Non-Inverting OpAmp Bias not acting as expected- UPDATE

Question

I have been experimenting with an opamp as a noninverting amplifier for a Piezoelectric vibration sensor 0-1Khz expected. I am only interested in the positive portion of the signal as an average hence the big Cap on the output signal. My problem is bias voltage injection. When the gain network is tied to the ground the circuit works as expected. but I have since tried to inject a 0.5v bias voltage via a divider network but I'm not getting the response expected.

I suspect it's the divider network because it's in the same order of magnitude as the gain network but I don't have larger values to test at the moment so I hope someone can shed some light on the issue. I feel like I'm missing something obvious. The circuit is supplied from a bench 3.0V supply so it's trustworthy.

The Opamp used has a 1.3mv offset and with an x101 gain, I expect to see 131mV on the output with no input signal and am currently getting 129mv so it seems accurate, when I apply a steady-state 7.5mv I get 760mV out which is also expected. but when I inject the biased voltage the output is stuck at a steady state of 35mV.

EDIT*https://www.ti.com/lit/ds/symlink/lmv321-n.pdf?ts=1626621566114&ref_url=https%253A%252F%252Fwww.ti.com%252Fstore%252Fti%252Fen%252Fp%252Fproduct%252F%253Fp%253DLMV321M5X%252FNOPB

So as an update thanks to the comments from members I have found a way that seems to work to provide Bias on a Noninverting amplifier. The Bias can be driven from the same supply rail on the condition that the output is buffered. in addition, the Piezo Sensor Negative ( if that's the correct term) has to be connected to the virtual ground of the same bias feedback path and not ground. Still need to test it on real circuit but simulation seems to be accurate

Answer 1

but I have since tried to inject a 0.5v bias voltage via a divider network but I'm not getting the response expected.

That's because you haven't accounted for -Vin being a virtual earth and it connects to the voltage divider (2 k and 10k) via the input resistor R3. R3 in effect, is partially grounding that node between the 2k and 10k (that otherwise would produce 0.5 volts): -

You need to factor it in. Maybe get rid of the 2 k and reduce the 10k to 5k.

However, that then upsets your piezo amplification gain (as it also does in your current circuit) so, another method should be found that both gives you the correct offset and doesn't alter the gain.

I don't have larger values to test at the moment so I hope someone can shed some light on the issue.

No, larger values are even worse. If you want to try something change 2k to 200R and 10k to 1k. If you can live with the smaller error then that's a viable solution.

I feel like I'm missing something obvious.

Hopefully, not any more.

Answer 2

An op amp works by driving its output to a voltage which makes the two inputs be at almost the same voltage as each other.

With a positive bias voltage at the left hand side of R3 the op amp is trying to drive its output negative with respect to ground in a desperate and failing attempt to make the inputs have the same voltage level.

The output can't actually go negative because there is no negative supply voltage and so it limits at the negative saturation limit. I haven't looked at the data sheet but the negative saturation limit is probably a few 10s of mV above ground when the op amp's negative supply rail is 0V.

Slowly reduce the 2k resistor and when it reaches the optimum value you will see the op amp's output just start to rise (leave saturation).