Electronics Basics #15 | BJT Fixed-Bias Circuit: Full DC Analysis Step by Step (IB, IC, VCE)

Instructor: Dr. Süleyman Burak ÇELİK

Our first real transistor circuit, solved completely — every current, every voltage, every single arithmetic step shown on screen. The fixed-bias circuit is where BJT DC analysis begins, and it teaches the method you will use in every bias circuit after it. Given: V_CC = 12 V, R_B = 240 kilo-ohms, R_C = 2.2 kilo-ohms, beta = 50. Find: I_BQ, I_CQ, V_B, V_C, V_BC, V_CEQ. (And don't let the Q scare you — quiescent just means the resting DC value.) What you'll learn along the way: - Why capacitors are OPEN circuits in DC analysis - How to WALK a Kirchhoff voltage loop: start at a known voltage, pay every obstacle - The trap almost everyone falls into: you do NOT have to walk all the way to ground — stop at any point whose voltage you know - The strategy question before writing any equation: does this line leave just one unknown? - Magnitude checks that catch errors instantly: base currents live in microamps, collector currents in milliamps — if you found milliamps for I_B, you forgot the kilo in 240k - The rule that pays rent forever: any double-subscript voltage is FIRST minus SECOND (V_CE = V_C minus V_E) - Why a negative V_BC is good news: it means the transistor sits in the active region Results: I_B = 47.1 microamps, I_C = 2.35 mA, V_CE = 6.82 V, V_C = 6.82 V, V_B = 0.7 V, V_BC = minus 6.12 V. Chapters: 0:00 Intro 0:03 Recap: the valve and the two golden facts 0:37 The fixed-bias circuit, piece by piece 1:49 Walk 1 — the input loop: finding I_B (47.1 microamps) 4:02 I_C in one line: beta times I_B = 2.35 mA 4:57 Walk 2 — the output loop: finding V_CE (6.82 V) 6:13 Who is left? V_C, V_B, V_BC and the first-minus-second rule 8:01 The method in your pocket + what comes next Next lesson: we put a resistor under the emitter and find out why this simple bias drifts — and how to pin it down. Reference: Boylestad, Electronic Devices and Circuit Theory