Electronics Basics #39 | MOSFET Common Source: Why Does the Output Fall?

Instructor: Dr. Süleyman Burak ÇELİK

Raise the gate by one millivolt: why does a common-source MOSFET's drain voltage fall even though the ideal gate takes no signal current? Derive input resistance, output resistance and the negative gain from the finite-output-resistance small-signal model. Based on original TR lesson 54. Supplemental example: IDQ=1 mA, gm=2 mS, RD=4.7 kohm and ro=50 kohm give Rout=RD parallel ro, about 4296.161 ohms, and gain about -8.592 V/V. A +1 mV input gives about -8.592 mV output. At infinite ro with the same gm and bias, gain approaches -9.4 V/V. The controlled-source current is not the entire finite-ro drain-current increment. Model-driven 3D instruments show signed drain-node current balance. Three current gauges share a fixed scale; output voltage has a separate stated scale. These are AC changes, not electron trajectories or total bias currents. Formulas and numbers appear after the corresponding spoken cue. Assumptions: an established N-channel saturation bias, grounded source, small signals, midband and negligible gate current/capacitance. Gate bias and protection networks can make practical input resistance finite. The ideal square-law gm example states its separate long-channel, ignored-channel-length-modulation assumptions. An external load reduces the output gain; amplifier output resistance is defined without that load. Chapters: 0:00 A voltage-controlled change 1:54 Find the input and output resistances 3:15 Derive gain and transconductance 4:43 Check one consistent operating point 6:55 State the limits and answer the question