Electronic Devices for Analog Signal Processing by Yu. K. Rybin

Author:Yu. K. Rybin
Language: eng
Format: epub
Publisher: Springer Netherlands, Dordrecht

However, it is unclear how these conditions can be understood, because they are nonsense for a closed self-oscillating system. Actually, if we consider Fig. 5.2d, where, say, LFDE is a well-known Wien bridge with the gain at the frequency of self-oscillations , and ANE is an amplifier with negative feedback and the gain then these conditions would take the more specific form

Check is it possible to satisfy these conditions in a closed-loop system. Let at some time the voltage at the amplifier input is 1 V. Pass on the feedback loop through the amplifier and the Wien bridge, this voltage becomes equal to 1.1 V at the same time. But 1.1 V is not equal to 1 V. What is the matter? The matter is that in the closed self-oscillating system the loop gain (if this concept can be applied here) is equal to 1 in both the steady-state mode and the excitation mode. Because the left-hand side of Eq. 5.7 it is always equal to the right-hand side. Then what is the amplifier gain or what is the gain of the Wien bridge during excitation? The oscillations increase in spite of these questions, but at what frequency? Obviously, it is not the frequency , since the gain at this frequency must be 3.3. Also it is absolutely unclear at what gain the oscillations are damped, when the loop gain, on one hand, must be less than 1, but on the other hand, it is equal to 1.

Thus the Barkhausen criterion and the amplitude balance and phase balance conditions have limited applicability only to the steady-state mode, but even in this case it is not always possible to determine K fb , for example, in the positive feedback in the amplifier. It is appropriate noting here that it is unclear what these conditions are at other frequencies. And, finally, these conditions are inapplicable to the oscillating systems generating non-sine-wave oscillations.

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