Custom Search


FILTERS In many practical applications of complex circuits, various combinations of direct, lowfrequency, audiofrequency, and radiofrequency currents may exist. It is frequently necessary to have a means for separating these component currents at any desired point. An electrical device for accomplishing this separation is called a FILTER. A filter circuit consists of inductance, capacitance, and resistance used singularly or in combination, depending upon the purpose. It may be designed so that it will separate alternating current from direct current, or so that it will separate alternating current of one frequency (or a band of frequencies) from other alternating currents of different frequencies. The use of resistance by itself in filter circuits does not provide any filtering action, because it opposes the flow of any current regardless of its frequency. What it does, when connected in series or parallel with an inductor or capacitor, is to decrease the "sharpness," or selectivity, of the filter. Hence, in some particular application, resistance might be used in conjunction with inductance or capacitance to provide filtering action over a wider band of frequencies. Filter circuits may be divided into four general types: LOWPASS, HIGHPASS, BANDPASS, AND BANDREJECT filters. Electronic circuits often have currents of different frequencies. The reason is that a source produces current with the same frequency as the applied voltage. As an example, the a.c. signal input to an audio amplifier can have high and lowaudio frequencies; the input to an rf amplifier can have a wide range of radio frequencies. In such applications where the current has different frequency components, it is usually necessary for the filter either to accept or reject one frequency or a group of frequencies. The electronic filter that can pass on the higherfrequency components to a load or to the next circuit is known as a HIGHPASS filter. A LOWPASS filter can be used to pass on lowerfrequency components. Before discussing filters further, we will review and apply some basic principles of the frequencyresponse characteristics of the capacitor and the inductor. Recall the basic formula for capacitive reactance and inductive reactance:
Assume any given value of L and C. If we increase the applied frequency, X_{C} decreases and X_{L} increases. If we increase the frequency enough, the capacitor acts as a short and the inductor acts as an open. Of course, the opposite is also true. Decreasing frequency causes X_{C} to increase and X_{L} to decrease. Here again, if we make a large enough change, X_{C} acts as an open and X_{L} acts as a short. Figure 113 gives a pictorial representation of these two basic components and how they respond to low and high frequencies. Figure 113.  Effect of frequency on capacitive and inductive reactance.
If we apply these same principles to simple circuits, such as the ones in figure 114, they affect input signals as shown. For example, in view (A) of the figure, a low frequency is blocked by the capacitor which acts as an open and at a high frequency the capacitor acts as a short. By studying the figure, it is easy to see how the various components will react in different configurations with a change in frequency. Figure 114.  Reaction to circuit by change in frequency.
As mentioned before, highpass and lowpass filters pass the specific frequencies for which circuits are designed. There can be a great deal of confusion when talking about highpass, lowpass, discrimination, attenuation, and frequency cutoff, unless the terms are clearly understood. Since these terms are used widely throughout electronics texts and References, you should have a clear understanding before proceeding further. HIGHPASS FILTER. A highpass filter passes on a majority of the high frequencies to the next circuit and rejects or attenuates the lower frequencies. Sometimes it is called a lowfrequency discriminator or lowfrequency attenuator. LOWPASS FILTER. A lowpass filter passes on a majority of the low frequencies to the next circuit and rejects or attenuates the higher frequencies. Sometimes it is called a highfrequency discriminator or highfrequency attenuator. DISCRIMINATION. The ability of the filter circuit to distinguish between high and low frequencies and to eliminate or reject the unwanted frequencies. ATTENUATION. The ability of the filter circuit to reduce the amplitude of the unwanted frequencies below the level of the desired output frequency. FREQUENCY CUTOFF (f_{co}). The frequency at which the filter circuit changes from the point of rejecting the unwanted frequencies to the point of passing the desired frequency; OR the point at which the filter circuit changes from the point of passing the desired frequency to the point of rejecting the undesired frequencies. LOWPASS FILTER A lowpass filter passes all currents having a frequency below a specified frequency, while opposing all currents having a frequency above this specified frequency. This action is illustrated in its ideal form in view (A) of figure 115. At frequency cutoff, known as f_{co} the current decreases from maximum to zero. At all frequencies above f_{co} the filter presents infinite opposition and there is no current. However, this sharp division between no opposition and full opposition is impossible to attain. A more practical graph of the current is shown in view (B), where the filter gradually builds up opposition as the cutoff frequency (f_{ co}) is approached. Notice that the filter cannot completely block current above the cutoff frequency. Figure 115A.  Lowpass filter.
Figure 115B.  Lowpass filter.
View (A) of figure 116 shows the electrical construction of a lowpass filter with an inductor inserted in series with one side of a line carrying both low and high frequencies. The opposition offered by the reactance will be small at the lower frequencies and great at the higher frequencies. In order to divert the undesired high frequencies back to the source, a capacitor must be added across the line to bypass the higher frequencies around the load, as shown in view (B). Figure 116A.  Components of a simple lowpass filter.
Figure 116B.  Components of a simple lowpass filter.
Figure 116C.  Components of a simple lowpass filter.
The capacitance of the capacitor must be such that its reactance will offer little opposition to frequencies above a definite value, and great opposition to frequencies below this value. By combining the series inductance and bypass capacitance, as shown in view (C), the simplest type of lowpass filter is obtained. At point P, a much higher opposition is offered to the low frequencies by the capacitor than by the inductor, and most of the lowfrequency current takes the path of least opposition. On the other hand, the least amount of opposition is offered to the high frequencies by the capacitor, and most of the highfrequency energy returns to the source through the capacitor. 
Integrated Publishing, Inc. 