[SOLVED] why this circuit oscillate?

[akbarza]

hi
i attached a circuit and i have question about it.
i before posted this circuit in allaboutcircuit.com( in that moment, the circuit did not oscillate. because the plus input and minus input were reverse. in that site, persons lead me and said that if i change the two input , the circuit oscillates.)
(in attached pic, the inputs are right now.)
my first question is that as we now the two inputs are equal, why in reverse state of inputs, it does not oscillate?
second, can any one explain why this circuit oscillate, and what happen?
third: we know positive feedback cause oscillation. in this pic, we have positive feedback(R1-R2) and negative feedback(R3-C branch).for having oscillation, namely positive feedback>negative feedback, what condition(s) must be valid?
thanks

 

[BigBoss]

If you connect (-) Input to GND then apply a Positive Feedback, it will always oscillate..

 

[KlausST]

Hi,

this circuit is already explained many thousand times.
You surely did an internet search before.
So please give a link to the explanation you used and tell us what exactly is unclear?

Klaus

--- Updated Sep 2, 2021 ---

Hi,

If you connect (-) Input to GND then apply a Positive Feedback, it will always oscillate..

I doubt this.

Klaus

 

[akbarza]

Hi,

this circuit is already explained many thousand times.
You surely did an internet search before.
So please give a link to the explanation you used and tell us what exactly is unclear?

Klaus

--- Updated Sep 2, 2021 ---

Hi,

I doubt this.

Klaus

hi klaus
what phrase must i search in net that results of search explain how this circuit oscillate?

--- Updated Sep 2, 2021 ---

If you connect (-) Input to GND then apply a Positive Feedback, it will always oscillate..

if i connect - input to GND, then the cap is eliminated,namely the cap is extra element!

 

[FvM]

A circuit with overall positive feedback down to DC won't oscillate but show bistable behavior. You need negative DC feedback and positive feedback at certain frequencies to achieve oscillations.

Loop gain details decide if the circuit works as harmonic or relaxation oscillator.

--- Updated Sep 2, 2021 ---

A detailed discussion of this circuit can be found at Wikipedia - Relaxation oscillator - comparator based relaxation oscillator.

 

[KlausST]

what phrase must i search in net that results of search explain how this circuit oscillate?

"comparator oscillator circuit explanation"
or
"OPAMP oscillator circuit explanation"
(While I strongly recommend to use a comparator instead of an OPAMP, because most OPAMPs are not designed for digital output)

Klaus

 

[G4BCH]

my first question is that as we now the two inputs are equal,

This assumption is not true. Ignoring C1 for the moment the -ve input will be at the same potential as he op amp output and the +ve input will be at vout/2.
The only condition where the two inputs are equal is for Vout=0V any deviation from this results in instability. Think about what happens to the output voltage and the input voltages if this condition is no met; assume an ideal opamp. The capacitor just slows things down. setting the oscillation frequency.

 

[RCinFLA]

Phase shift (delay) on R3,C1

 

[Easy peasy]

at start up, C1 is discharged, there fore neg input is low, opamp output will be at least a little above gnd, hence + input will be above neg, hence output will be forced high, which charges C1 thru R3, when Vc1, reaches Vdd/2, due to resistor ratio, and slightly exceeds it, Vout will then flick to near Vss, causing C1 to discharge until it gets just below Vdd/2, so the hysteresis band is very small and it will oscillate at a rate governed mainly by the slew rate of the op-amp. It would work better if you add an R from Vout to junction R1 & R2

 

[KlausST]

Hi,

the idea of both inputs have equal input and thus the output is zero is rather "idealistic".
Let´s consider this "metastable" state exists. Then the unavoidable noise and unavoidable drift will cause the output to move.

Even a microvolt of modified input may cause a volt of output signal.
This output signal makes the oscillation to start. It will start. Safely.

Thus RC oscillators are known for their fast start up behaviour .. in opposite to XTAL oscillators.

Klaus