巴特沃思型滤波器最早是由英国工程师斯蒂芬·巴特沃思（Stephen Butterworth）描述的。 PI滤波器是一种具有串联元件和两个并联元件的滤波器，其连接形状为希腊字母PI。 巴特沃斯滤波器的特征是电路中频带范围内的增益恒定（响应平坦），电路中每个极点每十倍频的滚降率为20 dB。 巴特沃斯滤波器被称为最大平坦幅度响应滤波器，针对通带内的增益平坦度进行了优化。在截止频率处衰减为-3db。在截止频率以上，衰减为-20db。 巴特沃斯滤波器是唯一一种在较高阶时保持相同形状的滤波器，而其他类型的滤波器（贝塞尔、切比雪夫、椭圆）在较高阶时具有不同的形状。 高通滤波器是一种通过高频但衰减低于截止频率的频率的滤波器。 The Butterworth type filter was first described by the British engineer Stephen Butterworth.  A PI filter is a filter that has a series element and two parallel elements connected in the shape of the Greek letter PI. Butterworth filter are characterized by a constant gain (flat response) across the midband of the circuit and a 20 dB per decade roll-off rate for each pole contained in the circuit. Butterworth filters are termed maximally flat magnitude response filters, optimized for gain flatness in the pass band. The attenuation is -3db at cutoff frequency. Above the cutoff frequency the attenuation is -20 db. The Butterworth is the only filter that maintains same shape for higher orders whereas other varieties of filters (Bessel, Chebyshev, elliptic) have different shapes at higher orders.  A High pass filter is a filter that passes high frequencies, but attenuates frequencies lower than the cutoff frequency.