A triangle-wave generator using discretes

[atferrari]

One thing I don't like is how glitchy the square-wave input is to the gate:

View attachment 89729

But the good news is that adding a load (remember: note to self: always model a load!) quickly stabilizes the output. Plus the current source can easily drive a 5k load (it'll go down to 1K, albeit with some distortion).

I did change R8 to 1K and R5 to 22K to try to isolate the current sink from the oscillator, which helped somewhat. Actually, that spike on the rising edge doesn't seem to make much difference anyhow.

Good that it worked.

Today I felt inclined to tweaking. Now no more weird scheme; just +Vcc.

I provided my own (simpler) astable driving the switching. To get the switching working properly with a single supply I needed to reference both current sources to a point halfway between ground and Vcc (R9 & R10 divider on the right).

If the second it takes to stabilize seems too long (as you say, is not), play with R9 & R10. They control that part upon start.

My current suggestion, astable included, is simpler and works.

 

[carbonzit]

I like it, but unfortunately it doesn't seem able to drive much of a load; it'll handle a 10KΩ load, barely, but nothing more. Oh, and I changed the integrator cap to 4.7μF.

Maybe further tweaking might improve things? Unfortunately I'm kinda over my head here, can't be of much help to you.

One thing I was wondering about is whether you really need 2 transistors to gate the current sink (Q5 & Q3); couldn't this be done with just one?

 

[atferrari]

I like it, but unfortunately it doesn't seem able to drive much of a load; it'll handle a 10KΩ load, barely, but nothing more. Oh, and I changed the integrator cap to 4.7μF.

Maybe further tweaking might improve things? Unfortunately I'm kinda over my head here, can't be of much help to you.

One thing I was wondering about is whether you really need 2 transistors to gate the current sink (Q5 & Q3); couldn't this be done with just one?

At the moment, I do not see how to do it with just one transistor. I will sleep on it.

 

[carbonzit]

At the moment, I do not see how to do it with just one transistor. Iwill sleep on it.

Well, it seems your insomnia is a more useful design tool, but we'll let you get some Zs.

 

[atferrari]

Those Zs, worked marvels and insomnia brought fresh ideas.

The astable now drives the switching transistor directly.

No more glitches; C4, down at the sinking source, seems to cater well for that. It would be good to see if it works OK in real HW. More if you make it variable in frequency. (Making it bigger to cover all ranges?)

Be well.

 

[alec_t]

If you wanted to eliminate the current mirrors .....

 

[atferrari]

If you wanted to eliminate the current mirrors .....
View attachment 89752

Hola Alec

Both of your ciruits are certainly simple. I like that! Could you venture to compare them with my last above? Pros & cons.

 

[audioguru]

Of course the simple circuit has a very high output impedance so it cannot drive anything except maybe an opamp. Oh, maybe an opamp should have been used to make the entire circuit in the first place.

 

[crutschow]

Of course the simple circuit has a very high output impedance so it cannot drive anything except maybe an opamp. Oh, maybe an opamp should have been used to make the entire circuit in the first place.

Yes, it may be a interesting academic exercise to design such a circuit with discrete devices but you can do it with many fewer parts along with better performance using op amps.

 

[carbonzit]

If you wanted to eliminate the current mirrors .....
View attachment 89752

I like it very much. For simplicity alone it should probably win the prize in this thread.

So, how does it perform? I modeled it with my multivibrator. It works pretty well.

Notice, however, how the waveform degrades with increasing load (decreasing load impedance). It seems to be able to drive about 20KΩ adequately. If one were to add a couple stages of amplification (a high-Z input CE amp plus an emitter follower, perhaps), one might make a pretty decent function generator out of it. (These are for 20K, 5k and 1K loads resp.)





Annoyingly, I had to modify the astable to get it to work with LTspice (by changing transistors and by adding a little "startup" capacitor). Dunno why it wouldn't start here when the same circuit starts elsewhere ...

 

[carbonzit]

If one were to add a couple stages of amplification (a high-Z input CE amp plus an emitter follower, perhaps), one might make a pretty decent function generator out of it.

What I mean by this is a DIY, homebrew, poor man's function generator, like this one.

 

[alec_t]

I think even a poor man would be better off using opamps rather than discrete transistor cicuitry .

 

[carbonzit]

Did you check out that link I gave above? It was about a square-wave generator someone made literally in a war zone, using only available parts.

 

[alec_t]

Yes, I checked it. A multi-range multivibrator with an R/C differentiator.

made literally in a war zone, using only available parts.

A pretty good parts selection for a war zone! I think real ingenuity was shown by POWs in WW2, who made radio receivers from little more than rusty razor blades and the odd scrap of scavenged wire.

 

[ltp_10]

If you wanted to eliminate the current mirrors .....
View attachment 89752

Hi, can you please explain the last part of the circuit? I know how the Multivibrator works, but I would like to know about the rest of it. Is it a buffer amplifier and an RC Integrator?

Thanks!