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 Homework Help Stuck on a textbook question or coursework? Cramming for a test and need help understanding something? Post your questions and attempts here and let others help.

#1
10-02-2009, 11:26 PM
 hasie Junior Member Join Date: Sep 2009 Location: South Africa Posts: 35

Hi Guys
Im new to this site, and im really looking forward to explore further. By the topics I have read sofar, this forum really seems informative.

Dont really know your full rules yet, but I would really appreciate it if anybody could give me help on my problem.
I did run a search on the forum, but did not get my answer that i am wondering about.

I am currently busy trying to build a RC Phase Shift Oscillator.

I see there is alot of info available on the net, but not any info sofar that I specifically found that I am looking for.

Most sites explain the works of the phase shift oscialltor with op amps.
My instructions are:
* Must be designed with BC547 transistor
* Frequency of oscillation must be 1KHz
* Must be as close as possible to a perfect sine wave
* Signal Amplitude must be atleast 2v(p2p)
* Must be operated with a 9v supply

I am not too farmiliar with these oscillators, so this week searched for as much info as I could.

I found a circuit at http://www.talkingelectronics.com.au...TrCcts.html#37 that somebody told me works perfectly.

I then used the formula to get the required resistance values and cap values for the required frequency.( F = 1/((2PI)(R)(C)(sqrt6))

an accepable value to the frequency to caps that I have with me, was 4.7nF capacitors, and 13.8K resistors(12k and 1k8 in series).

The other 2 resistors I left as is at 1M and 3k3.

On my simulation program it runs smoothly and just short of 1KHz, allthough when I built the circuit it gives a saw tooth(triangular wave) waveform.

I have no clue as to how to continue. Can anybody maybe shed abit of light that maybe know, or maybe have a better circuit for me?

ANy help much apprecated

Many Thanks
#2
10-03-2009, 12:46 AM
 Audioguru Banned Join Date: Dec 2007 Location: Ontario, Canada Posts: 9,411

The simple single transistor phase-shift-oscillator should give a clipped sine-wave output. The output is clipped because the circuit has nothing to prevent the transistor from going into saturation. The transistor does not have much negative feedback so it produces distortion.

Check the pins of your transistor and the wiring of your circuit to see why it produces a sawtooth/triangle waveform.
#3
10-03-2009, 02:14 AM
 Bill_Marsden Super Moderator Join Date: Mar 2008 Location: Dallas, TX (GMT-5 w/ DST) Posts: 19,022 Blog Entries: 5

A simple light bulb can be used to create an AGC (automatic gain control) effect. Why not post your simulation?
__________________
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"Good enough is enemy of the best." An old engineering saying, Author unknown.

General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input. Thanks for the verbage Wookie.
#4
10-03-2009, 08:50 AM
 hasie Junior Member Join Date: Sep 2009 Location: South Africa Posts: 35

My simulation on proteus?

How do I post my simultion? WIth a screenshot?

Many thanks guys, will recheck my circuit again quickly allthough I am almost sure its right..

How do I post simulation? Sorry im new to this

Many thanks
#5
10-03-2009, 09:49 AM
 hasie Junior Member Join Date: Sep 2009 Location: South Africa Posts: 35

And is this circuit relatively stable and accurate? Or is there a better circuit I can use to get a stable 1KHz sine wave?

Many thx
#6
10-03-2009, 09:51 AM
 Bill_Marsden Super Moderator Join Date: Mar 2008 Location: Dallas, TX (GMT-5 w/ DST) Posts: 19,022 Blog Entries: 5

An RC oscillator is never going to be as accurate as a crystal oscillator, but it is accurate enough. I was asking for the schematic locally with the adjusted values. It might give us some insight as to what's happening.

Looking at your schematic, try adding a variable resistor to the emitter. This is better negitive feedback than the base resistor going to the collector. It will control the gain much better.

This transistors biasing needs a lot of work. This is the fundimental reason the waveform is so off.
__________________
..
"Good enough is enemy of the best." An old engineering saying, Author unknown.

General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input. Thanks for the verbage Wookie.

Last edited by Bill_Marsden; 10-03-2009 at 09:57 AM.
#7
10-03-2009, 10:15 AM
 hasie Junior Member Join Date: Sep 2009 Location: South Africa Posts: 35

A RC oscillator was askes so I just hope to get a good design.

Here is my circuit at this moment

Ill play around with a variable resistor on the emitter.

many thanks for the help sofar

Any other suggestions?

Thanks
#8
10-03-2009, 10:20 AM
 hasie Junior Member Join Date: Sep 2009 Location: South Africa Posts: 35

When adding a 100ohm var resistor to the emitter the amplitude on the oscilloscope gets smaller.

And then when I remove the 1M resistor going from the base to the collector it does not oscillate at all?

Im going to rebuild this circuit again quickly on breadboard. and test it again. any input appreciated.

Many Thanks

Last edited by hasie; 10-03-2009 at 10:42 AM.
#9
10-03-2009, 11:02 AM
 hasie Junior Member Join Date: Sep 2009 Location: South Africa Posts: 35

Rebuilt the whole circuit on different breadboard, different components. Still doesnt work.

See also the thing that doesnt make sense to me, is that there are 3 caps in the RC network but only 2 resistors. is the f = 1/((2PI)(R)(C)sqrt6) still correct?

I can use this circuit. http://www.electronics-tutorials.ws/...scillator.html

But I have no idea what to make R4, R5, R6 and C4?
Would this circuit be better to use?

Any ideas appreciated,

Last edited by hasie; 10-03-2009 at 11:15 AM.
#10
10-03-2009, 12:13 PM
 Bill_Marsden Super Moderator Join Date: Mar 2008 Location: Dallas, TX (GMT-5 w/ DST) Posts: 19,022 Blog Entries: 5

I like the second circuit much better, it addresses my points about the transistor bias. Thing is, a 3 leg RC oscillator should have matching RC legs, the first design only had 2, the third they were hoping the transistor would be close enough to the correct impedance.

The second circuit actually puts a 3rd RC leg in there. Try using similar bias resistors, and see it if works. You can adjust the gain of this circuit by putting a variable resistor in series with the capacitor, which will allow you to adjust the gain of the transistor amp. While I don't know what the resistor needs to be it will be quite low in value.

Here's Wikipedia's take on it.
http://en.wikipedia.org/wiki/RC_Phase_shift_Oscillator
__________________
..
"Good enough is enemy of the best." An old engineering saying, Author unknown.

General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input. Thanks for the verbage Wookie.

 Tags oscillator, phase, shift

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