A triangular signal generator using only transistors. Is it possible?

[AlexRG]

Hello everyone. Creating a triangular signal generator using an operational amplifier, or a comparator, or a flip‑flop is quite simple. But is it possible to make it using only transistors? For example, BC556 or BC546?

If possible, please draw the circuit; preferably, it should be possible to simulate it in Multisim.

Here are the parameters needed (see the picture):
1. Signal frequency: 80–90 kHz, accuracy is not important.
2. The triangle amplitude is 1V (a deviation of up to 5–10% is allowed).
3. The triangle signal must have a constant offset of +2.5V.
4. The circuit must be powered by +5V.
5. As I already said — no chips, only transistors.
6. If possible, the edges of the triangle should be as linear (straight) as possible.

[themadhippy]

course ya can,after all whats inside an op amp a comparator, or a flip‑flop

[Analog Kid]

It absolutely is possible, yes.

Here's an example, from when I was on another forum years ago. We went back and forth with several iterations trying to get a decent-looking triangle wave; this one was pretty good:





LTspice file attached so you can run the simulation.

I'm still thinking of implementing something like this for a homebrew class D amplifier, which needs a very accurate triangle wave for low distortion.

[ledtester]

based on the comments by "crb3" in this reddit thread:

https://www.reddit.com/r/AskElectronics/comments/1qqcczg/generate_a_triangle_wave_of_a_controllable/

I found the schematic for the Moog 901. See the attachment below.

It uses a unijunction transistor to create a sawtooth wave and from that a triangular wave but there are other ways of creating a sawtooth with BJTs.

[Analog Kid]

Here's another iteration from that same session.
Single supply instead of split; has its own multivibrator for square-wave generation.
Triangle generator is the six transistors on the right.
It uses two current mirrors to create the up- and down-ramps.

[PGPG]

But is it possible to make it using only transistors? For example, BC556 or BC546?

The first problems to solve are how to get resistors and capacitors using only transistors.
Do Miller effect can be used to get enough big capacitance?
May be such capacitance can be connected between collector and base of second transistor to multiply it.
To get something replacing resistances germanium transistors could be better thanks to their higher Ic0 currents, I think.

[AlexRG]

Here's another iteration from that same session.
Single supply instead of split; has its own multivibrator for square-wave generation.
Triangle generator is the six transistors on the right.
It uses two current mirrors to create the up- and down-ramps.

Thank you for the circuit. Now let’s try to optimize it for my task. I assembled the circuit in Multisim, it worked, and here are the parameters:

1. The signal frequency is 237 Hz, but I need 80–90 kHz, preferably 80 kHz.
2. The amplitude of the triangle is 422 mV, but I need 1000 mV (1 V).
3. The offset is +4.06V, but I need +2.5V.

And regarding linearity. Perhaps this is a problem with the Multisim modeling, but the non‑linearity of the saw’s slopes is visible to the eye. I have superimposed perfectly straight green lines on the graphs.

[Analog Kid]

I can only be of limited help here; no electronics expert am I.

Regarding the frequency, that's set by the RC time constant of R2/C1 and R3/C2 in your sim.
Lower values of both R and C will increase the frequency.
You can play around with them, probably get somewhere in the ballpark.

Others will have to address the linearity issue, which is important. Likewise the low voltage output.

Good luck!

[Analog Kid]

Try this out in your simulator:



(this is just the square-to-triangle converter)

Much higher output.
Frequency is low here (500Hz) but can be adjusted later.
Output looks pretty linear, but I didn't check it carefully.
Very simple circuit.

[AlexRG]

Try this out in your simulator:

No, this is definitely not what I need)). I think it’s better to stick with the previous scheme with current mirrors. Just pick all the elements that suit my needs.

[Analog Kid]

Hmm; not saying this is the problem for sure, but when I changed the voltage in my sim (LTSpice) to 5 volts I also got a horrible waveform. As built with 9V supply it give a very nice-looking triangle wave.

Try increasing the supply voltage to 9 and see what happens. (Yes, I know you want to run at 5 volts, but let's see if this circuit works or not.)

[AlexRG]

Try increasing the supply voltage to 9 and see what happens. (Yes, I know you want to run at 5 volts, but let's see if this circuit works or not.)

I raised both the supply voltage and the amplitude of the input square wave up to 12V. Now the signal already looks like a triangle. However, the linearity is very poor at the top.

[Analog Kid]

OK. I'll be watching this thread going forward as this is something I'm interested in as well.

