Help Figure Out This Circuit?, series multivibrator

[ACHIEVEIT]

Hi guys,

i am new here but i really like this place. I am currently studying electrical engineering at the university so i am still learning. I have a problem figuring out a circuit and i wonder if anyone could help me. The circuit is this https://www.4qdtec.com/mvbz/smv1.gif and some info about it can be found here 4QD-TEC: Multivibrators and Relaxation Oscillator Circuits . It is a series multivibrator that produces sawtooth signals. However, i can't get it to work right. I connect a dc supply between the two horizontal lines on the top and bottom of the circuit (+ goes to the top line) as well as a ground to the bottom line. I take the output from the collector of the npn transistor (the upper one). Can you help me understand how this circuit works? Also, can you help me choose the transistors and diode to make it work? Thanks a lot guys

 

[duffy]

Use a 2N3904 for the NPN, a 2N3906 for the PNP, a 1N914 for the diode.

 

[ACHIEVEIT]

Use a 2N3904 for the NPN, a 2N3906 for the PNP, a 1N914 for the diode.

What about the dc supply?

The best results were in 9.7V and 10.2V but again the peak-to-peak is very very low. The professor asked for 1V or more peak-to-peak. Any idea? I have attached the output for the two values + the circuit

 

[duffy]

Are you using 4,700Ω resistors for the "4K7"?

Are you using .1µF capacitors for the "100n"?

 

[ACHIEVEIT]

I think PSpice recognizes 4K7 as it is - value: 4700 Ω. There shouldn't be a problem with 100n (0.1 μF) neither. But i'll check it right now

 

[Roff]

The simulation runs fine in LTspiceIV.
I think the oscillations you are seeing are numerical oscillations, not circuit oscillations.

 

[duffy]

Doesn't oscillate in Electronics Workbench. Hangs a lot, too.

 

[Roff]

You might want to try making the maximum step size 10uS. I didn't have to do that in LTspice, but it might help in other simulators.

 

[ACHIEVEIT]

Just downloaded the program.

* UPDATE: I received some help from 4qd. They use BC556C/BC557C/BC558C for PNP, BC546C/BC547C/BC548C for NPN and 1N4148 for diode *

How did you get it to work?
I have attached my attempt but like PSpice the peak-to-peak is very small. What have you done differently? Thanks

 

[Roff]

Just downloaded the program.

* UPDATE: I received some help from 4qd. They use BC556C/BC557C/BC558C for PNP, BC546C/BC547C/BC548C for NPN and 1N4148 for diode *

How did you get it to work?
I have attached my attempt but like PSpice the peak-to-peak is very small. What have you done differently? Thanks

It's working. That's how it works. What do you mean, "the peak-to-peak is very small."? Your waveform is over 4V p-p. If you want to see sawtooth waveforms, look at the bases of the transistors.

 

[ACHIEVEIT]

Really? I was expecting the sawtooth to be visible from the collector of either transistor because that's what was written on the web site

"A nice, well behaved oscillator circuit. The diode isn't really needed but it does improve the waveform available on the emitters so that a good sawtooth is available. The values shown give a pulse of about 30µSec every 37mSec. The pulse output may be taken from either collector."

So by saying the peak-to-peak was small i meant that the "ripple" of the output was very small - it was a few mV only. I expected the linear part to have bigger incline.

In the meantime i tried moving around the "probe" and i got the best result when measuring voltage between the collector of the npn and the base if the pnp. I have attached the graph. (that's pretty much what you said about the base)

Given that's how it works, can you explain to me the step-by-step procedure of how this circuit works?

Thanks a lot for the help

 

[ACHIEVEIT]

It's working. That's how it works. What do you mean, "the peak-to-peak is very small."? Your waveform is over 4V p-p. If you want to see sawtooth waveforms, look at the bases of the transistors.

Now i see what you meant. I ignored the long vertical lines because i thought it was a LTSpice (and a PSpice as well) bug. But it turns out that it was the actual output. Only instead of the lowest and highest values be connected with a straight line, that line started from way high - almost as high as the highest value.

 

[ericgibbs]

"A nice, well behaved oscillator circuit. The diode isn't really needed but it does improve the waveform available on the emitters so that a good sawtooth is available. The values shown give a pulse of about 30µSec every 37mSec. The pulse output may be taken from either collector."
Given that's how it works, can you explain to me the step-by-step procedure of how this circuit works?

Thanks a lot for the help

Hi, I think you are misreading the web text.

 

[ACHIEVEIT]

Sorry for the dumb questions but can you explain what this means? English is not my native tongue and it seems i am missing something here

 

[ericgibbs]

Sorry for the dumb questions but can you explain what this means? English is not my native tongue and it seems i am missing something here

The sawtooth waveform is on the emitters, the square output pulse is on the collectors.

EDIT: look at this method.

Circuits-Lab.com Oscillators

 

[Roff]

The sawtooth waveform is on the emitters, the square output pulse is on the collectors.

To get sawtooths (sawteeth?) from the emitters, you need to add a resistor (10k works) from the emitter of Q1 to GND, and from the emitter of Q2 to +V.

 

[ericgibbs]

To get sawtooths (sawteeth?) from the emitters, you need to add a resistor (10k works) from the emitter of Q1 to GND, and from the emitter of Q2 to +V.

hi Ron,
I expect he will have to buffer any 'off circuit' load, so that he dosn't load the timing.

 

[ACHIEVEIT]

Oh i see. Well do you think the output graphs are normal? The one from the emitter of the npn is not very linear (1st attached). Also, the one from the collector of the npn seems more like an horizontal line with instant drops than a square (shouldn't the high and low times be even?) (2nd attached). The most interesting though is NPN Collector minus PNP Base (differential voltage)(last one).

 

[ericgibbs]

Oh i see. Well do you think the output graphs are normal? The one from the emitter of the npn is not very linear (1st attached). Also, the one from the collector of the npn seems more like an horizontal line with instant drops than a square (shouldn't the high and low times be even?) (2nd attached). The most interesting though is NPN Collector minus PNP Base (differential voltage)(last one).

give a pulse of about 30µSec every 37mSec.

As the text states a 30uS pulse every 37mSec, not an equal mark/space.

About 1200:1 ratio.

 

[ACHIEVEIT]

I have to practice my English!

I tried what Ron said (tampered the capacitors a bit to make the sawtooth last longer) and it works well. I might add it as an "Appendix". I have attached the output from the PNP emitter.

Could you explain to me how this circuit actually works? (capacitors charging, then transistor goes on, etc)? Thanks

Do you think i suggest taking the output between the NPN collector and the PNP base (it was the best graph) or would it be 'theoretically' wrong?