Electronics Basics #44 | Gate Held Still, Output Rises? Common-Gate MOSFET

Instructor: Dr. Süleyman Burak ÇELİK

Can a MOSFET amplify while its gate is held still? Identify source input, fixed gate reference and drain output before following the voltage difference and current. Covers original TR lesson 60. The supplemental depletion-MOSFET bias uses VDD=12 V, VT=-2 V, k=1 mA/V² including one half, RS=1 kohm and RD=4.7 kohm. The valid point is ID=1 mA, VS=1 V, VGS=-1 V, VDS=6.3 V and gm=2 mS. Gate AC grounding is distinct from gate DC bias. Do not bypass the source input. With infinite ro and no body effect, vgs=-vin, id=-gm vin and vout=+gm RD vin. Source-node current balance gives Rin=RS parallel (1/gm), about 333.3 ohms. Output resistance is RD, 4.7 kohm. The dependent current source remains in the model; its value follows its control voltage. A +1 mV source-terminal input produces a -1 mV gate-to-source change, -2 microamp drain-current change and +9.4 mV drain output. The input provides +3 microamps; total drain current stays positive, about 0.998 mA. Adding 1 kohm external source resistance reduces the transistor input to one quarter of the generator voltage and gives +2.35 mV output for a 1 mV generator. The calculated 3D instruments and signed values use the same model. Input-side gauges share a ±1 mV scale; output uses a declared ±9.4 mV scale. A concluding comparison distinguishes common-source, common-drain and common-gate ports and gain signs. Assumptions: small signals, midband, saturation bias, ideal grounding/coupling, infinite ro, no body effect and no external output load. The closing experiment answers the opening question. Chapters: 0:00 Move the source, not the gate 1:21 Follow the control voltage and signs 3:29 Why the source input draws current 4:58 Check a complete signal example 6:34 Compare the three common arrangements https://acedumy.com