creating rectangle oscillator with op-amp problem

Thread Starter

akbarza

Joined Dec 22, 2013
3
hioscillator_problem.png
i upload a schematic that is opened in ltspice. i saw it in a page and want to know how it works?
in that page( i don't know the page address) was said that this circuit oscillate and create rectangle waves. how does create rectangle waves ?
also in ltspice i can not see oscillation.
in schematic, i choose the universal opamp in ltspice, is it cause the problem?
thanks
 

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LowQCab

Joined Nov 6, 2012
1,340
Now that You are aware of the problems with the first schematic,
what do You need the Circuit to do ?

Are You aware that this Circuit also produces a Triangle-Wave at the same time ?
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LowQCab

Joined Nov 6, 2012
1,340
True, but it is still quite useful in a PWM Scheme.

It's shape can also be adjusted to a degree by varying the relationship between the
Positive-Feedback-Resistor, and the Non-Inverting-Bias-Resistor.
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Thread Starter

akbarza

Joined Dec 22, 2013
3
You had the op amp inputs reversed.
Also that circuit needs a plus and minus supply for the op amp (simulation below).

View attachment 246514
thanks for attention
is my below opinion about the circuit true?
we have positive and negative feed backs.
when positive feedback is larger than negative feedback , then we will see oscillation.
i want to anyone explain to me how this circuit works and how it oscillate.
thanks again
 

crutschow

Joined Mar 14, 2008
28,201
when positive feedback is larger than negative feedback , then we will see oscillation.
That applies to standard types of linear feedback oscillators.
This is a non-linear relaxation oscillator, which operates differently.
i want to anyone explain to me how this circuit works and how it oscillate.
Simulation below:

Start with the op amp output saturated at the positive rail voltage (green trace).

C1 then charges positively through R3 (red trace) until the inm op amp input slightly exceeds the voltage determined by the R1-R2 divider (yellow trace, 1/2 the positive output voltage).

At that point [since the inm op amp input is more positive then the inp input] the output switches from the positive rail voltage to the negative rail voltage.

This causes the inp voltage to now go to 1/2 the output negative rail voltage and C1 to discharge.
When the C1 voltage (inm) goes slightly below this inp voltage the output goes back to the positive rail and the cycle repeats.

Make sense?

1630334096494.png
 
Last edited:

LowQCab

Joined Nov 6, 2012
1,340
If You don't have a good understanding of how an Op-Amp works,
the following explanation will just cause You more confusion.
If You did have a good understanding of how an Op-Amp works,
the function of this Circuit would be obvious to You.

The Op-Amp is "acting as" a Comparitor.

Not all Op-Amps will operate well in a Comparitor configuration.
An Op-Amp with "Rail-to-Rail" Inputs and Outputs will
have the least likelihood of creating any undesirable behavior
when used in this type of Circuit.

The Capacitor severely delays the Feedback to the Inverting input,
this alone will cause very high frequency Oscillation,
which which would be totally disastrous if
the Op-Amp was being used as a normal Amplifier.

So the Op-Amp is now permanently "unstable", the Oscillation can not be stopped.

Now, the Oscillation has to be "controlled",
this is done with Positive-Feedback.
The Positive Feedback will set the Voltage that must be achieved at the
Inverting-Input before the Output will, again, switch to the opposite polarity.

The Ratio of the 2 Resistors connected to the Non-Inverting-Input
will set the Triangle-Wave Voltage-Swing which is available at the Inverting-Input.

The Op-Amp-Output will always be a Square-Wave,
and will always attempt to go "Rail-to-Rail",
however, many Op-Amps are not capable of Output-Voltages that actually
get very close to the Rails, this will be specified in the Data-Sheet of the Op-Amp.

The Oscillation Frequency will be determined by the
Capacitor and the Feedback-Resistor on the Inverting-Input,
and by,
the Ratio of the Non-Inverting-Input Resistors,
which are usually selected to provide a Triangle-Wave that swings between
1/3 and 2/3 of the Supply-Voltage to the Op-Amp,
thus simplifying the Math-Formula for achieving the desired Frequency of Oscillation.
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