Hobbies And Interests
Filters
Low-pass filters allow only frequencies below a certain level -- called the cut-off frequency -- to pass. High-pass filters only pass frequencies above their cut-off points. Band pass filters pass only frequencies in between upper and lower cut-off limits. The type of filter is determined by the configuration of the components. The frequencies allowed to pass through the filter are determined by the values of the resistors and capacitors used in the circuit.
Simple Low-Pass Filter
An op-amp has two inputs, inverting and non-inverting. A simple-low pass filter uses a resistor (R̀1;) in series with the inverting input of the op-amp and a capacitor (C) and resistor (R̀2;) connected in parallel between the inverting input and the output. The noninverting input is tied to ground (connected to the ground rail of the circuit). The cutoff for a simple low-pass filter using an op-amp is given by: f˶0; = 1/(2πR̀2;C), where f˶0; is the cut-off frequency.
Simple High-Pass Filter
You can construct a simple high-pass filter using an op-amp by connecting a capacitor (C) and a resistor (R̀1;) in series with the inverting input; and a second resistor, R̀2;, between the inverting input and the output. the noninverting input is tied to ground. The cutoff frequency (f˶0;) for a high-pass filter using an op-amp is given by: f˶0; = 1/(2πR̀1;C).
Two Op-Amp Bandpass Filters
One method of building a bandpass filter is to connect the output of a high-pass filter to the input of a low-pass filter. The high-pass filter is designed to block frequencies below your desired bandwidth and the low-pass filter is designed to block frequencies above it. Between them, the two filters will block all but your chosen frequency range. This circuit requires two op-amps.
Single Op-Amp Bandpass Filter
You can construct a bandpass filter using a single op-amp, two capacitors with equivalent values and three resistors. You will need to calculate the center frequency to find the correct component values to use. The center frequency is in the exact middle of the range of frequencies -- the band -- that you want your filter to pass. The center frequency is given by multiplying the upper frequency and the lower frequency together, and then taking the square root of the result. You can then use this value to find the values for the resistors.