The audio spectrum range of human speech is most frequently on 300 Hz to 3 kHz. In phone, this range should be limited. Usually, DSP chip is used to perform this limitation but to reject high-frequency components, it requires an anti-aliasing filter. We can solve this problem by using a single-supply phone speech filter circuit. Here is the circuit :
This circuit consist of four resistors, two op-amps and five capacitors. This circuit is designed to be compact and low-power. It also scalable for even lower power consumption. You can use a passive (resistors) voltage divider for VCC/2 or you can use an active virtual ground circuit. [Source: Analog Application Journal]