Again, good luck with your project, whatever it may be.

[PCB.Wiz]

I raised both the supply voltage and the amplitude of the input square wave up to 12V. Now the signal already looks like a triangle. However, the linearity is very poor at the top.

Yes, if you inject two almost identical charges into a cap it will drift toward one of the rails. There is no defined DC point.


The must use transistors restriction imposes more design effort than simply using a good opamp, as was already given in another thread.

There are two broad approaches to  arbitrary must use transistors restriction :

a) You can make a schmitt with thresholds at 2V and 3V and feed that back to two current sources.
This uses a single Cap and self centres on the schmitt thresholds, but such a schmitt is not trivial to get right.

b) You can create a square wave (as above) and use that to switch currents into a cap.
That is simple to define frequency (with the caveat some cross coupled monostables can fail to start), but it has no defined DC average.
Thus you need to add an additional DC centre circuit.

[PCB.Wiz]

Here is a Single Gate CMOS Schmitt gate, with an idealized 2V and 3V switching points.
Zoomed on the triangle current slope, to show the linearity.
You get better linearity with cascade current mirrors, and there is just enough headroom to manage this on 5V rail.

[MariuszD]

I once found such a schematic. On the left side, there is an adjustable current source that needs to be replaced with a constant one.

[Analog Kid]

Here is a Single Gate CMOS Schmitt gate, with an idealized 2V and 3V switching points.
Zoomed on the triangle current slope, to show the linearity.
You get better linearity with cascade current mirrors, and there is just enough headroom to manage this on 5V rail.

That's fine, very interesting actually, except that it violates the "no chips" requirement.
I suppose you could build those Schmitt triggers with discretes; how difficult would it be to get the hysteresis points right?

[PCB.Wiz]

That's fine, very interesting actually, except that it violates the "no chips" requirement.

Perhaps, but that 'rule' is an entirely arbitrary arm-wave anyway.
A real design is likely to use matched transistors, and those are now 'integrated circuits' (oops) with two transistors, and come in the same package size as a single gate part.

I suppose you could build those Schmitt triggers with discretes; how difficult would it be to get the hysteresis points right?

It's just more parts.
You can use differential pairs to get more stable set/reset points, but any Schmitt tends to have soft lead-ins to currents prior to the snap-over, and it is not easy to keep those artifacts out of the current envelope.
A CMOS Schmitt has a very well behaved drive, with no skirt effects.

[Doctorandus_P]

I'm struggling a bit with the word "only".

Are capacitors and resistors allowed? a PCB (breadboard). Wires, Solder?

[AlexRG]

Are capacitors and resistors allowed? a PCB (breadboard). Wires, Solder?

No, only transistors. Without a printed circuit board, without solder and flux, transistors are prohibited from being connected to each other in any way.

It was a joke. Of course, resistors, capacitors, and diodes are allowed. “Only transistors” — and this was mentioned at the very beginning of the topic, meaning without the use of chips.

[AlexRG]

I once found such a schematic. On the left side, there is an adjustable current source that needs to be replaced with a constant one.

From which element in this circuit should the triangular signal be taken? From Ct?

[AlexRG]

I once found such a schematic. On the left side, there is an adjustable current source that needs to be replaced with a constant one.

To begin with, I assembled the circuit exactly as you did. If the signal is taken from Ct, then the circuit didn’t work in Multisim.

[MariuszD]

From which element in this circuit should the triangular signal be taken? From Ct?

From Ct or after the source follower.

It works for me. This circuit was designed to work as a VCO with a wide tuning range. I found it on the seventransistorlabs website.

[AlexRG]

It works for me. This circuit was designed to work as a VCO with a wide tuning range. I found it on the seventransistorlabs website.

Could you please select the circuit elements so that the Ct results in:
- 80 kHz (80..82)
- amplitude 1 V
- offset +2.5 V
And also post the circuit in better quality. Thank you in advance!

[AlexRG]

It works for me.

Please note that you swapped the connections of the collectors of transistors Q9 and Q10 (if you look at my diagram). In the very first diagram, their collectors were connected incorrectly. Then you posted a new diagram with graphs. There, the collectors were connected correctly.

I assembled this new circuit. I “played around” with the component values and got an almost perfect triangle. 80 kHz, 1 V peak‑to‑peak, 2.5 V offset, linearity — perfect.

Are there any pitfalls?

p.s. And the current consumption is 34 mA.. Isn’t that too much?