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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
 

Sceadwian

Banned
What research have you done?
 

BrownOut

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

smanches

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

BrownOut

Banned
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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Sceadwian

Banned
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.
 

Sceadwian

Banned
There are a LOT of 555's out there.
 

MikeMl

Well-Known Member
Most Helpful Member
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.
 

kinarfi

Well-Known Member
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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mneary

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

RCinFLA

Well-Known Member
You have to define how perfect is perfect.

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

kinarfi

Well-Known Member
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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