Bias circuit and signal amplification device

The bias circuit comprises a current mirror circuit, an operational amplifier and a bias signal generating circuit. The current mirroring circuit includes a reference branch circuit and at least one mirroring branch circuit. The reference branch circuit generates a reference current according to the reference current, and the at least one mirror branch circuit generates at least one mirror current according to the reference current. The operational amplifier is coupled to the reference branch circuit and the at least one mirror branch circuit, receives the first voltage and the second voltage, and generates a control voltage according to the second voltage to adjust the first voltage. The bias signal generating circuit is coupled to the at least one mirroring branch circuit and generates a bias signal according to the at least one mirroring current. The first voltage is the voltage on the reference branch circuit, and the second voltage is the voltage on the at least one mirror branch circuit or the adjusted