Simple Programmable Bandpass using twin-t bridge

The Simple Programmable Bandpass using twin-t bridge circuit gives a programmable bandpass where both the cut-over frequency and the gain, A, are controlled independently. In the twin-T bridge the resistors R and R/2 are replaced by two double FETs, 430, the channel resistance of the first one in the series, the channel resistances of the second one are in parallel as to stimulate the resistance R/2. Both these resistors are controlled by Vc which ranges from 0 V to about 1 V. The gain of the circuit is set by means of the resistors R2 and R3. 

Simple Programmable Bandpass using twin-t bridge Circuit Diagram