Amplifier with adjustable gain

An amplifier with adjustable gain, comprising:

• a plurality of differential amplifiers, each of the differential amplifiers having at least one differential pair and two transistor strings, wherein two current terminals of each of the at least one differential pair are coupled by a connection structure, and the connection structure provides a negative feedback resistance, the differential amplifiers commonly receive a differential input signal pair, output terminals of the differential amplifiers are coupled to each other, the two transistor strings are respectively coupled between two load terminals and the two current terminals of each of the at least one differential pair, and the two transistor strings are turned on or turned off according to a control signal and a reference voltage;
• and
• an output stage circuit, coupled to the output terminals of the differential amplifiers, and inverting a voltage on the output terminals of the differential amplifiers to generate an output voltage,
• wherein a direct current gain of the amplifier with adjustable gain is determined by adjusting at least one of working numbers of the differential amplifiers and the differential pairs.