job breaking it down!!!!
https://www.youtube.com/watch?v=BXcJxeEQo-M[br][/br]
Post Merge Complete
Added 1 day 18 minutes 27 seconds after previous.
In the process of trying to learn more about this myself, I think I have came to understand the reasoning behind choosing the reactance of the individual tank components. Obviously, capacitors have less loss than inductors, so the main focus is on the inductor value. The goal with the inductor is to have the highest Q possible, stated another way, to have the ratio of reactance to resistance as large as possible. As you increase the inductance, you will be increasing the number of turns needed and simultaneously reducing the wire diameter to keep the size practical. Therefore, increasing the reactance has the consequence of increasing the resistance, and at some point, the Q (XL\R) will start to degrade and loss becomes significant. I believe this is the limiting factor in determining the practical values of L (and thus C). In the video above, the bypass/blocking capacitors have a reactance of about 1.3Ω (which makes sense and suggest I was wrong about Cin in my first reply) and the tank components around 160Ω. I would speculate that, short of testing the inductor Q yourself given your frequency and coil design, these might be good values to initially shoot for.
https://www.youtube.com/watch?v=BXcJxeEQo-M[br][/br]
Post Merge Complete
Added 1 day 18 minutes 27 seconds after previous.
In the process of trying to learn more about this myself, I think I have came to understand the reasoning behind choosing the reactance of the individual tank components. Obviously, capacitors have less loss than inductors, so the main focus is on the inductor value. The goal with the inductor is to have the highest Q possible, stated another way, to have the ratio of reactance to resistance as large as possible. As you increase the inductance, you will be increasing the number of turns needed and simultaneously reducing the wire diameter to keep the size practical. Therefore, increasing the reactance has the consequence of increasing the resistance, and at some point, the Q (XL\R) will start to degrade and loss becomes significant. I believe this is the limiting factor in determining the practical values of L (and thus C). In the video above, the bypass/blocking capacitors have a reactance of about 1.3Ω (which makes sense and suggest I was wrong about Cin in my first reply) and the tank components around 160Ω. I would speculate that, short of testing the inductor Q yourself given your frequency and coil design, these might be good values to initially shoot for.