Voltage controlled filter - using the super OTA 13600

7-84 — elektor july/august 1982 Since their introduction in the early 70"s OTAs have become a classical component for voltage controlled filters. This is especially true of the dual OTA XR 13600 because it already contains the necessary buffer stages. The dual version has an excellent synchronous operation and is ideal for second order filters. The circuit diagram here shows a low-pass filter of this type. A modulation range over several decades can be obtained, with a good linearity. The 3 dB cutoff frequency of the filter depends on the transconductance (gm) of the OTAs, and on the values of the resistors R and RA and the capacitors C and 2 • C. The value of fg can be calculated from the following: RC must be multiplied by two, as the The next question is: How do we know the gm value? This is really quite simple; at room temperature the gm = 19.2 • IB, where IB is the current that flows into pins 1 and 16 of the IC (across RC). The voltage at these pins is approximately 1....