Variable gain amplifiers for communication systems

A variable gain amplifier device comprising:

• a first transistor and a second transistor, the first transistor comprising a first gate terminal coupled to a first input signal and a first drain terminal coupled to a first output node, the second transistor comprising a second gate terminal coupled to a second input signal and a second drain terminal coupled to a second output node;
• a first common mode resistor coupled to the first output node;
• a second common mode resistor coupled to the second output node and the first common mode resistor;
• a common node configured between the first common mode resistor and the second common mode resistor, the common node is characterized by a common mode voltage;
• a third transistor and a fourth transistor, the third transistor comprising a third gate terminal coupled to the first output node and a third drain node coupled to a third output node, the fourth transistor comprising a fourth gate terminal coupled to the second output node and a fourth drain terminal coupled to a third output node;
• and
• a fifth transistor comprising a fifth gate terminal coupled to a control voltage and the common node between the first common mode resistor and the second common resistor, the fifth transistor being configured between the third transistor and the fourth resistor, the resistance value based on the control voltage and the common node voltage.

A serializer/deserializer (SerDes) apparatus comprising:

• a data communication interface for receiving a first input signal and a second input signal;
• a continuous-time linear equalizer (CTLE) section configured for providing a common mode voltage and a first equalized signal and a second equalized signal based at least on the first input signal and the second input signal, the CTLE section comprising a pair of common mode resistors, the common mode voltage being configured between the pair of common mode resistors;
• a digital-to-analog (DAC) section configured for generating a control signal based on a control code;
• a variable gain amplifier (VGA) section comprising a transistor and configured to generate a first amplified signal and a second amplified signal based on the first equalized signal and the second equalized signal, the VGA section being characterized by a gain ratio being at least based on a degeneration resistance value of the transistor, the degeneration resistance value being a function of the common mode voltage and the control signal;
• and
• a clock data recovery module being configured to generate a clock signal using at least the first equalize signal and the second equalized signal.