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Simple op-amp

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mynameisdan

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I'm trying to design a simple op-amp circuit to boost the voltage from a thermistor (0.2v - 0.4v) to 0.2v - 5v.

However, with this circuit I don't seem to be getting any gain at all. I'm using this as a guide.

I've tried changing the value of R2 to a few values from 1M to 100R. I'm stumped here and would certainly appreciate advice!

See attached.
 

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hi,
The 741 with a 5V supply will only give an output of +1.5V thru to 3.5V.

Its a really poor spec OPA

EDIT:
With an input signal like this: (0.2v - 0.4v) and output like this 0.2v - 5v. not only do you have to amplify the signal you have to apply an offset voltage.

eg: when amplified by 12.5 you would get 2.5V thru 5V
so you need to offset the 0.2V part and amplify by 25 , so that would give approx 0V thru approx +5V

The important item is the OPA, you need a rail to rail OPA. they are not expensive.
Look at the MCP6002 datasheet
 
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I see, but for some reason I am getting no gain. Even a swing from 1.5v to 3.5v would be useful.

OK, the gain for a non inverting OPA is

1+ Rf/Rin, where Rf is the feedback and Rin is the input resistor.

So for a gain of 12.5V
use a 4k7 for Rin and a 54K for Rf,,,,, Gain = 1 + 54/4.7 = ~ 12.5

So when the input is 0.2V the output will be 2.5V and for 0.4Vin the Vout will be 5V

BUT as I said the 741 out will only swing from 1.5V to 3.5V

That means you have to amplify the 0.2Vin to get 1.5Vout by 7.5...........and the 0.4Vin will be 3Vout

So, for a new Gain of 7.5 , use a 4.7K for Rin and a 30K for for Rf......... thats 1+ 30/4.7 =~ 7.38

It will work , just, but I dont think you will be satisfied with the result.
 
I see, but for some reason I am getting no gain. Even a swing from 1.5v to 3.5v would be useful.
You are getting no gain because the 741 will not work down to much less than a couple volts from ground input or output when using a single supply voltage. You need a rail-to-rail op amp as Eric stated.

Edit: Eric beat me by a second.
 
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Ok, thanks. Although I'm not sure why I didn't get something more modern in the first place.

Can anyone point me to a more suitable for my application? Having a quick poke around I see the LM324 is a good place to start. Or a RC4558 linky
 
Ok, thanks. Although I'm not sure why I didn't get something more modern in the first place.

Can anyone point me to a more suitable for my application? Having a quick poke around I see the LM324 is a good place to start. Or a RC4558 linky

Hi,
From earlier.
The important item is the OPA, you need a rail to rail OPA. they are not expensive.
Look at the MCP6002 datasheet

The Lm324 will go from close to 0V to Vs-1.5v [ thats 3.5V for a 5V supply]

the 4558 is designed for dual supplies, its not a rail to rail
 
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