How does adding negative feedback help to stabilize the DC operating point?
Adding a negative feedback by placing a resistor in the transistor’s source circuit means that the terminal is no longer grounded (or 0 volts) but is placed at a small potential above.
If the drain current increases, the corresponding source current must also increase causing the voltage drop across Rs to increase. This voltage drop across Rs due to the drain current provides the necessary reverse biasing condition across the gate resistor, effectively generating negative feedback.
In other words, the addition of the source resistance helps control the transistors gate bias using negative feedback, which negates any attempted change in drain current with an opposing change in the gate bias voltage and so the circuit tends to be stabilized at a fixed level.
For the source degeneration circuit setup, what is the effect on the voltage gain, A, by increasing RS?
Following source degeneration gain equation presented in the “Adding source degeneration” section of the lab activity:
We notice that the Rs value is inversely proportional with the voltage gain A, therefore increasing the value of Rs will lead to a smaller at the output of the common source amplifier.