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List of Schematics - Low-pass Active Filters

This circuit uses a switched capacitor filter IC from National Semiconductor to filter signals with frequencies higher than the 3KHz needed for voice audio. The schematic includes an audio amplifier that is designed to drive a standard audio head phone. The circuit is described in more detail in the receiver section of Dave Johnson's Handbook of Optical Through the Air Communications.
(this link is off-site)
This circuit is the audio amp section for a complete optical transmitter. The circuit amplifies and filters the voice audio signals from an electrets microphone. The circuit is described in more detail in the transmitter circuit section of Dave Johnson's Handbook of Optical Through the Air Communications. (this link is off-site)
This schematic is the power supply and front-end sections of the field monitor that is discussed in more detail at Electric Field Disturbance Monitor. The system can detect human and animal motion by the electric fields they disturb.
This is a collection of inverting and non-inverting active low pass filter circuits.  I included one, two, three, four and six pole filter circuits.  You can change the component value ratios shown, to achieve any frequency cut-off you may need.  The circuit does not specify an operational amplifier.  The circuits should be used to select the needed resistor and capacitor components for a particular frequency knee and roll-off slope.
Active 2nd Order Filters:
Active Filter has Wideband Tuning Range: EDN-Design Ideas
Adjustable Filter Provides Lowpass Response: EDN-Design Ideas  PDF contains multiple circuits - scroll to find the one of interest
Basic Introduction to Filters: application notes on active, passive and switched capacitor filters, document in PDF format
Butterworth Filters:
Chebyshe Volt / Butterworth Filters:
Chebyshe Volt Bandpass Filter:
DC Accurate, Active RC Lowpass Filter:
Low Pass Active Filter:
Lowpass filter discriminates step input from noise :  EDN-Design Ideas /  Numerous applications exist in industry, particularly with control systems, in which it is desirable to remove all but the lowest frequency components from a signal to effectively yield a dc voltage. This voltage may, for example, serve as a setpoint to a PID controller in a process-control or an HVAC application, in which the cable that is carrying the analog signal is exposed to a wide spectr......
Lowpass filter uses only two values: EDN - Design Ideas /  In recent years, image-parameter design of LC filters has received new consideration (references 1 and 2). The composite lowpass filter uses interior constant-k full sections terminated by m-derived half-sections. For best passband response, you usually select m to equal 0.6. However, m=0.5 can still give useful filter performance while reducing the number of component values..
Single resistor tunes lowpass filter :  EDN-Design Ideas / Any tunable, second-order, active RC-filter section requires at least two thoroughly matched variable resistors. But the lowpass implementation in Figure 1 provides for wide-range cutoff-frequency control using only a single variable resistor, R. In addition to the resistor, this filter comprises an operational amplifier, IC2, which serves as a unity-gain buffer; two capacitors, C1 and C2; ......
Single Supply Twin T Butterworth Low Pass Filter:

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