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: