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Old 16th March 2015, 10:19 PM   #1
npn is offline npn  Germany
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Default Which resistor value for Gain

I need a Gain of 1 in my feedbackloop. But which values are better, lower or higher? R1=120k, R2 120k or R1=10Ω and R2=10Ω?

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Old 17th March 2015, 04:00 AM   #2
ammel68 is offline ammel68  United States
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No schematic of the circuit you're asking about??

How is anyone supposed to help you?
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Old 17th March 2015, 05:14 AM   #3
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Inverting, or non-inverting? (I'm guessing it's inverting.)

What happens if you make the feedback resistors ten ohms? The answer is super simple, and glaringly obvious.
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Old 17th March 2015, 12:59 PM   #4
Mooly is offline Mooly  United Kingdom
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You might want to look at those examples Amp 3 has a gain of 3.55.
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Old 17th March 2015, 01:03 PM   #5
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Quote:
Originally Posted by Mooly View Post
You might want to look at those examples Amp 3 has a gain of 3.55.
Awrk! Right. Correcting that, thanks.
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Old 17th March 2015, 01:19 PM   #6
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Generally you use kohm (k = 1000) resistors in a feedback network. You wouldn't use a value like 10 ohms unless you had a specific reason.

For audio the general procedure is to keep to lower values, say under 50k. But any values will work.

After that, "gain of one" can be a confusing way to put it. It's better to say unity gain, or else state the gain figure, as shown below.

Again referring to the illustration below, R2 is generically called Rf, for feedback resistor. R3 is generically called Rg, for gain resistor. So the gain formula is generically stated as Gain = 1 + (Rf / Rg).

But in a real schematic these components would always be numbered like any other, not called Rf or Rg.
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Old 17th March 2015, 03:42 PM   #7
npn is offline npn  Germany
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Thanks for the explanations. Its a non-inverting Amp. I read about Johnson Noise and thought about the right selection of the gain resistor values. then i will use Rf=Rg=1k to obtain a gain of 2.
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Old 17th March 2015, 04:23 PM   #8
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Right idea. But wait, you thought it would be that simple? C'mon.

Don't panic, things are going to get back to simple right away. But first I have to tell you that you need to consider the load-driving ability of your op amp. Data sheets generally state this as "drives 600 ohm loads," or words to that effect.

But what's a "load"? There's never--or rarely--just one resistor connected to an op amp's output, and the calculations can get complex.

Complex calculations are for engineers. We mere humans just use rules of thumb. So getting back to simple, the rule of thumb is never require an op amp to drive less than 2k unless it's the final output (to speakers or headphones).

In this particular case Rf would be 2.2k (the closest standard value), and Rg would be sized to match.

You're right to consider Johnson noise, but broadly speaking this is not something to worry about until you get up around 100k, in that area.

Addendum: remember that rules of thumb are, of necessity, very broad statements. You'll see this one--and all of them--broken from time to time. Not a problem, it'll work fine.
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Old 17th March 2015, 04:37 PM   #9
npn is offline npn  Germany
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My headphones are 40ohm impedance.
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Old 17th March 2015, 06:20 PM   #10
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Hmmm. You're totally winging it, aren't you? Well, nothing wrong with that. When in doubt, take a shot, see what happens.

Most op amps won't drive a 40 ohm load. You might be well served to take a look at the very well known Cmoy, which is here:

A Pocket Headphone Amplifier | HeadWize

Enclosures don't have to be an Altoids tin, or on the other hand something elaborate. I personally have been known to use tuna fish cans, or if you or your neighbor has a cat you're home free.
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