RC phase shift produces sine wave right?

Only if the amplifier stage is correctly biased and distortion-free and has the appropriate gain.

 

I ha
RC phase shift produces sine wave right? And I have to make 5khz frequency Waveform how to tackle it plz make necessary adjustments in it

Hi,

Yes, an RC phase shift oscillator produces sine waves, but the gain of the amplifier section (which you are trying to use a MOSFET for) has be a certain value to get sustained oscillations. We can calculate that with a given set of R and C in the circuit. For 3 sections it would be one value and for 4 sections it should be another value. If it is not the required value, the circuit either oscillates for a little while then dies out or oscillates until it clips, or the output gets pinned to one of the supply rails. This is even when you use a linear amplifier in place of the MOSFET.

The way this is usually handled is a certain amount of clipping in introduced on purpose. It's not placed directly on the output though because that affects the output as well. It is placed such that the signal that reaches the RC sections is clipped, but not the output. That provides some automatic gain control which keeps the oscillator going properly.

You can get it to work without clipping, but the amount of feedback around the MOSFET has to be very close to the right value or else the output oscillations die out or get clipped. There is a chance that it won't work at all if you consider long term stability, however. That is for several reasons like temperature changes, but it is very hard to get the exact value anyway, and there is no known oscillator that can do that except in theory of course.

What you need to do then is use feedback so that you get more gain than would be really required, then add clipping to the signal that gets to the RC sections, or possibly after just the first RC section. It's a bit of a trick to get this to work right, but we can go over some calculations, so you know where you stand with this a little better.
The RC sections act as a filter to filter the clipped waveform, so you don't have to worry about it getting to the output anymore.

So the way i see it is you have to do two things to get this to work, if you want a sine wave output:
1. Make the MOSFET part of the circuit as linear as possible using drain to gate feedback, while keeping the output bias point at a reasonable level (like 1/2 of Vcc).
2. Add clipping such that it does not affect the output, only the signal getting to the RC sections. It may be possible to add this after the very first RC section which would then isolate the clipping action from the output while still automatically adjusting the gain. You'll have to experiment with this or do some calculations.

I'll try to show you a Root Locus diagram of this system but not sure if I can get to it today. What that shows us is how the oscillations vary with the change of gain in the amplifier part of the circuit (the MOSFET in your circuit), and why it has to be of a certain value to get it right.

This is all if you really want a sine wave output, but there is still the question if you need a sine wave output or not. Most RC phase shift oscillators are made to be sine wave output oscillators because you don't need 3 (or more) RC sections to get a non-sine wave output oscillator. To use 3 or 4 RC sections to create a non-sine wave output oscillator would be a waste of components. This means there is probably the presumption that it has to be a sine wave output, but I'll leave that for you to decide.

There is also the possibility of adding a passive bandpass filter to the output to get a sine wave output, but I do not think that is a good way to handle this as that adds more components that probably are not needed.

 

Hi,

Yes, an RC phase shift oscillator produces sine waves, but the gain of the amplifier section (which you are trying to use a MOSFET for) has be a certain value to get sustained oscillations. We can calculate that with a given set of R and C in the circuit. For 3 sections it would be one value and for 4 sections it should be another value. If it is not the required value, the circuit either oscillates for a little while then dies out or oscillates until it clips, or the output gets pinned to one of the supply rails. This is even when you use a linear amplifier in place of the MOSFET.

The way this is usually handled is a certain amount of clipping in introduced on purpose. It's not placed directly on the output though because that affects the output as well. It is placed such that the signal that reaches the RC sections is clipped, but not the output. That provides some automatic gain control which keeps the oscillator going properly.

You can get it to work without clipping, but the amount of feedback around the MOSFET has to be very close to the right value or else the output oscillations die out or get clipped. There is a chance that it won't work at all if you consider long term stability, however. That is for several reasons like temperature changes, but it is very hard to get the exact value anyway, and there is no known oscillator that can do that except in theory of course.

What you need to do then is use feedback so that you get more gain than would be really required, then add clipping to the signal that gets to the RC sections, or possibly after just the first RC section. It's a bit of a trick to get this to work right, but we can go over some calculations, so you know where you stand with this a little better.
The RC sections act as a filter to filter the clipped waveform, so you don't have to worry about it getting to the output anymore.

So the way i see it is you have to do two things to get this to work, if you want a sine wave output:
1. Make the MOSFET part of the circuit as linear as possible using drain to gate feedback, while keeping the output bias point at a reasonable level (like 1/2 of Vcc).
2. Add clipping such that it does not affect the output, only the signal getting to the RC sections. It may be possible to add this after the very first RC section which would then isolate the clipping action from the output while still automatically adjusting the gain. You'll have to experiment with this or do some calculations.

I'll try to show you a Root Locus diagram of this system but not sure if I can get to it today. What that shows us is how the oscillations vary with the change of gain in the amplifier part of the circuit (the MOSFET in your circuit), and why it has to be of a certain value to get it right.

This is all if you really want a sine wave output, but there is still the question if you need a sine wave output or not. Most RC phase shift oscillators are made to be sine wave output oscillators because you don't need 3 (or more) RC sections to get a non-sine wave output oscillator. To use 3 or 4 RC sections to create a non-sine wave output oscillator would be a waste of components. This means there is probably the presumption that it has to be a sine wave output, but I'll leave that for you to decide.

There is also the possibility of adding a passive bandpass filter to the output to get a sine wave output, but I do not think that is a good way to handle this as that adds more components that probably are not needed.

Don't know my question is to design RC phase shift Oscillators using MOSFEt having fruqeucny 5khz and Feedback stages is 4

 

Don't know my question is to design RC phase shift Oscillators using MOSFEt having fruqeucny 5khz and Feedback stages is 4

Hi,

Yes my post explains the key points you have to deal with when making such an oscillator:
1. Proper feedback and DC output bias.
2. Added clipping in the right place.

Oh so you are not sure if it has to have a sine wave output.
Well, as I also explained, you don't need 4 stages to create a non-sine wave output oscillator so if part of the task is to use 4 stages, then I would think it should be sine wave out. The only way to be perfectly sure is to ask the instructor that gave you this assignment.