What is source degeneration in a MOSFET amplifier, and what is its effect on gain and linearity?

• A. Source degeneration via Rs adds negative feedback, reducing gain while improving linearity and input stability.
• B. It increases gain and reduces linearity.
• C. It has no effect on gain.
• D. It decreases bandwidth only.

Answer: (A)

Source degeneration means inserting a resistor in the MOSFET’s source path. That resistor develops a voltage that grows with the drain current, which feeds back to the gate as a reduction in the effective gate-source voltage. This local negative feedback lowers the stage gain (the small-signal gain is reduced roughly by a factor of 1 + gm Rs) but improves linearity because the feedback makes the transistor’s transfer more proportional to the input. It also helps bias stability by making the gain less sensitive to device parameter variations and temperature. So the best description is that source degeneration adds negative feedback, reducing gain while improving linearity and input stability. The other options don’t fit: it does not increase gain, it does affect gain, and its primary effect is not only on bandwidth.