Oscillators using Op. Amp. How they start to oscillate ?

Hello. This is my first topic in this forum.

I wonder if someone can help me with a question I have from some time ago.

When studying about oscillators using Op. Amp., they always teach you about how to calculate the value of resistors, capacitors, and other external elements, to get a particular frequency. But they never talk about the "External Voltage Supply" or voltage signals to make the amplifier to start generating an output voltage that will then generate an oscillation.

The question is:

Do Operational Amplifiers generate by themselves an output voltage enough to start an oscillation, without having any external voltage signal in any of its inverter or non inverter Input pins?

Of course I know that the Op. Amp. needs a V+ and V- voltage source in each of its V+ and V- pins. But those voltages are the ones that will be shown as the output voltages of the Op. Amp. after it senses an external voltage in its inverter or non inverter pins.

Why is that if not having any INPUT voltages, the amplifier generates an oscillating voltage (done with the resistors, capacitors and other elements attached to the circuit) ??? Where does the Op. Amp. gets voltage to generate OUTPUT voltage to start the oscillation ??

It's just like when I see a "crystal-with-Op.-Amp." oscillator schematic. It only has the crystal, two capacitors, the Op. Amp., one or two resistors, and that's it. Where is the Power Supply to activate the Op. Amp. or to activate the crystal ??


Well, that's the great question I have in my mind this days. Because after reading so much books and looking in internet pages, I couldn't find anything that can answer me this question.

Hope someone can help me "see the light" through this topic, jejejeje.

Thanks for any help you can give me.

Regards. :spoko:

Oscillation theory comes from that
each electrical componets Resistor ..etc
have noise signal with the dc voltage applied in ...
and we all must know that the noise signal is random
siganl in both the amplitude and frequency ..
if we take the fourier tranformer of the Random signal we will see many harmonics frequencies into.
with diffrent frequecies and amplitudes
so if we choose the selector or the tuner circuits to choose one of the harmonic frequencies in the Random signal (noise )to generates it and amplifying the signal we will get what


I need some schematics and diagrams to explain this to u very will

well to start an oscillator the gain around the loop should be greater than 1. but as soon as the oscillation starts the gain around the loop should drop to 1. this can be done in a number of ways. it could be done by using a simple variable resistor. turn the pot a few turns so that the gain is greater than 1 and then turn it back once the oscillations have started. the oscillator actually starts in the first place due to transients when the power is first turned on.

there is another method that automatically does this. you can use two back to back zeners connected in parallel with a resisitor in the feedback path. when the oscillator will start up the gain will be high. as soon as the output reaches a certain level the zeners will start working and short that resistor lowering the gain.

there is another method that uses automatic gain control (AGC) by a JFET. the JFET is operated in the ohmic region in which the gate voltage determines the drain to source resistance.

if you have an idea about op amp circuits and circuits in general you would understand what i am saying. but if you dont, all i have said will sound like greek to you. if you dont understand then tell me ill try to get some schematics

Thank you both, electronicman and samcheetah, for your answers.

But my problem isn't understanding Oscillators basics or Fourier analysis.

My doubt is about the Operational Amplifiers generating the initial voltage in its output so, with feedback in its INPUT pins (inverter and non inverter), the Op. Amp. can sense voltage to amplify and change its output to the levels it will work in the oscillation.

If I don't make myself clear, I'll put an schematic diagram in my next post.

But for now, just to tell you that my problem is to understand how the operational amplifier starts working (to oscillate) if it doesn't have any EXTERNAL VOLTAGE SIGNAL in its input pins.
Because, as an oscillator, Op. Amp. are just connected to resistors and capacitors, and feedback between its output and input pins, but nothing else. No voltage to start charging the capacitors, No voltage for the Op. Amp. to sense in its input pins.

See you later.

Hello Enrique,
If you are talking about a opamp circuit for a sinusoidal oscillator then the circuit is designed in such a way that the gain across the closed loop of the opamp circuit is 1 or for that matter the loop gain is -1 (FOR A PARTICULAR FREQUENCY). The frequency for which this is true, the circuit theoretically has an infinite gain. So when power is switched on the noise in the components is enough signal to start the oscillations going since noise contains all frequency components the particulat frequency is amplified more and more until it reaches the sustained oscillation mode.
If you are talking about the square wave oscillator circuit then that circuit is simply made ina way so that the inputs get an opposite positive feedback after sometime. So when power is switched on, since the opamp is connected in positive feedback, its output saturates and that when goes to the input after a delay (set by the RC circuit) it reverse saturates it and the cycle continues.
I hope these explanations were clear.

Thanks aryajur for the explanation. It was clear.

So, to see if I understand:

when connecting the elements to the Op. Amp. connecting the DC power supplies to the V+ and V- of the Op. Amp., and after starting the DC supplies, the Op. Amp. will have an output even though not having anything in its Input pins ???

Thanks again for the help.

See you later.

Thats right, even if the voltage difference between the input pins is 0 the opamp will have some voltage output at its output pins which may be due to offset voltage, or it may be because the opamp is designed to give a particular voltage for 0 input voltage between its pins (Commn Mode Feedback).

yes, as i have already said that with passive filters the thing that starts the oscillation is the small power supply transients that appear when the power is turned on. with opamps its the small offset voltage that cause the start of the oscillations. no matter how perfect the opamp is, it will have some offset voltage. and that is enough to start oscillations.

the basic concept is that to start oscillation you first need some noise. that could be in microvolts or anything. then you need a gain of more than 1. this amplifies the noise to the level where equilibrium will occur. the moment the circuit attains a state of equillibrium the gain is reduced to 1 and the noise is now a predetermined wave shape. even with crystal oscillators in microcontrollers you first need a small amount of noise which is then amplified to start oscillations.

i apologize for not making this clear in my first reply

in order to generaste self oscillating on opamp circuit yuo must put opamp in feedback mode (some kind like that idon't know the term) and the feedback gain should have -1 magnitude in order to make positive feedback so if there is little bust voltage either from change at power supply or noise spike (maybe) could generate voltage on opamp output

Thank you very much ARYAJUR, Samcheetah and E-EJA, for helping me understand finally the cause, the source, of the oscillation in circuits using Op. Amp. as the oscillator.

I'll have to buy someday an Op. Amp. and build a real circuit to "see it and believe it", jejeje :sm15:

The sad part is that I don't have an oscilloscope to watch the waveform of the oscillation. That's the primary reason why I haven´t build circuits, and so why I couldn't understand what I asked here in this topic.


Thanks a lot for the help. :spoko: