Electronics Basics #27 | Depletion MOSFET Self-Bias: Full DC Analysis

Instructor: Dr. Süleyman Burak ÇELİK

A grounded gate does not guarantee zero gate-source voltage. Work through the original n-channel depletion MOSFET self-bias circuit: 20V supply, RD6.2kΩ, RS2.4kΩ, RG1MΩ to ground, IDSS8mA and VP−8V. Derive the quadratic step by step and test both current roots. Both currents are below IDSS, but the larger root produces a gate voltage below cutoff and must be rejected. Verified ideal operating point: ID≈1.767mA, VGS≈−4.240V and VDS≈4.805V. The original 4.804V uses early-rounded current; we preserve the original circuit and retain full precision internally. Confirm KVL and the saturation condition rather than trusting current magnitude alone. Ideal saturation model, negligible gate leakage, suitable device ratings assumed. No-signal DC analysis; this is not an AC-gain or maximum-output-swing calculation. Chapters: 0:00 The goal: understand what a grounded gate really sets 1:05 The source resistor creates the control voltage 2:23 Combine the circuit relation with the device model 3:39 Check both roots, not just their current magnitudes 5:32 Calculate the node and terminal-pair voltages 6:47 Verify saturation and close the voltage loop 8:04 A self-consistent operating point, not a guessed current