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OP amp help - amplify low voltage

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fruct0se

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OP amp help

I am using a 741 op amp to try and amplify a low dc voltage.

+ = 5vdc
- = 0v
+ non inverting input = 0 to 0.8vdc
- inverting input = 100k to 0v / 20k to Output

No matter what I try I get around 4.2 to 4.6 on the output even if I use 0 feedback.

I would like to amplify a range 0 to 0.8v to a 0 to 5v range, please help.
 

snafflekid

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your output should be 20% larger than your input. Double check your connections

well my computer won't load a damn pdf now. I think you may be below the common mode voltage range of the 741
 

fruct0se

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Ah, the minimum supply is 10v... Would this explain why I was getting a constant 5v out though?
 

snafflekid

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That is probably part of the problem. Looks like it is spec'd for +/-15V although it might run on 0V/10V. I doubt 5V would work.

Also the input will not amplify signals that close to ground. Looking at the schematic I think you would need at least 3V common mode input voltage.
 

fruct0se

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Any ideas on how I could amplify 0-0.8 to 0-5?
 

snafflekid

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You need an opamp that is "ground sensing" and single supply. I'm looking at something like MAX4122. It operates at 5V
 

fruct0se

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Thanks for the great responses, I am going to try that out.
 

Audioguru

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An LM358 is a dual opamp with a minimum power supply voltage of 3V and inputs that work at 0V up to 3.5V when the supply is 5V. With a 5V supply its max unloaded output is 3.8V.
 

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