Electronic – Op-amp unexpected behaviour; amplification regime breakdown

amplifieroperational-amplifier

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

schematic

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.

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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\$:

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

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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\$?

Best Answer

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