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colpitts oscillator design problem

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ppat2006

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hi,


i have designed a bjt colpitt oscillator in matlab for 5 Mhz freq. but it shows damped oscillation since i have used collector resistance in place of rfc choke.what value of inductor to use as rfc choke. i have also another ckt for 2mhz but dont know how to choose value for rfc.??

anybody help?
 

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  • colpitt1.bmp
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The impedance of Ce is about 30 ohms in 5MHz. It can heavily reduce the open loop gain of feedback. Therefore, increasing the value of Ce to much higher value, in micro range, may solve your problem.

It is difficult to have choke at low frequency like 5 MHz.
 

i tried to change the Ce value in uf but it doesn't work. i have to use rfc or inductor in place of Rc bcoz rfc is necessary component for high frequency oscillator. due to Rc the output is getting damped means the amplifier is not working.

so how much value of inductor to use??
 

You don't need Ccc1, your L in the 5MHz osc is far too small and the caps are far too big. The impedance of the tuned circuit is L/CR (R being the loss resistance). make your L about 10X bigger (at least) and both the caps 1/10 of their values. 450 ohms is a low value anyway use 10K ohms and re-adjust the bias so the transistor takes about 1mA.
Frank
 

I don't see how the oscillator can work, because it implements negative rather than positive feedback. As chuckey already mentioned, the LC circuit has an unreasonable low impedance of 1.5 ohm, considering expectable Q of real inductors, it won't work.
 

actually i have made the design as per an electronic textbook of engineering. if u have design notes plz give. anyways thanks i will try to change the cap and inductor values.
 

i have made the design as per an electronic textbook of engineering
Either the textbook is wrong or you copied it incorrectly. A colpitts oscillator in common emitter circuit has a different feedback topology.
 

The oscillator will oscillate, but L2MHz = 6.8microH, L5MHz = 47 nanoH. You want to increase the frequency by 5/2, so reduce the coils value by 4/25 will retune the circuit to 5MHz but as explained will reduce the circuits impedance too much so, reduce the coil's L by 5/2 or .4 X 6.8 = 2.8 microH and the two capacitors by 5/2 as well ( 22nF -> 10nF, 1nf -> 400 pF).
In your 5MHz circuit you got the values of the capacitor the wrong way around.
Frank
 

The oscillator will oscillate.
Which circuit you are referring to, anything I missed besides post #1? I'm really curious to see an oscillator working with negative feedback.
 

FvM, You are right and I am wrong. last night I was sure I had seen an oscillator like the circuit that was linked to. Since then I have been wracking my brains to try and figure out how the thing worked. I have come to the conclusion that it can't.
Its a case of I would not use that configuration,i would connect the top end of tuned circuit to the base. The capacitive tap going to the emitter, with no emitter decoupling capacitor. The collector resistor provides a good semi-isolated output.
Frank
 

plz give me a colpitt oscillator schematic with values that shows sustained oscillation and tht can run in matlab/simulink.


also my problem is tht in matlab i have to select the bjt already provided by simulink tht means just like in pspice i cannot select any particular transistor number. so i have to adjust the parameters of bjt to act as an amplifier bcoz if amplifier is not working then i will get damped oscillation bcoz tank ckt always gives damped oscillation.


the problem of feedback i have solved. there was little error in connection.
 

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  • bjt parameters.bmp
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  • bjt1.bmp
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  • bjt2.JPG
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