What kind of oscillator is this

[abbeyromy]

Hey guys, give a look at this circuit. I was observing oscillations so just wanted to know how is this happening.
adding a filter after this stage will improve sine wave shaping? Sallen-key would work?

 

[vfone]

This is a relaxation type oscillator.
Yes, you can try adding a filter to improve the output shape, but better option is to make other type of oscillator, for example Wien Bridge.

https://www.ti.com/sc/docs/apps/msp/journal/aug2000/aug_07.pdf

 

[keith1200rs]

A few observations:

The LMV921 is obsolete.

Your maximum timestep is too large.

The maximum supply voltage of the LMV921 is 5.5V

The output should be a square wave but your time constant is far smaller than the opamp can manage.

Keith

 

[abbeyromy]

@vfone Thanks for the link!
@keith1200rs
well you are right... The output expected was a square wave but I was getting a sine wave. Can you suggest some other opamp? or should I try to increase C value to increase the time constant?

 

[LvW]

@vfone Thanks for the link!
@keith1200rs
well you are right... The output expected was a square wave but I was getting a sine wave. Can you suggest some other opamp? or should I try to increase C value to increase the time constant?

Hi, abbeyromy.
*What is your goal? Squarewave or sine wave?
*The circuit shown is very common, however, it is a very interesting one. Normally it acts as a relaxation oscillator with a squarewave output. But within a certain range of positive feedback factor it can produce sinusoidal signals. In this case, the dominant opamp pole is exploited.

 

[abbeyromy]

@keith1200rs
Instead of LMV921 I came down to uA741 as I was having its ready made Pspice symbol. Can I use LM118?
I was able to get square waveforms on increasing the value of c to 1uF but then the square wave was going from +15V to -15V (supply for uA741). Now this is what will happen if amplitude stabilization is not done right? Any more advice on the type of op-amp or circuit is welcomed!

@LvW
Well my goal is to get a square wave oscillation.
Also honestly I am not getting what is happening
Please if you can help me understand the simulation it would be great. Any more suggestions?. Thanks!

Thanks seniors for all the help. I am just stuck with too many problems today and nothing is working out for me

 

[LvW]

abbeyromy,

your circuitry can do nothing else than to switch between both extreme values (supply rails). This leads to a squarewave. You do not need any "amplitude stabilization" - as for example for sinusoidal oscillators.

 

[abbeyromy]

@LvW
Oh so for a square wave we do not need amplitude stabilization and it is a normal operation switching between extremes Right?

Ok suppose I am using the same circuit for sinusoidal output then low C like 1nF is making that happen but as keith pointed it out that this may not be the right operation of the circuit. what you have to say on this.
One more question-> I just found a similar circuit on web and tried to implement this one, honestly I dont know even why the oscillations happening because I dont know the feedback factor for this and I have no math to support any of the observations. Do you have some solid reasoning for the oscillations?

 

[LvW]

Oh so for a square wave we do not need amplitude stabilization and it is a normal operation switching between extremes Right?

Yes, that`s the only function which can be (normally) performed. One exception (sinus) I have mentioned earlier, but this applies not to you as you are interested in a squarewave.

Ok suppose I am using the same circuit for sinusoidal output then low C like 1nF is making that happen but as keith pointed it out that this may not be the right operation of the circuit. what you have to say on this.

I don´t understand: Do you want sinus or sqarewave?
Concerning Keith: Even for a squarewave, the circuit performance limits are determined by the opamp (slew rate). If the period is to small in time the opamp cannot switch fast enough.

One more question-> I just found a similar circuit on web and tried to implement this one, honestly I dont know even why the oscillations happening because I dont know the feedback factor for this and I have no math to support any of the observations. Do you have some solid reasoning for the oscillations?

You can calculate by yourself:
1.) Determine the dc bias at the + input.
2.) Assume the output at its positive maximum (supply)
3.) Calculate the loading effect of the cap in the neg. feedback loop (e-function)
4.) At the time t=to voltages at both opamp inputs are equal and opamp switches
5.) Period of squarewave: 2*to.

 

[keith1200rs]

Concerning Keith: Even for a squarewave, the circuit performance limits are determined by the opamp (slew rate). If the period is to small in time the opamp cannot switch fast enough.

I thought that was what I said!

Keith

 

[LvW]

Concerning Keith: Even for a squarewave, the circuit performance limits are determined by the opamp (slew rate). If the period is to small in time the opamp cannot switch fast enough.

I thought that was what I said!

Keith

Yes, I know. But, apparently, Abbeyromy did not understand quite well.

 

[abbeyromy]

Hey LvW/keith Thanks for the support!

The calculation for oscillation seems complex
Any good book to get this stuff?

 

[LvW]

Hey LvW/keith Thanks for the support!

The calculation for oscillation seems complex
Any good book to get this stuff?

It's a basic circuit. So, I think each book dealing with this subject will serve.
Why dont you try it by yourself? Or look at an appropriate webside. There are a lot of them!
Another hint: Your circuit has single supply. In most of the books you only will find relaxation circuits with double supply. I don`t know if you are obliged to use only single supply.

 

[abbeyromy]

@LvW
I had changed it to double supply..thanks for the info..
Ok I will explore some ebooks that I have for the subject..
You are really helpful man.. Thanks a ton!!

 

[keith1200rs]

The positive feedback provides hysteresis. How much hysteresis you have determines the switching thresholds. Equal values will give one third/two thirds Vcc thresholds I think. The negative feedback charges and discharges the capacitor between the thresholds. The equation for charging/discharging a capacitor v=V e^(-t/cr) will come into it.

Keith