ELEC 202 Circuit Analysis II ANALOG FILTERS ELEC 202 Circuit Analysis II

Definition A frequency-selective device or circuit designed to pass signals with desired frequencies and reject or attenuate signals with unwanted frequencies Limit the frequency spectrum of a signal to some specified band of frequencies Applications in communications and control systems

Types of Filters Lowpass – passes low frequencies and stop high frequencies Highpass – passes high frequencies and rejects low frequencies Bandpass – passes frequencies within a certain band and blocks frequencies outside the band Bandstop – passes frequencies outside a certain band and blocks frequencies within the band

Lowpass Filter Highpass Filter Bandpass Filter Bandstop Filter

Passive vs. Active Filters A passive filter consists of only passive elements (e.g., R, L, and C). An active filter consists of active elements (e.g., transistors and op amps) in addition to passive elements. 5

Cutoff Frequency The frequency at which the frequency response drops in magnitude to 70.71% (or 3dB) of its maximum value. Or, the frequency at which the output power of the filter is half of the maximum input power  half-power frequency Also called corner frequency or roll-off frequency Obtained by setting the magnitude of to 6

Lowpass Filter Designed to pass only frequencies from dc up to the cutoff frequency.

Highpass Filter Designed to pass all frequencies above its cutoff frequency.

Example What type of passive filter does the following circuit represent? Also, calculate its cutoff frequency.

Example For the circuit shown, identify the type of filter it represent by obtaining and calculate its corner frequency.

Bandpass Filter Designed to pass all frequencies within a certain Band of frequencies.

Bandpass Filter

Bandstop Filter Designed to stop all frequencies within a certain band of frequencies.

Bandstop Filter

Narrowband vs. Wideband Rule of thumb: If the 3-dB bandwidth of a bandpass filter is more than twice the center frequency, the filter is said to be wideband. Examples of narrowband filters: Resonator, Comb filters, notch filters, inverse comb filters 15