Low-gain feedback has found several applications in constrained control, robust control, and nonlinear control. In this paper, we first generalize the existing low-gain design methods by introducing the notion of L-infinity-vanishment and by providing a full characterization of feedback gains that achieve such a property. We observe that L-infinity low-gain feedback can lead to energy peaking, namely, the control energy required by L-infinity low-gain feedback increases toward infinity as the low-gain parameter decreases to zero. Motivated by this observation, we consider the notion of L-2-vanishment and establish several of its characterizations, based on which a new design approach referred to as the L-2 low-gain feedback approach for linear systems is developed. Different from the L-infinity low-gain feedback, the L-2 low-gain feedback is instrumental in the control of systems with control energy constraints. As an application of L-2 low-gain feedback, the problem of semiglobal stabilization of linear systems with control energy constraints is solved in this paper. The notions of L-infinity and L-2-vanishment also allow us to establish a systematic approach to the design of L-infinity and L-2 low-gain feedback. The advantage of this new design approach is that it results in a family of control laws, including those resulting from the existing design methods.