# Electronic – Op-amp theory vs practice: what have I done wrong

amplifieroperational-amplifier

I'm currently trying to use the MCP6142 op amp in an ultra-low power project (sub-uA ideally). This op-amp has been chosen because it has the highest GBW in the set of the sub-uA op-amp we've reviewed. However in the range of the frequencies we'd like to amplify (3800-4000 Hz) the gain of this op-amp is far from "ideal". See this figure from the datasheet: So we've come up with this circuit to amplify the signal: simulate this circuit – Schematic created using CircuitLab

Considering the following equations:

``````Vout = G(w)*exp(j * phi(w))*(V+ - V-)
V- = R1 / ( R1 + R2 + 1/(jwC1) ) * Vout
``````

We've derived the transfer function `H(w)`:

``````H(w) = G(w) * (1+j*w*(R1+R2)*C1) / (G(w) + j*w.*G(w)*R2*C + exp(-j*phi(w)) + exp(j*(pi/2-phi))*w*(R1+R2)*C);
``````

We've used `G(w)` and `phi(w)` plots from the datasheet to compute `H(w)` and plot its curve: So we expected a peak gain around 4000 Hz… that we never observed when pluging a waveform generator at the input of the op-amp. Of course, we tried to find the peak by tuning the waveform generator frequency, but we never found it. What could we have done wrong? Have me made a good use of the Gain-Phase plots from the datasheet?