Hi i try to simulate this circuit by using pspice but it did not work so can some one help me what is the problem
thanks

 

Hi i try to simulate this circuit by using pspice but it did not work so can some one help me what is the problem
thanks

What means "did not work"?
As a most import aspect in simulating a circuit - one should approximately know in advance what the result should be.
In case you know that - it would be wise to inform us about your expectations.

What do you want? An oscillator (with sinusoidal output) or a multivibtator (relaxation oscillator)? What frequency? And what do you get from the simulator? Nothing? You see - a substantial answer can be expected only in case of a good and detailed question with all necessary information.

 

What means "did not work"?
As a most import aspect in simulating a circuit - one should approximately know in advance what the result should be.
In case you know that - it would be wise to inform us about your expectations.

What do you want? An oscillator (with sinusoidal output) or a multivibtator (relaxation oscillator)? What frequency? And what do you get from the simulator? Nothing? You see - a substantial answer can be expected only in case of a good and detailed question with all necessary information.

the out put should be square wave and frequency about 30khz

 

I believe that r6 might be meant to feed hysteresis back positive amp input and not connect to the positive supply.

I have managed to play with the values and get the amplifier to start to work in LTSpice by making that change.



It oscillates but the waveform is very triangular. That could be because I wasn't using an LM139 as the Op Amp.

 

Hello,

@ Patato Pudding,
The resistor at the output to the power supply is needed for the LM139, as the comparator has an open collector output.

@TAKYMOUNIR,
Patato Pudding is right about the positive feedback resistor.

Take a look at this picture, taken from the National datasheet:

Bertus

 

Hi i try to simulate this circuit by using pspice but it did not work so can some one help me what is the problem
thanks

Apparently most oscillator types are kicked off by resistor noise or some imbalance due to tolerances on individual components.

Usually - in a simulation; you have to give it a prod to get it going. Probably a voltage source programmed for a single narrow pulse.

 

Apparently most oscillator types are kicked off by resistor noise or some imbalance due to tolerances on individual components.

Usually - in a simulation; you have to give it a prod to get it going. Probably a voltage source programmed for a single narrow pulse.

This is typically not necessary in a hysteretic oscillator, which this.
It oscillates in simulation for me in LTspice. The frequency is about 185kHz.

 

I believe that r6 might be meant to feed hysteresis back positive amp input and not connect to the positive supply.

Yes - that´s correct. R6 must be changed to provide positive feedback.

 

Hmm I sometimes see this kind of resistor in a wien bridge oscillator . (output to positive rail with a 10k)

 

Hmm I sometimes see this kind of resistor in a wien bridge oscillator . (output to positive rail with a 10k)

Yes? I never did!
Such a resistor (if it is not a pull-up resistor) acts primarily as a load - that`s all.

 

Yes - that´s correct. R6 must be changed to provide positive feedback.

R4 provides the positive feedback. R6 is the pullup for the open collector output of the LM139.

The circuit is a hysteretic oscillator, similar to the one posted by bertus in post #5. The only difference is the addition of R7 and D1, which change the duty cycle.

 

R4 provides the positive feedback. R6 is the pullup for the open collector output of the LM139.

Oh yes - sorry for my mistake. I overlooked the positive feedback path (perhaps because it is shown at the bottom of the drawing).