Operational amplifier high pass filter

-a summary and overview for the design of an operational amplifier or op-amp active high pass filter with ciruit details.

Op-amp circuits include:
    •  Operational amplifier circuits
    •  Inverting op-amp
    •  Non-inverting op-amp
    •  Op-amp high pass filter
    •  Op-amp low pass filter
    •  Op-amp bandpass filter
    •  Op-amp variable gain amplifier
    •  Op-amp notch filter circuits
    •  Operational amplifier multivibrator
    •  Op-amp bistable
    •  Op-amp comparator
    •  Op-amp Schmitt trigger
    •  Op-amp integrator
    •  Op-amp differentiator

Operational amplifiers lend themselves to being used for active filter circuits, including a high pass filter circuit. Using a few components they are able to provide high levels of performance.

The simplest circuit high pass filter circuit using an operational amplifier can be achieved by placing a capacitor in series with one of the resistors in the amplifier circuit as shown. The capacitor reactance increases as the frequency falls, and as a result this forms a CR low pass filter providing a roll off of 6 dB per octave. The cut off frequency or break point of the filter can be calculated very easily by working out the frequency at which the reactance of the capacitor equals the resistance of the resistor. This can be achieved using the formula:

where:
  Xc is the capacitive reactance in ohms
  pi is the greek letter and equal to 3.142
  f is the frequency in Hertz
  C is the capacitance in Farads

Two pole low pass filter

Although it is possible to design a wide variety of filters with different levels of gain and different roll off patterns using operational amplifiers, the filter described on this page will give a good sure-fire solution. It offers unity gain and a Butterworth response (the flattest response in band, but not the fastest to achieve ultimate roll off out of band).

The calculations for the circuit values are very straightforward for the Butterworth response and unity gain scenario. Critical damping is required for the circuit and the ratio of the resistor vales determines this.

When choosing the values, ensure that the resistor values fall in the region between 10 k ohms and 100 k ohms. This is advisable because the output impedance of the circuit rises with increasing frequency and values outside this region may affect he performance.

By Ian Poole

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