Open-loop gain of Op-amp - LT6015

Question

I was recently reading a technical article from AllAboutCircuits by Robert Keim about Op-amp Stability, https://www.allaboutcircuits.com/technical-articles/negative-feedback-part-8-analyzing-transimpedance-amplifier-stability/

I am simulating the LTSpice circuit, as seen below, of the transimpedance amplifier using the LT6015 Op-Amp as depicted in the article.

However when I run the simulation I am not able to get the same Open-loop frequency response as stated in the article.

I obtain a max open-loop gain of ~145dB, whereas he has ~102dB. Furthermore I created some matlab code to check the open-loop gain using the approximation of the op-amp as a first-order low-pass filter. Taken from the datasheet of the LT6015 the open-loop gain is 3,000,000 and the open-loop bandwidth is therefore 1.1Hz (calculated from the gain bandwidth product, with a value of 3.2MHz). However this outputs a open-loop gain of ~129dB.

Here is the code of my matlab program:

    Ao = 3000*10^3 %Open-loop gain
    Wb = 1.1 %Open-loop bandwidth of op-amp

    w = .1:.1:100000;
    h = 20*log10(Ao*(abs(1./(1+(j*w)/(Wb)))))

    figure
    semilogx(w,h) %% Plot on log scale
    hold on
    xlabel('\omega(rad/s)')
    ylabel('|H(j\omega)|dB')
    legend('1','2')

I am unable to see what I am doing wrong to create the discrepancy between these three plots, with the primary focus being the former two, the last one acts as check. Perhaps the last one is incorrect because the LT6015 cannot be modelled as first-order low-pass filter?

Any help would be greatly appreciated!

Answer 1

ConfusedCheese, here is a simple method for simulating the loop gain for an operational amplifier with feedback. This simulation scheme provides DC feedback and a stable operational point (for double and symmetrical voltage supply only!).

• The dashed box contains the feedback factor, if any!.

• Without this box (direct connection to the inv. input) you simulate the open-loop gain Aol of the opamp only.

• The loop gain is simply T(s)=V(node2)/V(node1). Of course, the frequency must be swept over the desired range (ac analysis).

^{simulate this circuit – Schematic created using CircuitLab}