Quantum key distribution (QKD) systems based on the modulation of weak-coherent states and homodyne detection has attracted much attention over last years due to its compatibility with existing telecom equipment. The coherent state required by most of the practical protocols for continuous-variable QKD can be produced in practice using a typical laser source followed by standard phase and amplitude optical modulators. On the other hand, the homodyne detector used to measure the quadratures of the modulated light field can be implemented with high-efficient PIN photodiodes. Moreover, in continuous-variable QKD systems the security of the secret key depends on the precise estimation by Bob of the loss and excess noise of the quantum channel. In this work we discuss the impact of imperfect state preparation by Alice and the role of homodyne detector imbalance on the performance of a continuous-variable QKD system. More specifically, we will consider that Alice phase and amplitude modulators are not ideal suffering from incorrect modulation calibration and optical imbalance, and that the positive and negative currents of Bob homodyne detector are not completely canceled due to a non-ideal beam-splitter and due to different quantum efficiencies of its two photodiodes.