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How to boost a sawtooth signal? Answered

Hi,

I need to make a sawtooth generator based on an NE555 (the power supply will be 15 volt - 5000mA), and I need to boost the output signal as follow:
http://s21.postimg.org/dwz6aynvb/Sawtooth.png

Which needs to go from 9.6 volt to 14.10 volt with a 3000mA current.

I've found a schematich, but I don't know if is valid:
http://s27.postimg.org/9ptmbsbkz/Signal_Boost.png

Would be a viable way, using this schematic, or there would be a better/different solution? If so, someone could suggest me another schematic or tell me how to modify the one which I posted?

Thank you.

Discussions

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seandogue

3 years ago

It's really quite simple. Use a signal amp to boost the signal level, then a transistor follower (FET, biipolar) for the elevated current loading.

the word, btw, is schematic.

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akopelumenuscuseandogue

Answer 3 years ago

Thank you.

I've also read, somewhere, that the better choice for the "current amp stage" would be a mosfet (fast switching, less heat), but there are various kind of mosfet; can you suggest me a good mosfet for that application?

(Obviously "schematich" was just a mistyping ;) Thank you however for the correction, since I'm not english. )

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seandogueakopelumenuscu

Answer 3 years ago

International Rectifier is one "goto" semiconductor company I like for FETs.

you could also check Digikey, Mouser, or if in the old world, someone like Farnell for FETs from other sources. Be prepared to do some foot work checking the spec sheets..

If you're new or new-ish to electronics, I'd suggest picking up a book like "The art of Electronics" as I am offering only general advise here and won't be providing many specifics.

BTW, you didn't mention the frequency range of the sawtooth. If not a low fixed frequency, you'll need to consider that in the design of your amp and follower.

Alternatively to an amp and a follower, you could simply opt for a power op amp. Power op amps, unlike their much lower load-capable signal amp little brothers, can carry relatively large loads without the need for follower stages.

I think I used a Texas Instruments OPA544 for a vaguely similar purpose a few years ago, although it might be a bit under powered for a 3A load. Again, Digikey, Mouser, Farnell, etc.... And then the semiconductor companies.

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rickharris

3 years ago

Odd, this is the second time this has been asked by "different" posters. The other one wanted 4 amps at the output.

Mmmm.

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rickharrisrickharris

Answer 3 years ago

Ah, no this is the same person. obviously didn't like the other answers.

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akopelumenuscurickharris

Answer 3 years ago

I'm sorry that I've given you this impression, but I'm just looking for a different solution, a different circuit: in this question I have submitted a different signal generator (although in the previous question I didn't submitted any schematic); in the previous question I have asked for a different thing:, how to calculate the base transistor (and in the previous question I was also using another transistor), so: this is a different question; here I've asked if this schematic could be OK.

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rickharrisrickharris

Answer 3 years ago

Be aware that unless you chose your components carefully you stand a good chance of distorting your nice saw tooth wave form. You will have to experiment as I said before.

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akopelumenuscurickharris

Answer 3 years ago

Thank you very much for your nice reply, Rick!

I'm also looking for some circuits which rely on an non inverting op-amp (for the first stage) and a transistor (for the second stage) placed as "emitter-follower".

I think that if I rely on an opamp for the first stage, maybe I can get rid of the "riddle" of the base resistor. I've found that the calculation of the two resistors for the op amp is more easy:

http://s13.postimg.org/8gnyje44n/Sawtooth_Gen_Op_Amp.png

The OpAmp will boost the voltage (from 7 volt to 14 volt); the BD441 will take care of boost the current. What do you think of this schematic?

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rickharrisakopelumenuscu

Answer 3 years ago

Maybe OK the data sheet says the transistor can provide 4 amps (in principle). - You really need to invest in a few components and try some of this. it will hardly cost you much.

A lot of electronics doesn't actually work as per the theoretical calculations and you need to tweak things to get what you want. Just one of those facts of life and tolerances.

Without wishing to sound harsh we can all theorize until the cows come home but the proof will be in building the circuit and see what is happening.

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Downunder35m

3 years ago

Don't see anything wrong with the schematics, pretty much standard amplifying output.
The fisr transistor provides the power and regulation for the second, which gives the high current output signal.
Be aware that you need a very well working power supply for this or at least some decent sized electrolytic capacitors on the power supply.
Otherwise the supply voltage might break down and ruin your output signal.

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akopelumenuscuDownunder35m

Answer 3 years ago

Hi, thank you for the reply.

I've read something about amplification transistors, (but consider the fact that I'm not very expert) and I have a pair of doubts: the base resistor R2 seems too much big (550 Kohm) to me, and on some forum I've also read that this configuration will cause to both transistors will be turned on all the time and do nothing.

However, since you told me (I consider the fact that you are an expert) that the schematic is good, so I can go with this schematic without doubts.

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Downunder35makopelumenuscu

Answer 3 years ago

If in doubt you can replace both resistors at the transistors with potentiometers, this way you can do some finetuning.
And, no I am no expert, just building all sorts of things ;)
But you can use a simple signal generator with an audio amp if the frequencies you need is in the normal operating range of the amp.
Might not be as perfect as a dedicated solution but should do the trick for most applications.
People even run tesla coils that way...

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steveastrouk

3 years ago

What load are you driving ? It sounds to me all you need is a simple amplifier.

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akopelumenuscusteveastrouk

Answer 3 years ago

Hi, thank you for the reply to you too.

Yes: I just need a simple amplifier.