Plotting the Open-loop Gain and Phase with an Integrator

Plotting the Open-loop Gain and Phase with an Integrator 

We are interested in plotting the gain and phase of the open-loop transfer function T(s). This is the product of the transfer functions of the G (plant) and H (feedback) blocks in cascade. Review the rules presented earlier in the section mathematics in the log plane.  

In particular, at lower frequencies we can clearly see that the open-loop gain is offset vertically by an amount equal to the plant gain. If the plant is a buck, then we know its 

Therefore, if we have a certain crossover frequency, fcross, in mind, this tells us the RC that the integrator section must have, to make that happen. 

However, we still have a problem. Though we can clearly set the open-loop crossover frequency almost where we want, we are still not crossing over with a —1 slope as desired. 

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