Op-amp unexpected behaviour; amplification regime breakdown

Question

I'm still trying to understand real-life op-amps. I've built this circuit:

^{simulate this circuit – Schematic created using CircuitLab}

Op-amp is an MCP6141 (datasheet) I've chosen for its low-power consumption.

\$V_{\text{in}}\$ is sourced by a waveform generator, and has the following equation:

$$V_{\text{in}} = 1.5\text{ V} + 5\text{ mV} \times \sin(2\pi \times 3.8\text{ kHz} \times t)$$

When \$R_1\$ is between 0 and \$5.9\text{ k}\Omega\$, I have a normal behaviour, signal is amplified, and gain increases. By the way, I obtain a gain of 40 dB for \$R_2 = 5.9\text{ k}\Omega\$, which is strange twice, because

1. \$20\log(5900/100) = 35\text{ dB}\$ which is lower than 40 dB
2. datasheet says op-amp gain at 3.8kHz is around 30 dB.

As we can see output signal (yellow) is slightly distorted but everything happens as expected (except the gain as explained above).

FTT shows the main frequency is amplified by 40 dB for \$R_1 = 5.9\text{ k}\Omega\$:

But suddenly for \$R_1>5.9\text{ k}\Omega\$, things change and output signal becomes strange:

Does somebody has any idea why there is this strong discontinuity in the regime of the op-amp when \$R_2\$ crosses \$5.9\text{ k}\Omega\$?

Answer 1

That's a very low impedance feedback path for an ultra-low power op-amp. The output resistance is probably of a similar order to the R1 which is causing you problems (you can 'measure' it from the SPICE model, they typically don't discuss such dirty little secrets on the datasheet).

I would suggest increasing it by at least an order of magnitude, taking R2 to 1K and C1 to 10nF, preferably by two orders of magnitude (R1-> 10K and C1->1nF). Gain is specified with a 50K load to ground.

Check your calculation of the C1 value too- it should have negligible reactance at 3.8KHz compared to R2, so I get more like several uF than 100nF for R2 = 100\$\Omega\$. So maybe 10K/39nF for R2/C1 and R1 from 0 to 2M