Hi everyone,

I am working on a ramp wave generator using this chain of components:

1. an op-amp integrator (with a capacitor C), which generates the ramp
   
2. an op-amp comparator, which detects if the step 1. integrator output reaches a fixed threshold and switches from 'high' to 'low'
3. a saturated BJT. The base is connected with 2. output

I want to pose the Vce output in parallel with the capacitor C to discharge it, but all my efforts are vain.

Here below is an image of the schematics:

Thank you in advance for your help!!!!

 

What is V3 doing? It's shorted.

 

It's the negative rail (or, at least, my personal concept of negative rail). Am I wrong?

 

Am I wrong?

Your drawing has an error. Too many ground lines on the voltage source.

 

Your drawing has an error. Too many ground lines on the voltage source.

Thank you for the reply.
I've deleted a line betweeen ground and Ve.
Talking about V3, what I want to do is draw the negative rail, which feeds my op-amps along with positive rail (expressed by V1).

Here is the new version:

 

V3 is drawn wrong, it should be Negative to the op amps, and its still shorted out.

 

V3 is drawn wrong, it should be Negative to the op amps, and its still shorted out.

Modified the voltage of V3 to -9V from 9V. Do I have to place a resistor in series with the V3?

Many thanks.

 

Modified the voltage of V3 to -9V from 9V. Do I have to place a resistor in series with the V3?

Many thanks.

Here's the new diagram. Still I don't know how to interface the BJT with the capacitor in order to discharge it.

 

V3 is shown as as a positive supply the +/- signs are wrong,as it stands its ok the output will go from zero to positive,

If you connect your emitter to the negative V3 rail, and take your output from ground and the collector, your output will swing negative and positive through zero.

 

V3 is shown as as a positive supply the +/- signs are wrong,as it stands its ok the output will go from zero to positive,

If you connect your emitter to the negative V3 rail, and take your output from ground and the collector, your output will swing negative and positive through zero.

This is what i get with the modifications you suggested:



Maybe I am missing something... (i.e. the connection of Ve with negative rail). Still I miss how to connect BJT to capacitor...

 

That looks better. What is the problem?
Note that the base voltage of the transistor will turn it on at any voltage above -8.3V.

 

Dont see why you need U2, if you connect the transistor to the output of U1, it should work.

 

I don't see much need for R2 either.

 

That looks better. What is the problem?
Note that the base voltage of the transistor will turn it on at any voltage above -8.3V.

What if I connect Emitter to ground? Anyway, putting cpacitor in parallel with Vce seems not to work (I don't get the desired oscillation). Maybe it's a timing problem...

 

Can you scope the output of U1 from ground, and the negative supply, what signal is it.?

 

Can you scope the output of U1 from ground, and the negative supply, what signal is it.?

It's the same of V(Vc)



there is a small oscillation: (better zoom)



but not the oscillation that I want (from 0 to threshold, ramp style)

 

Here you go. Note the positive feedback resistor R4. The ratio R4/R2 determines the ramp amplitude (and also affects frequency).

Here's the asc file:

 

Here you go. Note the positive feedback resistor R4. The ratio R4/R2 determines the ramp amplitude (and also affects frequency).
View attachment 99286
Here's the asc file:

Thank you very much! The problem was the lack of 'hysteresis' in the comparator, I guess. I have to study this topic a little bit more...

Thank you all again!!!