Amplifiying Millivolt to Volts

I'm afraid nobody on this list can string together an op amp and two resistors.

dering if anyone knew how to do that. I'm really in a rush so please resond quickly.

The classic answer used to be the Fairchild uA725 op amp

Farnell don't stock it any more, but it had the two necessary properties fo r your application - a low input off-set voltage (which you could trim down to zero with an external potentiometer if you wanted to) and a low frequen cy gain of about one million, which left enough loop gain over to give resp ectable closed loop performance at a closed loop gain of 100.

There are "chopper amplifiers" around with much lower input offset voltages , but none of the ones with which I am familiar have the high low-frequency open-loop gain that you got from the uA725.

The Analog Devices AD797 is a more modern device, which you can buy - for $

15 each. It's gain is just as high, it's input offset voltage is a lot lowe r, and you can trim out what offset there is.

It's really intended for looking at low impedance sources - less than 1k - and the data sheet lists suggestions for higher-impedance inputs.

It has got a very respectable bandwidth, so careful layout, proper power-su pply decoupling and so forth are required to stop it acting as an oscillato r.

Hope this helps.

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Bill Sloman, Sydney

Almost any op-amp will do. Google "basic op amp circuits".

On a sunny day (Mon, 17 Feb 2014 21:28:50 -0800) it happened miso wrote in :

Those wil use transformers.... ;-)

e

A 100:1 ratio transformer would be mores stable and more accurate - at abou t 1 part in 10,000,000 - than any resistive divider you could buy.

If the OP wanted AC amplification, that would be a way to go. Making up a 1

01-wire bundle, winding the transformer with it, and identifying which of t he 100 wires coming out of one end matched connected to any given one of th e 100 wires coming out the other could be tedious.

I have through about using ribbon cable for the job, on a decent sized toro id, which would make the job easier. Two 64-way ribbon cables plus IDC conn ectors eliminate all the tedious manual labour, though the winding density would be rubbish.

Come to think of it, I'd throw in one extra - thicker - wire (to carry the exciting current) and use an op amp to make sure that the voltage across on e of those 100 identical windings matched the voltage I wanted to amplify.

Resistive drops are predictable and correctable, but why correct for someth ing you can eliminate?

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Bill Sloman, Sydney

What is the frequency of your signal? From what source impedance? To what load impedance?

What are your power supply requirements? Line powered, battery, etc?

How much noise can you stand?

In other words, you need to provide more detail.

hth

I suppose a version of the Baxandall circuit would fit nicely for this application ;-)

Cheers

Klaus

Tin all the wires and use a LED in series with headphones and a 3V supply as a continuity probe

You can then do sweeps with the probe to find the other end of each wire easily, (use one led lead as probe tip: that keeps it in view)

--
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Cheers

Klaus

wondering if anyone knew how to do that. I'm really in a rush so please re sond quickly.

plication ;-)

I'm sure that Peter Baxandall would have come up with something appropriate . His tone control is much more famous than his class-D oscillator, and he was well-known in U.K. audio circles for his enthusiasm for the proposition that a good audio amplifier was one that looked good when you measured it' s performance as an amplifier. The "subjectivists" liked amplifiers that so unded good to them. Some of their preferred amplifiers added appreciable od d-order distortion.

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Bill Sloman, Sydney

dering if anyone knew how to do that. I'm really in a rush so please resond quickly.

Hi guys, it's not really a signal i'm trying to convert 10 millivolts witho ut any other source of electricity. I've tried using op amp but it doesn't seem to work. Where do I put ground on my load. It only has one wire coming from the op amp for the output. I'm trying to amplify it to dc or if I can amplify it enough I'll go with ac(120). I basically trying to show that 10 millivolts is usable.

I mean 110v

ondering if anyone knew how to do that. I'm really in a rush so please reso nd quickly.

thout any other source of electricity. I've tried using op amp but it doesn 't seem to work. Where do I put ground on my load. It only has one wire com ing from the op amp for the output. I'm trying to amplify it to dc or if I can amplify it enough I'll go with ac(120). I basically trying to show that 10 millivolts is usable.

Hi Caiden. This really is a very silly request. What is your 10mV source?

It's got to have two wires coming out of it to count as a 10mV (or any othe r kind of voltage) source.

You are going to have to learn enough about electricity to understand that before you get to be in a position to take advantage of any higher level ad vice.

If you thought that you were in a rush before, you now know that you bit of f way more than you can chew.

On the basis that you are some kind of troll, who knows more than he is let ting on, you might try reading

odfnYA3A

Their stuff won't harvest anything below 20mV, but it's in the ball-park of what you now might be asking for, if you had any idea of what you were tal king about.

--
Bill Sloman, Sydney

ondering if anyone knew how to do that. I'm really in a rush so please reso nd quickly.

You might have better luck if you describe what you are trying to accomplis h. Depending on what you need in the way of stability , frequency response , power etc there are many ways to go from simple circuits to ones that pas s high frequency thru one channel and low frequencies thru a chopper stabil ized channel.

A hi fi amplifier will do what you describe , but probably not what you wa nt.

Dan

ondering if anyone knew how to do that. I'm really in a rush so please reso nd quickly.

hout any other source of electricity. I've tried using op amp but it doesn' t seem to work. Where do I put ground on my load. It only has one wire comi ng from the op amp for the output. I'm trying to amplify it to dc or if I c an amplify it enough I'll go with ac(120). I basically trying to show that

10 millivolts is usable.

Well concerning the current return from your load, there should be a ground on the power supply that is powering the opamp. Attach your load return t o the same point. It would be nice if you included a circuit diagram. (on e worries that you are making some other "small" mistake.) You can just sc ribble something on paper, take a picture and post a link to one of the pic ture hosting sites.. (drop box is also an option.)

To get up to 120 volts is harder... (maybe start with getting to 10 Volts.) George H.

her kind of voltage) source.

Sound like background RF radiations, like generating power from cell phone signals that we discussed here before. You can just build a big enough wir e loop to get some mV. However, one source is not enough. You have to pul l in multiple sources in series, perhaps from parallel universe, ;-)

Nice book!

You are trying to extract power from your 10mV source? How much power? If you want to power a 40W light bulb from 10mV, and you could by some miracle do the conversion at 100% efficiency, you'd need to flow 4000 amps, which means that even a bus bar the size of your arm would drop

10mV.

I think you have a chicken-and-egg problem here.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com