A bias-current-control circuit is provided. The bias-current-control circuit includes a transconductance circuit, a constant-current source, and a current-mirror circuit. The transconductance circuit is connected to a node and detects a voltage signal to generate a first current. The constant-current source is connected to the node and generates a tail current. The current-mirror circuit includes a reference current terminal and a bias current terminal, and the reference current terminal is coupled to the node. A second current which flows through the reference current terminal is determined by a current difference between the tail current and the first current. A bias current which flows through the bias current terminal is generated based on the second current. Furthermore, the second current and the bias current are in a predetermined ratio.