Colpitt Oscillator using opamp-uncontrolled oscillations

[abbeyromy]

Which software are you using? I am afraid that I can only help you for pspice. but there are many seniors here who know almost everything. You can post your doubts about simulator working in appropriate section of the forum.

Refer the MODIFIED COLPITTS OSCILLATOR.doc

Well placing of back to back diode is a method of amplitude stabalization.. refer any book or internet for detailed explanation. In a nutshell for very small signals diode does not connect and the resistor across which these are placed comes into circuit increasing the gain (as it acts along with a feedback resistor). When amplitude starts to increase diodes get fwd biased and thus eliminate the resistor from circuit and thus stabilizing gain.

The feedback resistors are chosen such that their ratio balances the feedback attenuation of LC network so as to mainatain the loop gain of 1. Refer theory and see that how C1/C2 ratio matches closely with R2/R1 ratio.

Well placing a 5K resistor across output side is just my imagination in order to get the circuit oscillating . I am not an experienced designer so use at your own risk. No book or internet site has it.

Use of 5K ohms resistor at output is done so as to match the input condition as input side has 5K resistor of inverting feedback arrangement. The more identical input and output stages are the better the chances of having a stabilized sine wave at output thats the thumb rule I have followed.

Well not every circuit shown in the text book would work practically. U need to apply the principles and select resistor, capacitor values logically. Believe only schematics and not diagrams!!

Best of luck!

-Hemanshu

 

[yasir_ali86]

well..... Thanks a tan to reply.
but i still have some questions that:

1- how did you design the feed back (LC Portion)? i designed that portion using a rule, i read some where that the reactance of the coil of colpitts Oscillator should be arround 140, so using that i determined the value L. and you know at resonance the the impedance of capacitor should be equal thats why i used the same 140 impedance for capacitor and found the value of Capacitor too. but i was not getting the output sine wave. so, i wanna ask you that how did you design the LC portion of your Oscillator? can you tell me your Mathematics?

2- in OP-Amp circuit, why did you change the value of Rin and Rf from 10k and 100k to 5k and 50k? both are satisfying the condition A*B=1 . pleas tell me.

3- i have designed a common emitter Amplifier and tested which is giving me a gain of very near to 10. how can i design a LC portion of this circuit to have an Oscillation of 1MHz.
-The design in attached please see it.

Thanks

 

[abbeyromy]

@yasir_ali86

For colpitt feedback arrangement the factor B = C2/C1 = 1/10 in my case. Now gain A =10 in order to make AB=1.

So your R2/R1 =10. You can select 100k and 10 k as well. but then if you are making R1=10k then you need to take care that between output, colpitt arrangement and input side similar condition exist in terms of current.

The derivation of B =C2/C1 assumes that the output current going through the colpitt arrangement is same that goes to the input of opamp. So just try to implement this.

How did I select the L value? Well I just took it as 50mH as I was not focused on any particular frequency of resonance.

"reactance of the coil of colpitts Oscillator should be around 140" can you please share the source of this statement?

Regarding transistor design make sure your gain RC/Re( refer books to find the equation for derivation of this) is near t ratio of C1/C2 if not exactly equal. You need to play with values of C for these. Once you have finalized the C1 and C2 use the formula to calculate what value of L can help you achieve desired frequency of operation.

I must say that you need to mix n match values of R,L,C in order to get the desired response. The math that I have used gets you near nothing more. I have been myself unable to find anything solid over internet/books for this. I have just surviving level knowledge of the concept

Keep trying and do post if you come up with something interesting!
-Best of luck!

 

[yasir_ali86]

OK. now what do you think what should we do? we both have stuck on the same place.

have you seen my Amplifier i designed?

and about impedance of coil, i told you i read it somewhere i think i read it in the forum but I'll try to search that again and will give you the link.

i am also working on it and you also should work on it.in this way we can help each other.
look at the attachment i found on internet. this is a working colpitts Oscillator with Common Emitter Configuration. work on it and find what we are missing in our oscillators. i am doing the same. okay.?
And please tell me any report regarding our Oscillator.

Wish you Very Good Luck

 

[yasir_ali86]

Hey! Guys I have designed my own colpitts Oscillator. please have a look at it. i have 2 Questions that:

1- How can i improve its sine wave? (like it's clipping from the edges).should i decrease the gain of the Amplifier? if change the gain, then condition for Oscillation goes false and whole Oscillation stops.i have tried changing the value of C3. first i increase it and then decrease it but the only effect was on amplitude, the wave as it is shown(no effect on wave).What should i do now.?

2- Is it possible to increase its Amplitude still more?(what i think is that, we cannot increase the amplitude, but I'm just confirming).

 

[FvM]

How can i improve its sine wave? (like it's clipping from the edges).

The waveform is not clipping. Your simulation uses an inappropriate large timestep. The difference can be easily seen from the
waveform. You have to force the simulator to a suitable time step to determine the actual waveform. With Multisim, you can use
the THD measurement to get the distortion.

Is it possible to increase its Amplitude still more?

Yes, by increasing the supply voltage. You have already 8 Vpp, which
is a lot with 9 V supply.

 

[yasir_ali86]

Hey! FvM! Thanks a lot