An apparatus is provided for monitoring heart rate. The apparatus comprises various components to effectively reduce photodiode capacitance. The apparatus includes: an amplifier； a first current source； a first pair of resistors coupled to the first current source and the amplifier； a pair of devices coupled to the first pair of resistors； a photo-diode coupled to the pair of devices； a second pair of resistors coupled to the pair of devices and the photo-diode； and a second current source coupled to the second pair of resistors.
