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Op amp non inverting amplifier not working

I14R10

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Hello

I'm trying to build an amplifier with Op amp (TL072). I built a simple circuit like this

opamp-opamp15.gif


But I don't get anything on output. Weird thing is, I get huge amplification if I don't connect R_f and R_2. As soon as I put any resistor there, my output drops to 0.

I've also simulated a random 2 op amps in LTSpice and this exact circuit doesn't work there either.


Screenshot_20220705_155749.png


What am I doing wrong?
 

danadakk

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You have to provide a bias return path for the NI input. Say a 1M to ground,
since you are AC coupling. Or DC couple and offset V2 waveform.

When no fdbk is used the OpAmp is open loop with a typical G of

1657030479301.png


Regards, Dana.
 

I14R10

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Yes, missing N.I. bias. To achieve useful operation point with given feedback network, it should be biased to e.g. 0.5 V.

I want only positive half of the signal to be amplified. Do I still need to do N.I.bias?

Also, I'm starting to doubt if my opamps are good. I've also made voltage follower (wire from output to - input) and my opamps are still getting a huge amplification on the output.
 

I14R10

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Thanks everybody. Somehow I thought op amps were some magical devices that doesn't need biasing.
 

crutschow

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I want only positive half of the signal to be amplified. Do I still need to do N.I.bias?
Thanks everybody. Somehow I thought op amps were some magical devices that doesn't need biasing.
They are very useful devices, but certainly not magical.
Both inputs need a path for their input bias current no matter how small it is.

Also if you want to operate them from a single-supply they need to be single-supply or rail-rail types, or the inputs need to be biased at 1/2 the supply voltage (virtual ground).
 

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