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I am looking to amplify a DC signal that will range from 0 to 400mV by at around 5-10X and read it with my DAQ.

I currently have a TLV271 and a UA741CP that I tried to use in a non-inverting circuit. I can't get it work as I would like.

The signal is coming straight from an electric guitar. Will either of the op-amps I have work for me? If not which one should I buy. Also I have another DAQ that can take up to 15V input to read the signal that I can use it that makes things easier.

In my circuit Rf was 4.7K ohm and R2 was 1K ohm with supply being 5V and 0V ground.

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JYelton
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ataveras64
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    "*cant get it work as I would like*" Elaborate. We cannot help if you do not tell us what went wrong. – DKNguyen Jun 08 '22 at 14:07
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    Electric guitars don't output DC. – JRE Jun 08 '22 at 14:09
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    you are using split ( plus and minus ) power supplies right? – danmcb Jun 08 '22 at 14:44
  • Have a look at some guitar preamp circuits - there are millions out there. Look at how they work. Note how they deal with amplifying an AC signal when you only have a single-ended power supply. – nekomatic Jun 08 '22 at 15:09
  • Sorry I was not clear. I initially tried to at my bench to test the op amp(the 741) by powering it with 12v and feeding the input with a voltage of 3v. The results seemed like it was only really shifting the voltage up by like 3v instead of amplifying by the gain set by the resistors. I would increase the input voltage by 1v and the output would only change by a little over 1V(maybe like 1.2V). Ans no I was not using a split power supply, I still have not tested it with it plugged into the guitar – ataveras64 Jun 08 '22 at 15:11
  • @ataveras64 The problem with your test case there is more basic than the other responses that address the nature of audio signals. A 741 cannot accept a voltage input close to its input rails (3V is too close to to 0V rail). Opamps generally can't input or output close to their voltage rails unless specified. So-called common mode input and output. Check the datasheet. – DKNguyen Jun 08 '22 at 19:53

2 Answers2

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ua741 goes right into ; can't use it at all with 5V. Also, please read this excellent answer, which also addresses a couple of things you forgot to mention in your question, but which would become a problem even if you had enough supply voltage:

Reasons not to use a 741 op-amp?

The signal you want to amplify is audio; that has as much positive as it has negative voltages. You only have a positive supply – so, even a rail-to-rail opamp, which can deal with voltages down to its negative supply and up to its positive supply couldn't help you.

You need to bias the signal you want to amplify somewhere into the middle of your supply range.

NB: As said above, a guitar does not produce a DC signal.

JYelton
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Marcus Müller
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  • Gotcha, seems like I was misunderstanding what I was trying to do. But it is possible to add an offset to the signal to bring into my 0-5v reading range correct(like a 2.5v offset to set it in the middle of my reading range). If so I still need an op-amp that can amplify this ac signal. The 741 is not an option, what op amps should I look into considering? – ataveras64 Jun 08 '22 at 15:18
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    "possible to add offset": yes. That's literally what I was recommending! – Marcus Müller Jun 08 '22 at 15:54
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An electric guitar is not DC. A guitar, however, is a high-impedance signal source so you want to pair it with a high-impedance low-current-noise opamp. TLV271 works with 5V of supply voltage, but of course since the signal is AC, you need to connect R2 not to ground but to a "virtual signal ground" at about half the supply voltage and decouple both input and output DC with a capacitor large enough to admit the interesting DC range.

This opamp is nice for battery operation; however, its equivalent input voltage noise at \$39\,\mathrm{nV}/\sqrt{\mathrm {Hz}}\$ is not particularly quiet and will noticeably increase the thermal noise from a guitar pickup. The spec sheet of the opamp does not mention audio applications and the detailed specs don't really point to this being a forte (check out things like capacitive load and other output behavior, common mode and power rejection).