RC FILTERS Analog Electronics IE2030

FREQUENCY

PRACTICAL USAGE OF FILTERS Filters have a wide application in electronics equipment. In hi-fi systems, filters are used to select particular bands of frequencies for amplification or attenuation as required by the listener. Also used to filter out noise, unwanted signals at frequencies resulting from equipment deficiencies or signal pickup from the surrounding medium. In communications equipment, filters are used to select the frequency band containing wanted information while rejecting unwanted frequencies.

OUTLINES OF FILTER DESIGN Filtering:  Certain desirable features are retained  Other undesirable features are suppressed

FILTERS Filter circuits are typically designed to select or reject a band of frequencies depending on the particular application. Manly two filter types; 1.Passive (using a series or parallel combination of R, L and C filter circuit design) – No need of external power 2.Active (using op-amps or transistors and R, L and C filter circuit design) – Need external power

CLASSIFICATION OF FILTERS

FILTER CATEGORIES There are four broad categories of filter – based on their frequency response: Low-pass filter This filter permits the passage of low-frequency signals while rejecting those at higher frequencies. f C is the cut-off frequency, the frequency above which the response falls below Vm/√2. fCfC

FILTER CATEGORIES High-pass filter This filter permits the passage of high-frequency signals while rejecting those at low frequencies. f c is the cut-off frequency, below which the response falls below Vm/√2. fCfC

FILTER CATEGORIES Band-pass filter This filter (also called a passband filter) permits the passage of signals in a defined frequency range, in which the response of the filter (Vout) lies aboveVm/√2, while rejecting those signals outwith this range of frequencies. The range of frequencies between f1 and f2 is termed the bandwidth.

FILTER CATEGORIES Stopband filter This filter prevents the passage of signals in a defined frequency range, in which the response of the filter (Vout) lies below Vm/√2.

FILTER CATEGORIES Allpass filter This filter passes signals of all frequencies. However, some frequency ranges are amplified while others will not.

LOGARITHMS Normally the frequency range considered is fairly large and it is thus usual to plot frequency on a logarithmic scale. Similarly, where the amplitude variations are large a logarithmic scale can be used. Alternatively, the output may be expressed in logarithmic form by using the decibel notation.

PLOT THE FREQUENCY RESPONSE DATA SHOWN USING SEMI-LOG GRAPH PAPER.

SOME IMPORTANT NOTES ABOUT LOGARITHMS

THE DECIBEL (dB) The decibel is a measure of power ratio and more usually a measurement of voltage and current ratio and is dimensionless. It is standard practice in industry to plot the frequency response of filters, amplifiers and systems in total against a decibel scale.

TERMINOLOGY IN FILTER DESIGN

RC FILTERS In combination with a resistor, a capacitor’s variation in reactance with frequency can be used to construct a simple low-pass or high-pass filter:

PASSIVE LOW-PASS FILTER

PASSIVE HIGH-PASS FILTER

PASSBAND (OR BANDPASS) FILTER There are a variety of techniques to produce a passband filter characteristic. 1.Using resonant circuit 2.using a low-pass and a high-pass filter in conjunction (highly used) A low-pass and a high-pass filter in conjunction connected together in series. The components of the two circuits are selected so that the cut-off (halfpower) frequency ω 1 of the high-pass filter is lower than the cut-off (halfpower) frequency ω 2 of the low-pass filter.

PASSBAND (OR BANDPASS) FILTER

STOPBAND (OR BANDSTOP) FILTERS A low-pass and a high-pass filter circuit may be used in parallel for this type of filter. Low frequencies, lower than the half-power frequency ω 2 of the low-pass filter, pass through the low-pass circuit. Frequencies above the half-power frequency ω 1 of the high-pass filter pass through the high-pass circuit. Frequencies between these two frequencies, ω 1 and ω 2, lie in the stopband of both filter circuits.

STOPBAND (OR BANDSTOP) FILTERS

QUESTIONS???