# Why doesn't this non-inverting amplifier work with a biased signal in a real circuit?

I have a signal with amplitude of 200 mV biased to 4.5 V. I would like to amplify that signal (AC only, no DC gain) after biasing with a single supply op amp. I have tried a non-inverting op amp circuit with 4.5 V connected instead of ground to inverting input, but it doesn't work in my real circuit.

simulate this circuit – Schematic created using CircuitLab

The op amp is supplied with 9-0 V. I tried 1M, 10k, 1.2k and 30 ohm resistors. I used a TL071. In LTspice simulation, it works well, in reality there is constant 4.5 V on the output without AC signal at all. The signals on IN+ and IN- comes from OUTs of another TL071 that works as a low pass filter (15 kHz.)

What have I missed or misunderstood?

• Do you really want to amplify x1 so you can drive 1 G$\Omega$? Make it clear your source is biased at 4.5V and verified there is a signal there on input. Sep 2, 2021 at 7:28
• Sounds like your +4.5 sources aren't both actually +4.5V. Since you don't want DC gain, why aren't you AC coupling the input and referring both inputs to the same +4.5V supply? Sep 2, 2021 at 11:52

If you want to amplify only the AC component and you want only the AC component at the output you need to add two extra capacitors as shown in the below circuit.

Select the capacitor and resistor of the HPF such that the cut-off frequency is very less compared to your input frequency. Here I took the cut-off frequency as 100Hz.

Please see the output waveform below

Ran it on your CircuitLab link (TL081). Looks ok to me. Make sure to run long enough time and small enough steps.

• Thx for your answer. It works in simulation but doesn't work in real circuit and idk why. Sep 2, 2021 at 6:22
• Ah. ok. I misunderstood the question. Well, for one the op amp is not identical in your simulation setup. Double check if they are the same specs. Your input might be loaded down. Can you just connect directly to a freq. generator?
– pat
Sep 2, 2021 at 6:25
• 07x is low noise version of 08x. Both inputs come form 07x outputs. You are right that I should check it with generator, but I will able to do it next week. Now I have only guitar. Sep 2, 2021 at 6:35

$$V_{out}=V_{-}\cdot (-G) + (1+G)\cdot V_+$$ $$G=\dfrac{R_2}{R_1}=1$$ $$V_{out}=-V_{-} + 2\cdot V_+ = -4.5V + 800\ mV_{pp}$$

So you shouldn't get +4.5V out of this circuit, rather -4.5V if you would have been using a dual power supply. Green: 400mVpp input signal, Blue: output.

EDIT:

Single supply

See, you get nothing.

EDIT 2:

I see now, the signal is 4.5V + 400Vpp, not just 400Vpp. In this scenario, an opamp with single supply should output:

• Isn't 4.5V virtual ground/0 for the op amp in my circuit? I thought about it and you made me wonder. My op amp is supplied with 9,gnd not 5,-5, and simulation in LTspice works as I want. Sep 2, 2021 at 7:06
• @Jomonger The virtual ground would have approx. the same potential as V+ input of opamp. I did the simulation with dual supply, because it can't work with a single, I have explained that in my answer. Sep 2, 2021 at 7:08
• This answer doesn't match his original post. He had a 4.5V bias on both the negative input as well as the sin source (biasing vcm at vdd/2 is typical for single rail). The non-inverting circuit gain is 1+Rf/Ri just as in the sim result (Av = 2) with single rail. I don't see anything in the data sheet indicating you can't use single rail. Your equation is omitting the source bias. Yep. Saw the correction. It might have been confusing because it doesn't show the bias in his schematic.
– pat
Sep 2, 2021 at 7:37
• @pat Corrected. The equation has all it needs, now it would become: $V_{out}=-V_{-} + 2\cdot V_+ = -4.5V + 2\cdot (4.5V + 400\ mV_{pp})=4.5V + 800V_{pp}$ Sep 2, 2021 at 7:52

You say that signals at IN- and IN+ come from the same source. That makes no sense in the context of the rest of this question, and the circuit you've shown us. This is a differential amplifier, and by applying the same signal where you've marked "4.5" and at the non-inverting input, the difference is zero, and regardless of the gain you set with R2 and R3, the output won't do what you expect.

If that's not how you've wired it up, then read on. I'll assume from this point onwards that the node marked "4.5" is a fixed +4.5V source, not some AC signal input.

If in the real circuit you have +4.5V output, then I am confident that your opamp is fine, and the input signal (at the non-inverting input) is also centered on +4.5V. With a gain of 2 (as you have it here) any fluctuation at the non-inverting input should be measurable at the output.

Barring any wiring errors, or other silly mistakes, the fact that you see no deviation from 4.5V in the output tells me that there is no AC component to the signal at the non-inverting input. In other words, the previous stage's output is stuck at +4.5V too.

If you really want to see if this stage is working, instead of attaching the input to another opamp source, first try manually applying +4.4V and +4.6V (using a potential divider, or potentiometer, perhaps) at your input, to verify that it's good. That way you isolate this particular stage, and you can rule out problems from elsewhere, like frequency or bias issues.

In LTspice simulation, it works well, in reality there is constant 4.5 V on the output.

That's pretty much what you will get. You are using a non-inverting configuration with the inverting feed biased at exactly the same DC voltage level as the bias on the signal input hence, there can be no DC gain (because you've balanced that out) and, that 4.5 volts is reflected onto the output. It sounds like it does as intended. If you are not getting the required AC gain then that's a different story but, you haven't given details about that.

Now the OP is clearer (following their amendment): -

in reality there is constant 4.5 V on the output without AC signal at all.

Make sure you are using a frequency that is in the operating capabilities of the TL081 - something less than 1 MHz should be fine. Also, double check that the signal is present on the non-inverting input pin of the op-amp. Preferably use a two-channel oscilloscope and take a photograph of the result. Quite possibly you may have made a wiring error so check that. It's also possible that the device is broken.

• Yes, I don't want DC gain only AC. I'm not getting AC on output at all, only 4.5V DC. Sep 2, 2021 at 7:18