Common-Base colpitts osc. help

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samy555

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Hi
I've built this oscillator, then I implement it using a simulation program LTC, which is used by most of you, but unfortunately it did not work permanently.
Here is the Circuit Diagram
Here is the simulation software file easier for you to check the circuit
www.edaboard.com/attachment.php?attachmentid=108250&stc=1&d=1407863886
Notes
Each coil has its internal series resistor
The 50 ohm load is an antenna which is matched to the output of the OSC. Through the tap.

I need your help
Any inquiry about the values of the elements I am ready to answer
Thank you
 

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  • 1.rar
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Remove C2. Also I think you might have reversed the values of R1 and R2. See if it will oscillate with the load removed, then connect the load and see if it continues to oscillate, because I feel that L1 should be smaller, this would increase the gain of the circuit and potentially the output. Fiddle with the value of L1 to get maximum output.
Frank
 

chuckey said:
Remove C2.
Click to expand...
C1 & C2 reflect the low input impedance to be high at the output.

Also I think you might have reversed the values of R1 and R2.
Click to expand...
no, R1 and R2 ensure the operationg point (IC=1.3mA, VCE=1.5V)
After removing the load, the circuit did not oscillate. L1 & L2 are clculated carefully to oscilate at 100MHz
thank you Frank
Waiting for more suggestions
 

Looks like you would like an oscillator in the range below 100MHz. I think you're missing a capacitor across the inductances.
 

rfmw said:
Looks like you would like an oscillator in the range below 100MHz. I think you're missing a capacitor across the inductances.
Click to expand...
Yes 100MHz.

no capacitor is needed across the inductances. The series C1&C2 are infact across the inductor.

thanks
 

No they are not. To be parallel capacitance you must ensure zero impedace across the supply rails and take into account the phase shift across the capacitors as well. Try connecting 20pF directly across the two inductors. I am assuming they are in fact a single inductor with a tap, if they are two real inductors you would also have to take into account their mutual inductance.

Brian.
 

My design based on this attached file
What I added was a tap in the coil to match the low impedance antenna
 

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  • 2.rar
    20.1 KB · Views: 130

My FM transmitter is the normal circuit with a capacitor parallel with the inductor and no emitter capacitor like hundreds of circuits on the internet. It oscillates at about 100MHz.
It stops oscillating when the load is too much for it (low impedance) and will work if the inductor is tapped.
 

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  • FM transmitter.png
    23.3 KB · Views: 88

Thank you Guru for participating
If you want to add any other information, you are welcome

I have equestion related to your design
Why you did not use the voltage divider in the dC part of your oscillator?
You told me once that the voltage divider reduces dependence on beta
 

samy555 said:
Why you did not use the voltage divider in the dC part of your oscillator?
You told me once that the voltage divider reduces dependence on beta
Click to expand...
This is not a low distortion linear audio amplifier. It is a simple oscillator and beta makes very little difference.
 

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