Wideband Two Pole High Pass Filter

The circuit provides a 10MHz cutoff frequency. Resistor R3 ensures that the input capacitance of the amplifier does not interact with the filter response at the frequency of interest. An equivalent low pass filter is similarly obtained by capacitance and resistance transformation.

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Variable Q Filter For 400Hz

A bootstrapped twin T notch filter in this circuit can yield an effective Q of up to 10. Rs adjusts the feedback, hence the Q. Values of C1 and C2 can be changed to alter the frequency. RF is a fine tune null control.

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Variable Bandwidth Bandpass Active Filter

This circuit has adjustable bandwidth with values for a center frequency of about 800 Hz. The 10 K pot adjusts bandwidth from approximately +-350 Hz to +-140 Hz at 3 dB down points.

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Variable Bandpass Audio Filter

This circuit is a variable audio bandpass filter that has a low cutoff variable from about 25 Hz to 700 Hz and a high cutoff variable from 2.5 kHz to over 20 kHz. Rolloff is 12dB/octave on both high and low ends. R2-a-b and R6-a-b are ganged potentiometers for setting lower and upper cutoff frequencies, respectively.

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Tunable Audio Filter

This circuit uses a Wien Bridge and variable negative feedback. R7 controls the gain and R8A and R8B controls the tuned frequency.

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Three Amplifier Notch Filter

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Speech Filter (300Hz - 3kHz Bandpass)

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Simple High Pass Active Filter For 1kHz

This simple 1 kHz filter uses a voltage follower and an RC section for a filter element. For other frequencies f3 dB - 1/6.28 R1C1. The response drops 6 dB/octave below f3dB.

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Second Order Low Pass Filter For 10kHz

This circuit uses equal value capacitors. The cutoff frequency (fc) is

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Second Order 100Hz High Pass Filter

This second-order filter can be scaled to change the cutoff frequencies.

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Scratch Filter Using LM387

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Rumble Filter Using LM387

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Razor Sharp CW Filter

The circuit consists of four stages of active bandpass filtering provided by two type- uA747 integrated-circuit dual op amps and includes a simple threshold detector (diodes D1 and D2) between stages 2 and 3 to reduce low-level background noise. Each of the four filter stages acts as a narrow bandpass filter with an audio bandpass centered at 750 Hz. The actual measured 3-dB bandwidth is only 80 Hz wide.

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Multiple Feedback Bandpass Filter

The op amp is connected in the inverting mode. Resistor R3 from the output to the inverting input sets the gain and current through the frequency-determining capacitor, C1. Capacitor C2 provides feedback from the output to the junction of R1 and R2. C1 and C2 are always equal in value. Resistor R2 may be made adjustable in order to adjust the center frequency which is determined from:


When designing a filter of this type it is best to select a value for C1 and C2, keeping them equal. Typical audio filters have capacitor values from 0.01 uF to 0.1 uF which will result in reasonable values for the resistors.

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Multiple Feedback Bandpass Filter (1.0kHz)

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High Pass Active Filter

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High Q Notch Filter 2

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High Q Notch Filter

A shows a twin-T network connected to an LM102 to form a high Q, 60 Hz notch filter. The junction of R3 and C3, which is normally connected to ground, is bootstrapped to the output of the follower. Because the output of the follower is a very low impedance, neither the depth nor the frequency of the notch change; however, the Q is raised in proportion to the amount of signal fed back to R3 and C3. B shows the response of a normal twin-T and the response with the follower added.

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Full Wave Rectifier/Averaging Filter

The input signal is rectified by D1 and D2 op amp U1-a, and fed to output amp U2. R8 is set for correct circuit calibration.

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Fourth Order High Pass Butterworth Filter

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