Bio-signals in living objects are the signals that carry physiological information from one part of the body to another. Studying bio-signals can extract data that maps health status or body activities for medical purposes. Devices for measuring different bio-signals are desired for medical applications where the devices measure the signals with further processing to provide feedback for diagnosis and treatment. Bio-signals produced by the body usually have a small amplitude requiring low noise and low power analog front-end amplifiers and signal processing circuitry. In this work, I have designed and implemented low noise and low power front-end circuitry to acquire bio-electrical signals and bio-magnetic signals as implantable devices or wearable devices. First, a neural recording amplifier with an innovative pulsing technique demonstrates an improvement in noise efficiency factor beyond that achievable using traditional design. A low NEF of 1.55 is achieved inclusive of the impacts of sampling and aliasing. Second, compared to bio-electrical recordings, bio-magnetic sensors are non-invasive and non-contact. Printed circuit board based readout electronics for strain modulated multiferroic sensors with a bandwidth of 3.4 kHz and a magnetic noise floor at 1 kHz of 98.5 pT/√Hz after demodulation is presented. The total power consumption is 440 mW. A readout circuit in 180 nm CMOS for the sensor is also presented. By utilizing a demodulator first architecture, measurements for the sensor-readout system demonstrate a 127 pT/√Hz magnetic noise floor at 1 kHz and a low power consumption of 5.9 mW. To further improve the noise performance at low frequencies, readout circuit in BiCMOS and a differential structure are implemented to achieve a magnetic noise floor of 85 pT/√Hz at 1 kHz, and 300 pT/√Hz at 10 Hz with a power consumption of 5.6 mW. In additional, a low intermediate frequency (low-IF) demodulation readout circuit is implemented and measured to eliminate the 1/f flicker noise and realizes a noise floor of 722 pT√Hz at 1 Hz with the power consumption of 2.9 mW. The noise and power consumption that the magnetic sensing systems have achieved are significantly lower than alternative magnetic sensor systems of similar volume.