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# sawtooth Generator Circuit

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#### lmartinel

##### New Member
hey guys,

Does anyone of you may be able to provide a simple discrete circuit for a sawtooth circuit....500k Hz frequency with a variable amplitude. I have done my research but have not found a reliable one. thank you

What research have you done?

Try looking up unijunction transistor oscillator. I think the output of those are a pretty good sawtooth.

555 works, but not sure if you could get it to 500khz.

I was thinking that too. 500 KHz isn't out of the range of the 555, depending on the particular technology ( not going to look it up ) I have one running at 500 Khz on a nasty breadboard right now.

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I happen to remember the specs from an NE555 PDF, max listed frequency is 500khz. Would depend on the exact 555 you had but I'm sure there's a bit of wiggle room there.

Mine says 1Mhz.

There are a LOT of 555's out there.

555 works, but not sure if you could get it to 500khz.

HOW!!!!!! Is the circuit made out with a 555.....

Now you want a perfect sawtooth?

This circuit is not being close to being a perfect sawtooth..Can someone help.....

As I remember I dont think that u mentioned u need a perfect saw tooth gen.

Anything that just charges a capacitor through a resistor (Unijunction, 555) will not produce a linear sawtooth beacause the voltage vs time is an exponential function.

It takes an integrator (opamp), or a current-source charging a capacitor to create a linear voltage vs time function.

There are many circuits out there that just a modicum of searching will uncover.

Anything that just charges a capacitor through a resistor (Unijunction, 555) will not produce a linear sawtooth beacause the voltage vs time is an exponential function.

It takes an integrator (opamp), or a current-source charging a capacitor to create a linear voltage vs time function.

There are many circuits out there that just a modicum of searching will uncover.
If you want linear from a UJT , 555, ete, you have to charge the cap with a current regulator. Here's one I'm using, J310, I didn't try for 500kHz yet, if I remember, I'll try today, but with a little adjustment you can get a pretty good saw tooth with a fairly sharp bottom tooth.
Kinarfi

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Nothing is perfect. OP needs to abandon 'perfect' and describe in detail what is good enough.

You have to define how perfect is perfect.

The ramp linearity is easy compared to an infinite reset slope.

Anything that just charges a capacitor through a resistor (Unijunction, 555) will not produce a linear sawtooth beacause the voltage vs time is an exponential function.

It takes an integrator (opamp), or a current-source charging a capacitor to create a linear voltage vs time function.

There are many circuits out there that just a modicum of searching will uncover.

If you want linear from a UJT , 555, ete, you have to charge the cap with a current regulator. Here's one I'm using, J310, I didn't try for 500kHz yet, if I remember, I'll try today, but with a little adjustment you can get a pretty good saw tooth with a fairly sharp bottom tooth.
Kinarfi

Well I tried to get to get 500KHz out of a PUTJ 2N6027 and only got up to 211KHz and that was with 3 10pf caps in series. Scope shot attached,
I also said that if you charge a cap with a constant current you would get a flat ramp instead of the rc time constant ramp, not so sure now, I know I have in the past but I couldn't do it today. I put a resistor in the circuit and measured the amps and took a picture of it, then replaced the resistor with a 3 terminal current regulator LM334 and adjusted it to the same current and took another picture, put the on the same picture, attached.
Kinarfi

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• 700mv211khz.PNG
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• 180uac.PNG
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