Automatically gain controllable linear differential amplifier using variable degeneration resistor

A linear differential amplifier for amplifying a difference between a positive input signal and a negative input signal to generate an amplified difference output signal, comprising:

• a first amplifier for receiving the positive input signal to generate an amplified positive input signal;
• a second amplifier for receiving the negative input signal to generate an amplified negative input signal;
• first and second bias current source, each connected to one end of said each first and second amplifiers, for providing a bias current;
• first and second output loads, each connected to another end of the first and second amplifiers for providing an amplified difference output signal, wherein the amplified difference output signal represents a difference between the positive amplified input signal and the negative amplified input signal;
• and
• a degeneration resistor for maintaining a steady resistance value for the bias current sources in response to the positive input signal and the negative input signal, wherein the degeneration resistor includes:
• a first MOS transistor having a first drain/source coupled between the first amplifier and the first current source, a second drain/source coupled between the second amplifier and the second current source and a gate for receiving the positive input signal;
• a second MOS transistor having a first drain/source coupled between the first amplifier and the first current source, a second drain/source coupled between the second amplifier and the second current source and a gate for receiving the negative input signal;
• and
• control means for generating a gain control signal to be commonly to the gates of the first and second MOS transistors.