filter, active low-pass

The circuit diagram shows an active-low- pass filter design which can be given any desired cut-off point - over a wide range - simply by calculating two values for four capacitors. The filter consists of an RC-network and an NPN/PNP transistor-pair. The transistor types shown can be directly replaced by many other types without spoiling the circuit`s performance. The supply voltage used should be between 6 and 12 V.