Electronics Basics #40 | MOSFET Source Degeneration: Where Did the Gain Go?

Instructor: Dr. Süleyman Burak ÇELİK

If a MOSFET gate rises by one millivolt, does the transistor feel the entire change when its source can rise too? An unbypassed source resistor creates local negative feedback. Derive vin=vgs+vs, gain=-gm RD/(1+gm RS), infinite ideal input resistance and Rout=RD under the stated infinite-ro model. Based on original TR lesson 55. Supplemental example: IDQ=1 mA, gm=2 mS, RD=4.7 kohm, RS=1 kohm and VDD=12 V. A +1 mV gate change divides into about +0.333 mV gate-to-source and +0.667 mV source-to-ground, producing about -3.133 mV output. Three common-scale 3D voltage gauges show the same input sum; output uses a separate scale. The approximation -RD/RS is not accurate merely because a source resistor exists. Here gm RS=2, not much greater than one: the approximation overstates gain magnitude by 50 percent. Comparing RS=0 with RS=1 kohm at the same drain current requires gate rebiasing from 2 V to 3 V in this example. Assumptions: N-channel saturation, small signals, midband, infinite intrinsic output resistance, no body effect, negligible gate current and no external output load. RS is the transistor source resistor, not a generator resistance; no bypass capacitor is present. Instruments show AC voltage changes, not total DC voltages or mechanical displacement. The closing experiment answers the opening question. Chapters: 0:00 The gate rises, but so does the source 1:27 Follow the controlling voltage 3:01 Gain and the two port resistances 4:37 A numerical experiment 6:48 Approximations and the answer