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I am going through the following guide for op amps : http://web.mit.edu/6.101/www/reference/op_amps_everyone.pdf

On page 87 (screenshot attached) there is a bode plot which has got me slightly confused. I've usually seen two bode plots on such plots, one for open loop gain (starts of high and then has a roll off based on the pole zero configuration) and then the closed loop gain which the designer sets using passive components is usually (ideally) much lower than open loop and then intersects with the closed loop to form the bandwidth of your op amp circuit.

In this image they state the flat bit is set by the DC gain. What are they referring too? This isn't the closed loop gain right? Is this the difference between closed and open loop? But that isn't the circuit gain? Also why is the slope 20 log (Abeta)? If this was loop gain wouldn't it be 20 log (1 + Abeta)

Thanks!

Screen shot from guide being referred to

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  • \$\begingroup\$ The shown BODE diagram is used to find stabiliy margins (gain and phase margin) . Therefore, we can conclude that it shows the LOOP GAIN (excluding the sign inversion due to negative feedback). \$\endgroup\$
    – LvW
    Feb 12 at 9:22
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That Bode plot is a plot of magnitude and phase of loop gain, plotted in dB and degrees respectively.

Loop gain (Aol x beta) is equal to open loop gain minus the noise gain when both are plotted in dB. If they're both plotted on a linear scale then Loop Gain = open loop gain divided by noise gain where noise gain is equal to (Rf + Rin)/Rin (for a purely resistive feedback network the noise gain plots as a horizontal line).

The difference between open loop gain and closed loop gain (both plotted in dB) is equal to 1 plus the loop gain, 1 + (Aol x beta), where 1 + (beta x Aol) is known as the feedback factor.

The difference between closed loop gain and noise gain is that, for a purely resistive feedback network, noise gain is constant over frequency where as the closed loop gain reduces as frequency increases, slowly at low frequency reducing at a faster rate at higher frequency after the closed loop gain plot meets the open loop gain plot.

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  • \$\begingroup\$ Thank you, that was a very clear explanation. Why is the slope after it meets the first pole 20 log(AB)? \$\endgroup\$
    – Hasman404
    Feb 12 at 22:44
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    \$\begingroup\$ The slope isn't 20 log(AB), the magnitude of the plot is 20 log (AB). The plot is a plot of loop gain which is equal to AB and converting to decibels uses 20 log (AB). The slope after the first pole becomes -20dB/decade which relates to a phase shift of -90 degrees in the phase plot. That is to say the loop gain drops 20dB for every 10 fold increase in frequency. This slope becomes -40dB/decade after the second pole when the phase lag reaches -180 degrees. How close the phase shift is to -180 degrees when the loop gain has dropped to 1 (0dB) determines the stability. \$\endgroup\$
    – James
    Feb 12 at 23:09

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