simple DC voltage doubler

[cypher]

Hi all!
I am looking for a really simple DC voltage doubler solution - in my case the ENV generator gives the peak output of approx. 2,5 V and I need to make the 5 V peak from it (or lets say, double the amplitude).
I have found some ideas already, but I am not sure that its the best solution for synth device.
Greetigs

[clee]

A simple non-inverting op amp with a gain of 2. Google "op amp gain calculator" and a schematic and resistor values should be easy to get.

[rutgerv]

Yes, a simple op-amp circuit with a gain of 2 sounds like the best solution. However, you could probably also modify the ENV generator to put out 5V directly. Most ENV generators have an op-amp buffer with a specific gain at the output. If you modify this gain, you can get the signal up to 5V instead of 2,5. But then it's not so easy to switch back to 2,5 if you need it of course.

Rutger

[JovianPyx]

Seeing the schematic for the envelope generator would help enormously.

I agree with rutgerv that it may be easiest to modify it.

As for returning to 2.5 volts, it could be done with 2 equal resistors forming a 2:1 voltage divider which would reduce 5volts to 2.5volts.

Usually, however, it is desirable to have a module's output be as high as possible within the limits of it's power supply and the receiving module would have a level pot on it's input to control the effect. That way, you simply adjust the knob to get what you want.

[cypher]

Hi all!
Thanks a lot.
Well, I tried an OP-AMP (I used the TL-064 JFET OP-AMP) and it works.
But there seems to be a little bug or something - the OP-AMP output is "high level" at the start and also ends at "high level" too...
For example:

When the key is not pressed:
ENV level is 0V; The non-inverted pin of AMP of course 0V; the inverted pin of AMP je 2,5V; the AMP output is 5V.

When the key is pressed:
ENV level:
0V 0V 0V 1V 2V 2,5V 2V 1V 0V 0V 0V 0V
AMP OUTPUT level:
5V 5V 0V 2V 4V 5V 4V 2V 0V 5V 5V 5V

I connected the non-inverted OP-AMP imput (audio input) trought 100K resistor to groud; the inverted OP-AMP input trought 10K res. to ground and trought 10K res. to feedback (output).
Maybe something else on the OP-AMP have to be grounded or what can be the problem? Why is the amp output by "default" at high level?

Greetings

[clee]

sounds like you are using an inverting configuration possibly, not sure.

Use this:

http://www.leapfrogsw.com/noninverting.php3

[JovianPyx]

yes, it looks like an inverter... but not really, an inverter on an opamp powered with a bipolar power supply should output negative voltages.

May we please see some schematics?

[cypher]

[clee]

Oops, I didn't know you were operating with a single ended supply. Being in a synth I just assumed you were using a split supply. It's not working because you are not giving the op amp a ground reference. What you describe as ground the op amp is seeing as the negative supply voltage. You need to set up a voltage divider (2 100k resistors) from 5v to ground and a cap (25uf electrolytic) from the junction of the 2 resistors to ground, negative end of the cap at ground. That would then be the virtual ground. All ground connections need to go there.

It gets complicated from there. Your op amp will output 2.5v with no signal applied. Plus, you may have trouble getting the output to 5v if that is the B+ voltage. Most op amps are not completely rail to rail.

Don't you have +/- 12 or 15v available? The circuit is MUCH simpler with a bipolar supply even if you only need positive output voltages.

[cypher]

hmm. its not a very good situation
I really would prefer just the existing 5V PS (as long as possible), because the device should be "portable" and all other blocks are already working at 5 V DC powersupply.
The Voltage divider you described OUTPUT will produce 2,5 Bias or -5V or ?

But you wrote also, that after using that voltage divider, the default output of the OP-AMP will still be just 2,5V ?

So there is no way to do "relative simple" gain of 2 amplifier in 5V DC device?
To get approx. 0-5V DC from 0-2,5V DC ENVELOPE GENERATOR?
Greetings

[clee]

Here is what you need I think. For some reason I can't insert the picture, you need to download it.

The ratio of RF to RG sets the gain. You want to set it to a gain of 2. I THINK this works... breadboard it to test it.

What I meant was that the output of the op amp will sit at 2.5v with no input. There may be an offset you need to adjust for. It's been a long time since I did single supply so I'm a little rusty

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single supply op amp.bmp
Description:		Download (listen)
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[JovianPyx]

This is in the LM324 datasheet as an example schematic. For a gain of two, both resistors would be 10K. The circuit is spec'd to run on 5volts single supply, but should work up to the maximum supply voltage allowed.

EDIT ADD: It should also be noted that in the graph to the right of the schematic, we can see that the voltage curve flattens off somewhat before it gets to 5 volts. This is because an LM324 is not a "rail to rail" opamp. So to use this to double a 2.5 volt signal, the power supply should be higher than 5 volts. I would suggest 8 volts or more since the datasheet also says that the maximum voltage the opamp can output is Vcc - 1.5 (this assumes that you are using an LM324, if other opamp, please check it's datasheet). The LM324 can, however, supply 0.0 volts at it's output.

Is that what you tried cypher?

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LM324.gif
Description:	Snippet of LM324 datasheet (National Semi)
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[cypher]

Hi all!
Well, the problem was just I had there a 100K resistor to ground from IN+ (I saw it in some similar circuit). I tried also adding the voltage divider, but its better working just without it in my case (and for ENVELOPE doubling usage).
Anyway, thanks to all.
Greetings