Adjustable AF Notch Filters

Add two op-amps to a Wien-Robinson bridge and we can construct a notch filter whose centre frequency can be set using a single resistor. This is a significant advantage when compared with the commonly-used double-T network.The notch filter described in this article employs the ST Microelectronics type TS925 quad operational amplifier. This part offers rail-to-rail (ground to US) operation on both inputs and outputs and has the additional advantage that an internal voltage divider and buffer IC1.E generate a virtual ground at a potential US / 2 (Figure 1).The circuit of Figure 2 shows a modified Wien-Robinson bridge with resonant frequency fo. IC1.B, together with C2, C4, R3, R6, R7 and R8 realise the complex branch of the bridge, while R1+R2, R5 and IC1.C form the real branch.