The course concentrates on classical Feedback with elements of modern Control theory.

A control systems is a system that regulates an output variable with the objective of producing a given relationship between in and an input variable, or of maintaining the output at a fixed value. In a feedback control system, at least part of the informations used to change the output variable is derived from measurements performed on the output variable itself. This is knows as closed-loop control and is often preferred to the open-loop control systems (where the output-variable is not used to influence the system). Closed-loop systems that use feedback have lower sensitivity to externally applied disturbances and changes in system parameters.

The principal advantage of feedback is a reduction in sensitivity of the system to changes in gain of certain elements. This reduction in sensitivity is obtained only in exchange for an increase in the magnitude of the gain of one or more of the elements in the system. In some cases it is also possible to reduce the effects of disturbances applied to the system. However, this could be achieved without a feedback solution (though feedback is almost always a better practical choice).

Because feedback reduces the sensitivity of the system to changes in open-loop gain, it can often moderate the effects of nonlinearities. The performance of the system can be determined by recognising that, since the nonlinear element is piecewise linear, all transfer relationships must be piecewise linear.

Feedback provides a mechanism to reduce the sensitivity of the system to certain kinds of disturbances. Three different sources of disturbances are applied to the system below. The disturbance V_d1 enters the system at the same point as the system input, and might represent the noise associated with he input stage of an amplifier. Disturbance V_d2 enters the system at an intermediate point, and might represent the changing load characteristics.

Therefore, we can derive the dependency between the output voltage, input and disturbances is as follows:

As could be seen from equation above, the disturbance V_d1 is not attenuated relative to the input signal. This, of course, is expected since V_i and V_d1 enter the system at the same point. Furthermore, this reflects the fact that feedback cannot improve quantities such as the noise figure of an amplifier.

How does this relate to the application of closed-loop control to a distributed system?

The disturbances that enter the amplifier at other points are attenuated relative to the input signal by amounts equal to the forward-path gains between the input and the points where the disturbances are applied.