• 3KHz LOW PASS FILTER PLUS AUDIO AMP
  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)

• AUDIO AMP + 3KHz FILTER
  This circuit is the audio amp section for a complete optical transmitter. The circuit amplifies and filters the voice audio signals from an electret 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)

• ELECTRIC FIELD DISTURBANCE MONITOR
  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.

• LOW PASS ACTIVE FILTER DESIGN COLLECTION
  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.

1MV Offset Clock-Tunable Monolithic 5-Pole Lowpass Filter:   DN67  Design Notes (Linear Technology) (app note added 6/06)

Akerberg-Mossberg AM Second Order Lowpass inverting:   (electronic design added 6/07)

Applications for a DC Accurate Lowpass Switched-Capacitor Filter:   AN20 Linear Technology Discusses  principles of operation of  LTC1062 and helpful hints for its application. Various application circuits are explained in detail with focus on how to cascade two LTC1062s and how to obtain notches. Noise and distortion performance are fully illustrated.

Bach Second Order Lowpass non-inverting:   (electronic design added 6/07)

Basic Introduction to Filters:   application notes on active, passive and switched capacitor filters, document in PDF format (app note added 4/02)

Berka-Herpy BH Second Order Lowpass non-inverting:   (electronic design added 6/07)

Centronics Port DA Converters and Lowpass Filter:   (circuit / schematic design added 6/06)

Chebyshe Volt/Butterworth Filters:   (electronic circuit added 7/03)

Chopper Amplifiers Complement a DC Accurate Low-Pass Filter:   DN9  Design Notes (Linear Technology) (app note added 1/06)

First Order Lowpass I non-inverting:   (electronic design added 6/07)

First Order Lowpass II non-inverting:   (electronic design added 6/07)

Fliege Second Order Lowpass non-inverting:   (electronic design added 6/07)

KHN Inverting Input Second Order Lowpass inverting:   (electronic design added 6/07)

KHN Non-Inverting Input Second Order Lowpass non-inverting:   (electronic design added 6/07)

Low-Pass Filter RFI Filter:   Circuit Ideas for Designers Application Notes Advanced Linear Devices, Inc.  (app note added 6/06)

Lowpass, 30-kHz Bessel filter offers high performance for audio applications:   2/16/2006    EDN Design Ideas  / (added 05/07)    Careful selection of amplifiers and components helps achieve the lowest levels of noise and distortion for higher-order filter.

Low-Sensitivity, Lowpass Filter Design:   National Semiconductor Application Note   (app note added 2/06)

LTC1560-1: Tiny 1MHz Lowpass Filter uses No Inductors:   DN169  Design Notes (Linear Technology) (app note added 1/06)

Mikhael-Bhattacharyya MB Second Order Lowpass non-inverting:   (electronic design added 6/07)

Multiple Feedback MFB Second Order Lowpass I inverting:   (electronic design added 6/07)

Multiple Feedback MFB Second Order Lowpass II inverting:   (electronic design added 6/07)

OA-27: OA-27 Low-Sensitivity, Lowpass Filter Design:   National Semiconductor Application Note   (app note added 2/06)

Replace Discrete Lowpass Filters with the LTC1563 Zero Design Effort Two Item Bom and No Surprises:   DN251  Design Notes (Linear Technology) (app note added 1/06)

Sallen-Key SK Second order Lowpass I non-inverting:   (electronic design added 6/07)

Sallen-Key SK Second order Lowpass II non-inverting:   (electronic design added 6/07)

SSB AF Filter:   This is a simple filter that restricts the LF response a little as well providing quite a heavy HF roll-off. The prototype has enhanced a Yaesu FT101B that was only fitted with an AM IF filter. The IC is an LM358. (added 02/05)

Tow-Thomas TT Second Order Lowpass non-inverting:   (electronic design added 6/07)

Twin-T Second Order Lowpass non-inverting:   (electronic design added 6/07)

Unique Applications for the LTC1062 Lowpass Filter:   AN24 Linear Technology Highlights the LTC1062 as a lowpass filter in a phase lock loop. Describes how the loop's bandwidth can be increased and the VCO output jitter reduced when the LTC1062 is the loop filter. Compares it with a passive RC loop filter. Also discussed is the use of LTC1062 as simple bandpass and band stop filter.

Active Filter has Wideband Tuning Range:   03/28/96 EDN-Design Ideas / (added 4/02)  

First Order Lowpass III inverting:   (electronic design added 6/07)

Adjustable Filter Provides Lowpass Response:   03/01/01 EDN-Design Ideas / (added 11/05)

Single Resistor Tunes Lowpass Filter:   08/21/03 EDN Design Ideas  /  (added 12/04)  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; ......

Lowpass Filter Discriminates Step Input from Noise:   09/18/03 EDN Design Ideas  /  (added 12/04)  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 spectra......

Lowpass Filter has Improved Step Response:   02/19/04 EDN Design Ideas  /  (added 12/04)  A common problem that arises when you design lowpass filters for signal conditioning is the filters' effect on the system's time-domain response. Because pushing the cutoff frequency lower slows the step response, the system may fail to recognize significant changes within a reasonable amount of time....

Precision Level Shifter has Excellent CMRR:   04/15/04 EDN Design Ideas  /  (added 1/05)  Most designers make level shifters with op amps and 1%-tolerance discrete resistors. Discrete-resistor mismatching limits the op amp's CMMR (common-mode rejection ratio) to 40 dB, so you cannot use op amps in circuits that require high CMRR. Differential amplifiers contain precision matched internal resistors, so ICs such as the INA133 can readily achieve CMRRs of approximately 90 dB....

Brick-wall lowpass audio filter needs no tuning:   9/14/2006    EDN Design Ideas  / (added 05/07)    Gyrator supercapacitors eliminate the need for inductors in this 15-kHz filter.

Brick-wall lowpass audio filter needs no tuning:   9/14/06  EDN Design Ideas  /  (added 10/06)  Gyrator supercapacitors eliminate  need for inductors in this 15-kHz filter.

Lowpass Filter uses Only Two Values:   07)/05/01 EDN Design Ideas  /  (added 12/04)  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....

Active 2nd Order Filters:   (electronic circuit added 7/03)

Butterworth Filters:   (electronic circuit added 7/03)

Chebyshe Volt Bandpass Filter:   (electronic circuit added 7/03)

Low Pass Active Filter:   (electronic circuit added 7/03)