Filters Passive

The ciliary body, situated posterior to the iris, performs several functions. It connects the anterior part of the choroid to the circumference of the iris, and contains the ciliary muscles necessary for accommodation. The ciliary body secretes aqueous humor into the posterior together with nutrients to nourish the lens. In the ciliary body, melanin is located only in the outer pigmented epithelium [24]. Aqueous humor is actively secreted and passively filtered by the ciliary body. Although the rate of secretion is about 2 pL/min the same volume is drawn-off via Schlemm s canal from where it is conducted into veins [25]. 

Harmonic filters are broadly classified into passive and active filters. Passive filters, as the name implies, use passive components such as resistors, inductors, and capacitors. A combination of passive components is tuned to the harmonic frequency that is to be filtered. Figure 4.22 is a typical series-tuned filter. Here the values of the inductor and the capacitor are chosen to present a low impedance to the harmonic frequency that is to be filtered out. Due to the lower impedance of the filter in comparison to the impedance of the source, the harmonic frequency current will circulate between the load and the filter. This keeps the harmonic current of the desired frequency away from the source and other loads in the power system. If other harmonic frequencies are to be filtered out, additional tuned filters are applied in parallel. Applications such as arc furnaces require multiple harmonic filters, as they generate large quantities of harmonic currents at several frequencies. 

Active filters use active conditioning to compensate for harmonic currents in a power system. Figure 4.23 shows an active filter applied in a harmonic environment. The filter samples the distorted current and, using power electronic switching devices, draws a current from the source of such magnitude, frequency composition, and phase shift to cancel the harmonics in the load. The result is that the current drawn from the source is free of harmonics. An advantage of active filters over passive filters is that the active filters can respond to changing load and harmonic conditions, whereas passive filters are fixed in their harmonic response. As we saw earlier, application of passive filters requires careful analysis. Active filters have no serious ill effects associated with them. However, active filters are expensive and not suited for application in small facilities. 

Passive filter A kind of usually simple filters composed of elements such as resistors (R), capacitors (C), and inductors (L) that do not depend upon an external power supply. There are different passive filters such as the so-called RC, RL, LC, and RLC varieties. Inductors block high-frequency signals and conduct low-frequency signals, while capacitors do the reverse. Resistors have no frequency-selective properties, but are added to inductors and capacitors to determine the time-constant of the circuit. 

However, by using higher order filter response can be improved further. The loss in the passive filter is expected to be in limit. Otherwise also an amplifier following the filter can make up the thing. It is expected that the loss in the passive filter will be affordable as the mixer has an in-built amplifier. The cutoff frequency is around IMHz. 

With whole-cell configuration, the series resistance plays an important role in the control of the voltage of the cell membrane. The series resistance problem has been described for the first time by Hodgkin, Huxley and Katz [31]. It produces a double effect on the voltage-clamp A passive filtering of the voltage pulses applied to the membrane, and a drop of the applied membrane potential when a current is flowing... 

A filter is a multiport network designed specifically to respond differently to signals of different frequency. This definition excludes networks which behave as filters incidentally, sometimes to the detriment of then-main purpose. Passive filters are constructed exclusively with passive elements (i.e., resistors, inductors, and capacitors). 

Two-port filter networks. The exclusion of some passive filters with multiple outputs is not as significant here as it would be in a discussion of active filters. 

Low-order filters. The unavoidable insertion loss associated with a passive filter increases with the number of elements in the filter. Consequently, it is usually more practical to use active filters, which can be designed to offset insertion loss (or even to provide insertion gain) for applications requiring higher than a third- or fourth-order transfer function. 

Passive filter An electrical network comprised entirely of passive elements and designed specifically to... 

The textbook by D.E. Johnson, Introduction to Filter Theory, mentioned in the references, is recommended as a compact but thorough seminal reference for the design of lumped-parameter passive filters. In addition, this subject has been the primary focus of a number of other textbooks and is included in virtually every basic circuit theory textbook. 

Cavities, combiners, and duplexers are all selective-tuned filters. They are composed of three types of passive filters ... 

All the filters described above are passive filters, which means that they have no amplifying modules. Only resistors and capacitors were used. Other inductive circuits are similar but are not commonly used, because, as in the case of low-pass filters, in the region of 10 Hz and lower, the inductances must be high and therefore the required coils would need to have relatively large dimensions. Some examples are given in Figs. 3-21, 3-22, 3-23. 

In the previous Section 3.5.4.1 we learned that capacitors and inductors (coils) are frequency-dependent elements. For low frequencies, it is easier to pass inductors, and for high frequencies, to pass capacitors. RC filtershave the disadvantage that for DC sieving a drop in potential occurs. Therefore it is suitable to combine passive filters with amplifiers. Nowadays, this idea can be easily realized with IC s (Fig. 3-25). 

In the circuit, the analogue stage replaces the passive filter, which is often required by switching amplifiers. That is, it performs the tasks of an analogue filter at the power level. The ripple of the output signal of the switching amplifier can be dampened by more than 20 dB. A passive coil filter usually... 

From Harrison, C., Passive filters, in The Electronics Handbook, Whitaker, J.C., Ed., CRC Press, Boca Raton, FL, 1997, p. 282. 

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