Positive feed-back

Here Kc = 0 represents open loop conditions, Kc < 0 represents positive feed back conditions, and Kc > 0 represents conventional negative feedback control. 

Nurse This is the issue does advancing mitosis — that is, by elevating String levels — positively feed back on increasing growth rate ... 

One of the present authors proposed a model for superoutbursts and superhumps of SU UMa stars [1], In this model, the normal outbursts of SU UMa stars are thought to be essentially the same as the ordinary outbursts of dwarf nova and they are supposed to be caused by the disk instability (see e.g., Smak [2]). Superoutbursts are explained in the following way. The heating of the secondary s atmosphere by strong far UV and soft X-ray radiation due to accretion and the resulting increase in mass-overflow rate from the secondary may lead to a positive feed back instability between accretion and mass overflow in a certain circumstance. This "irradiation-induced mass-overflow instability" is the suggested mechanism of "superoutburst" of SU UMa stars. 

The bifurcation diagram represents an imperfect pitchfork diagram, where the middle steady state persists over the entire range of Kc, even for negative values of Kc, i.e., even for positive feed back control, which would destabilize the system. 

Of course the thyroid does not simply produce T-4 continuously. This is due to the "checks and balances" nature included called "feedback-mechanisms". In the care of thyroid function the feedback-mechanism or loop involves the hypothalamus (secretes TRH), pituitary gland (secretes TSH), thyroid gland (secretes T-4), and the liver (converts T-4 into T-3). A feedback-mechanism or loop can trigger the release of another hormone (positive feed-back), or inhibit its release (negative feed-back) thus maintaining that balance. This means high levels of T-4 or T-3 initiate a negative feed-back loop that tells the hypothalamus to produce less TRH, and low levels of T-4 or T-3 initiate a positive feed-back loop that tells the hypothalamus to produce more TRH. 

Protein kinase A can phosphorylate phosducin [33]. This phosphorylation markedly inhibits the effects of phosducin on G proteins [28, 34]. Since phosducin inhibits the function of Gg, and protein kinase A prevents this inhibition, Gg, adenylyl cyclase, protein kinase A and phosducin form a positive feed-back loop. The biological implication of such positive feed-back are as yet not understood. 

The leitmotif of this chapter that biological concepts beget synthetic drug discoveries which, in turn, give rise to new biological discoveries and concepts and that these exert a positive feed-back on the creation of new structural drug prototypes, is shown schematically in Figure 1. 

The method of positive feed-back has been suggested by Lauer and Osteryoung [64] and is commonly utilized in most commercial equipments. This method can be employed using either the four-wire or the three-wire technique, as it permits compensation for any type of resistive load between the working and reference electrodes. A schematic diagram is given in figure 10. 

The mode of application of this method need not be described in this article, since it is thoroughly illustrated in many user s manuals of commercial potentiostats. It should be recalled, however, that when this technique is adopted use must be made of an oscilloscope and a function generator because for an optimum choice of the feed-back amplitude it is necessary that the wave shape of the current peak be strictly controlled. It may be added that the use of the Solartron mod. 1286 electrochemical interface makes it possible also to choose the amount of the positive feed-back numerically. 

However, ohmic drop compensation by positive feed-back has some disadvantages it can only he used in the potentiostatic mode and it requires the use of a fixed current range. This is because, in order to reduce the effect of the presence of R, on the polarization potential of the working electrode, a voltage signal (E/) coming from the output of the... 

Figure 10. Schematic diagram of ohmic drop compensation by the positive feed-back technique in potentiostatic mode.

Thus compensation for the resistance ii, is achieved by increasing the absolute value of the actual polarization potential. As a result, the choice of the current range cannot be optimized because any change, at a fixed current intensity, after the amplitude of the positive feed-back has been optimized involves a variation of the potential E/. 

An analogue display system Incorporating A and B scans was chosen, the B scan drawn on an oscilloscope was scaled using positional feed back from the manipulators and Information from the A scans on the flaw detector. A composite B scan could be drawn for each scan sequence (Fig 4) and erased after each complete cycle. Using video equipment the A and B scans were then mixed on to a monitor with digital Information Inlaid through a number Inlay unit. This Information was then recorded on video tape. 

The hydrodynamic instability, where a small increase in die pressure leads to a larger local residence time, which in turn, through conversion, results in a larger viscosity. This viscosity increase will successively increase the die pressure even further. The positive feedback will be counter balanced by the back flow, because an increased viscosity also increases the pressure built up ability of the extruder. An influence on the stability may be expected if the interaction parameters and the local viscosities are such that the positive feed back dominates. 

One word of warning the evaluation of kinetic parameters as described above is correct only for the treatment of AC polarograms which are free from the influence of the solution resistance (usually this is assured by the positive feed-back loop of a potentiostat). The iR drop in AC techniques is a vector that causes severe phase errors and the reader is referred to details in the review by Sluyters and co-workers [15]. The consequence of the phase error is that the apparent estimated... 

It can easily be shown that if there is a positive feed-back it is necessary to have fast control, and if there is a fast control the system responds quickly to any variation, and in particular to the noise. No clear idea of the effect of noise in such a reactor seems to be available. It is one of the reasons which make it difficult to fix a practical limit to the positive void coefficient, but qualitatively it is possible that a problem exists. 

Another important question is the importance of correcting for the uncompensated Ohmic resistance in experiments of the ORR. This problem is not new, but it is often disregarded. It was elaborated by van der Vliet et al. [68] who showed that if one omits to correct measured current for IR drop erroneous values of specific activity and E a is obtained. This is a consequence of the fact that no matter how close Lugin capillary is to the working electrode, it is difficult to reduce electrolyte resistance below 10 Q. For measured ORR current of approx. 1 mA this would shift electrode potential by 10 mV. For ORR Tafel slope of 120 mV dec", this potential shift induces approx. 25% underestimated ORR activity (expressed as A,esa or 7k,mass a d corrected for mass transfer) and also incorrect value of Tafel slope. For these purposes, positive feed-back scheme is proposed [52]. 

By contrast, the Calvin cycle in photosynthesis illustrates a positive feed-back mechanism. The concentration of Calvin cycle intermediates persisting in photosynthetic cells in the dark is very low. When photosynthesis commences the availability of ribulose diphosphate (RuDP) may limit the rate of COj assimilation. However, the formation of phosphoglyceric acid (PGA) immediately raises the level of other intermediates in the cycle, including that of RuDP, and this in turn speeds up the rate of CO assimilation. Similarly, when cells... 

---