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RC Oscllator

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ProFPGA

Banned
HI ,

I need help with this RC oscillator i am trying to make .
its dead and i dont know where i am wrong .
 

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BrownOut

Banned
What do you mean by "dead?" It won't simulate? Prototpe not working? Was it working and stopped???? Details are important.

Edit: R2 is pretty high. How did you arrive at that value?

Also, your amp had a gain of less than 3, which may not be enough to sustain oscillations. You may need a little more.
 
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MikeMl

Well-Known Member
Most Helpful Member
Bias resistor is way too high. It likely wont oscillate because gain is too low. Make R1 bigger, and C1 bigger.
 

dougy83

Well-Known Member
Are you missing a junction dot between R1, Q1 and the output/feedback line?
 

audioguru

Well-Known Member
Most Helpful Member
Look at the values of R2 and R4. They turn off the transistor so it doesn't work.
 

john1

Active Member
Yes.
Two things.
1
Each stage of phase-shift costs about a third of the power, maybe a little more.
The gain is insufficient, i recommend strapping another Tr across the existing one,
darlington fashion.
2
Also that four and a half megohms is too high. (As has been mentioned.)

Biassing.
Disconnect that train of caps, adjust till you get between 3 and 6 volts on the collector
of the output, then reconnect the train of caps.

Best of luck with it, John :)
 

BrownOut

Banned
Look at the values of R2 and R4. They turn off the transistor so it doesn't work.

Good point. Although it usually isn't shown on schematics, it might be a good idea, for your circuit, to capacitively couple the feedback network to your base. Alternatively, some oscillators connect the 3rd stage resistor to the base, rather than ground.
 
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colin55

Well-Known Member
R2 should be connected to the collector. The 560R's should be about 22k. R1 should be about 4k7. If you follow the convention of putting the "k" between the figures, we won't have the problem of trying to work out what values you have used.
 
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ProFPGA

Banned
Thanks for all those tips , i will try to test each and every one of those .
By "dead" i meant i made a prototype and i dont see anything on the oscilloscope !!!
i will give you feedback after adjusting some the R's and C's.
 

ProFPGA

Banned
here is the new schematic and its results simulated in altium .
The missimg junction really helped , i wasnt able to simulate it first .
I wont alter the prototype as yet , once i get it running in simulation.

As the simulation shows oscillations die down after some time ???
 

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MikeMl

Well-Known Member
Most Helpful Member
Insufficient gain. Gain of the amplifier stage MUST be > 29.

Look at this circuit provided as an example with the LTSpice download. Note the bias to the transistor. Note the emitter bypass.
 

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ProFPGA

Banned
Ok , but the supply is 12v , how do i translate the same %age of gain using 9V DC ??
i am a bit rusty on analogue design .
How to calculate the values of emitter bias and base bias Resistors??
 

ProFPGA

Banned
the built ckt works , i have made the ckt as posted by MikeMI .
the output frequency is 546.5hz .
to make this ckt for higher frequencies i am using the formula : f=1/2*pi(2N*RC)^0.5.
is this rite ??
 
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