Electronics Basics #41 | MOSFET Bypass: AC Ground, but Still One Volt?

Instructor: Dr. Süleyman Burak ÇELİK

How can a source resistor set a MOSFET bias while a capacitor makes that same source nearly stationary for the changing signal? Separate the DC source voltage from its AC change. This lesson combines original TR lessons 56 and 57: self-bias with a source bypass, then the unbypassed source-resistor limit. Use a depletion N-channel MOSFET with negative threshold -2 V. A supplemental example uses k=1 mA/V² (including the one-half coefficient), RS=1 kohm, RD=4.7 kohm, RG=1 Mohm, ro=50 kohm and VDD=12 V. The physical DC solution is IDQ=1 mA, VSQ=1 V, VGSQ=-1 V, VDSQ=6.3 V and gm=2 mS. This zero-gate self-bias must not be assigned to an enhancement device with positive threshold. With an ideal AC bypass, Rin=RG, Rout=RD parallel ro and Av approximately -8.592 V/V. A +1 mV gate signal gives approximately -8.592 mV output while the source retains its 1 V DC bias. The 3D experiment uses a finite 100 microfarad capacitor at 1 kHz, not an exact short: its reactance magnitude is about 1.592 ohm, and computed resistor current plus capacitor current equals drain current. Capacitor charge, current and voltages come from one steady-state phasor model. Disconnecting the capacitor preserves the settled DC bias but restores source feedback. With finite ro the gain is approximately -3.019; only after taking ro to infinity does it become -3.133. The schematic and formula change together. Assumptions: saturation, small signal, settled sinusoidal response, no body effect or external output load, negligible gate current. This is not a switch-on transient or a claim that an AC ground has zero DC voltage. The closing experiment answers the opening question. Chapters: 0:00 A resistor with two different roles 0:37 Keep a physically possible bias 2:07 Separate DC from the changing signal 2:48 Measure the two ports and gain 4:52 Test the model with numbers 6:27 Know when the bypass is an approximation