Op amp non inverting amplifier not working

[I14R10]

Hello

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

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.

What am I doing wrong?

 

[danadakk]

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

Regards, Dana.

 

[FvM]

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

 

[I14R10]

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.

 

[FvM]

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

In this case you can try with zero volt bias. For precise rectifier operation you might prefer a dedicated rectifier circuit.

 

[I14R10]

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

 

[crutschow]

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).

 

[danadakk]

https://www.ti.com/lit/an/sboa059/sboa059.pdf?ts=1657100540571&ref_url=https%253A%252F%252Fwww.google.com%252F

https://www.ti.com/lit/an/slyt189/slyt189.pdf

https://www.ti.com/lit/an/sloa030a/sloa030a.pdf?ts=1657100901569&ref_url=https%253A%252F%252Fwww.google.com%252F

Regards, Dana.