A programmable gain amplifier with a large extent for the variation of gains made by a manufacturing process of compensated metal oxide semiconductor or a bipolar junction transistor is disclosed. The structure of the programmable gain amplifier is divided into two stages, a first stage and a second stage. The first stage is formed by connecting a plurality of transistors in parallel, and the second stage is formed by a further plurality of transistors connected in parallel. The first stage and the second stage are connected in series. The current output end of transistors in the first stage each are connected to respective switches, respectively. Another, the current input end of transistors in the second stage each are connected to respective switches, respectively. Each of the switches is connected to a decoder. Therefore, through the control of the decoder, the on and off conditions of the switches can be determined so as to derive the required gain current.