Opamp multivibrator not oscillating

[TheRedDevil18]

I'm trying to simulate this circuit in multisim

The frequency should be 200 kHz. Used this site to calculate R1 and C1, http://www.electronics-tutorials.ws/opamp/op-amp-multivibrator.html

All I get is this from the oscilloscope

Is the circuit wrong ? or is it just the simulator

 

[NascentOxygen]

That's a very low feedback resistor, might not function very well in a real circuit.

But I think your immediate problem is that the oscillator is not reliably self-starting. In a real construction you might just have to switch it on and off a few times, but in a simulation this may still not work. Can you put an initial voltage on C1? Say, 3.5V.

 

[jim hardy]

my GUESS is the oscillations are tiny and centered around 1 volt.
Observations leading me to that suggestion:
1. 100 ms/div timebase is WAY too slow to see 200 khz. Might as well use a calendar.
2. At 200 khz , 100 nf is 8 ohms?(check my arithmetic)... So you're asking that poor little opamp to drive a load that's less than 30 ohms?

I think you're trying to drive a railroad spike with a tack hammer . That's why it doesn't move enough to see with your 'scope settings where they are.

So,
to what sweep speed , vertical sensitivity and coupling mode should you set your 'scope to see if that's indeed what is going on ?

Look at the datasheet for that opamp. At what load do they specify its drive capability ? How does yours compare?

old jim

 

[TheRedDevil18]

That's a very low feedback resistor, might not function very well in a real circuit.

But I think your immediate problem is that the oscillator is not reliably self-starting. In a real construction you might just have to switch it on and off a few times, but in a simulation this may still not work. Can you put an initial voltage on C1? Say, 3.5V.

I tried that but it still won't oscillate. I also tried a 125 ohm feedback resistor. Would the feedback resistor need to be higher ?, if it doesn't work then I'll just try building it on a breadboard

At 200 khz , 100 nf is 8 ohms?

Yes correct, but since impedance is 1/wC then I would have to make C smaller to increase the impedance (I can't change the frequency, it has to be 200k).

Also the voltage is fixed at that value seen in the image. I checked because I moved that little green bar

So,
to what sweep speed , vertical sensitivity and coupling mode should you set your 'scope to see if that's indeed what is going on ?

Not too sure what those are

At what load do they specify its drive capability ? How does yours compare?

http://www.ti.com/lit/ds/slos200g/slos200g.pdf

It says "Large-Capacitance Drive Capability . . . 10,000 pF" which is 10nF. I'm using a 100nF capacitor, so should be able to drive it ? or maybe not must it be smaller than 10n ?

 

[Baluncore]

The circuit is a Schmitt trigger and an integrator. That type of circuit should start without a problem. The resistor and capacitor that make the integrator need to be changed from 25R with 100nF to higher impedance components with the same time constant. e.g. 2k5 with 1nF.

There is one problem I see. The Schmitt trigger is referenced to the ground connection in a single supply application. It should be referenced to the mid-point of the supply. So add another resistor with the same value as R3 = 30k to the circuit. Connect it from the R2 - R3 junction, to the Vcc rail. Then it will run. You may need to adjust R2 to correct the amplitude and frequency.

 

[NascentOxygen]

I can't change the frequency, it has to be 200k

Yes, but a better approach may be to first get it working properly at some low frequency, then scale it upwards in steps, so that you can monitor its deteriorating performance as you increase the demands on it.

 

[Stuschreib]

Set Vee at -5v and see what happens.

 

[jim hardy]

Not too sure what those are

wow you really need to spend some time learning to use the oscilloscope.

It says "Large-Capacitance Drive Capability . . . 10,000 pF" which is 10nF. I'm using a 100nF capacitor, so should be able to drive it ? or maybe not must it be smaller than 10n ?

so R+jwXc needs to be higher, as Baluncore pointed out.
The capacitor should charge to about midpoint of whatever AC is at output.

Also the voltage is fixed at that value seen in the image. I checked because I moved that little green bar

Green Bar? What is its function ?

Looks to me like you have trigger set to "none" so scope may be just waiting for instructions what to do.

Were it an old fashioned analog scope i'd suggest set trigger to AUTO which in absence of valid trigger makes one sweep every line cycle.
I'd expect this computerized scope to be showing the very last trace it ever drew, perhaps even before you connected it to your circuit.

 

[rbelli1]

Might as well use a calendar.

I'll have to remember that one!

BoB