Parallel elastic actuation utilizing an elastic spring to reduce the required actuator torque is one energy-efficient strategy suitable for various mechatronic applications relating to asymmetric torque requirements. This paper presents a novel concentric drivetrain configuration of a parallel elastic actuator (PEA) embedding a torsion spring in the intermediate gearing stage of a planetary gear mechanism between the sun gear and the carrier, connected to the input shaft and the external load respectively. In addition to the input torque amplified by a gear ratio N, the output torque of the actuator is enhanced by the spring moment amplified by the ratio N-1. An alpha prototype was developed and tested with and without the controlled input torque in order to evaluate the output torque varying against the spring deformation. This proposed concept provides an alternative design solution of PEA using a low-inertia motor in combination with a low-stiffness spring. Connecting the spring arms to the carrier instead of the fixed ring gear not only allows the elastic component to support the load directly but also helps reduce the backlash between the sun gear and the planet gears through the loaded spring.