# Op-amp unexpected behaviour; amplification regime breakdown

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 $5.9\text{ k}\Omega$?

• Try changing C1, and see if the "break-point" changes. Try several different values, and see if there is a relationship. Oct 19, 2016 at 19:11
• Yeah C1 looks small. Also the GBW of that opamp is 100kHz! you've got a gain of ~60 and a freq of 3.8kHz... ~220 kHz. That might be causing an issue.... what's it look like at lower frequency? Oct 19, 2016 at 19:30

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