Colpitts Oscillator

M

miki

Member
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.
 
Sorry, the answer is a bit late!
The frequency of oscillation will be the product of the coil inductance and to total value of the two capacitors in series. For example 112pf in series with 112pf = 56pf, with a coil inductance of 18uH will osc at around 5Mhz For speed of calculation, you can use one of the many online resonant frequency calculators. Your equation is correct but the common mistake is people put the units of inductance and capacitance into the equation using the wrong units. The units of division uH, mH, pF, nF, uF mF must be the same for the C and the L.

Example 56 pF in tank with 18uH (this is where numbers get mixed up) 56pf = 0.000 000 000 056 F 18uH = 18000000 pH Next multiply these numbers together to get 0.001008 then sqroot this to get 0.03174 multiply this by 2xpi = 0.199383 the the reciprocal of this = 5.015 Mhz Recap, if the C is in pF convert the L to pH before calculating and your result will be in Mhz. I hope this helps. Check your results using an online resonant frequency calculator. I work with this stuff all the time and still make mistakes when using manual calculation! It`s so easy to get an extra zero. Just for interest, I prefer the Colpitts oscillator.
 
Dresden-Works said:
Sorry, the answer is a bit late!
The frequency of oscillation will be the product of the coil inductance and to total value of the two capacitors in series. For example 112pf in series with 112pf = 56pf, with a coil inductance of 18uH will osc at around 5Mhz For speed of calculation, you can use one of the many online resonant frequency calculators. Your equation is correct but the common mistake is people put the units of inductance and capacitance into the equation using the wrong units. The units of division uH, mH, pF, nF, uF mF must be the same for the C and the L.

Example 56 pF in tank with 18uH (this is where numbers get mixed up) 56pf = 0.000 000 000 056 F 18uH = 18000000 pH Next multiply these numbers together to get 0.001008 then sqroot this to get 0.03174 multiply this by 2xpi = 0.199383 the the reciprocal of this = 5.015 Mhz Recap, if the C is in pF convert the L to pH before calculating and your result will be in Mhz. I hope this helps. Check your results using an online resonant frequency calculator. I work with this stuff all the time and still make mistakes when using manual calculation! It`s so easy to get an extra zero. Just for interest, I prefer the Colpitts oscillator.
Click to expand...
That's a very in-depth analysis. This surprisingly will help me as well.
 
Back
Top