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BJT Bipolar Junction Transistor

Basic Concepts

BJT (Bipolar Junction Transistor) — a semiconductor device that uses a small current to control a large current.

NPN:                    PNP:
      C (Collector)              C
      │                      │
    ┌─┴─┐                  ┌─┴─┐
  B │   │   Ic           B │   │   Ic
  ──┤   ├──              ──┤   ├──
    │   │                  │   │
    └─┬─┘                  └─┬─┘
      │                      │
      E (Emitter)             E

Arrow direction = Forward current direction of the emitter junction
NPN: Arrow points outward  (N→P→N)
PNP: Arrow points inward   (P→N→P)

BJT Essence

Consists of two back-to-back PN junctions:

NPN:  N ─ P ─ N
      E   B   C
      ↑   ↑
    BE junction  BC junction

BE junction forward biased + BC junction reverse biased → Amplification mode
Collector current Ic is controlled by Base current Ib:
  Ic = β × Ib

β (hFE): Current gain, typically 100~400

Three Operating Modes

Mode	BE Junction	BC Junction	Ic	Application
Cutoff	Reverse biased	Reverse biased	≈0	Switch OFF
Active	Forward biased	Reverse biased	β·Ib	Amplifier
Saturation	Forward biased	Forward biased	Vcc/Rc	Switch ON

Switch Mode

Cutoff: Ib = 0 → Ic = 0 → Equivalent to open circuit
Saturation: Ib > Ic/β → Vce ≈ 0.1~0.3V → Equivalent to closed circuit

When used as a switch:
  Rb = (Vdrive - Vbe) / Ib
  Ib ≥ Ic / β_min × 1.5  (Ensure deep saturation)
  Vbe ≈ 0.7V (Silicon transistor)

Amplification Mode

Ic = β × Ib
Ie = Ic + Ib = (β+1) × Ib

Vce = Vcc - Ic × Rc

Basic Amplifier Circuits

Common Emitter — Most Common

          Vcc
           │
           Rc
           │
        ┌──┴── Vout
        │
      C │
  ──┤├─B   NPN
  Rb   │
       E │
         │
        GND

Gain: Av = -gm × Rc  (Inverting!)
      gm = Ic / VT ≈ Ic/26mV (Room temperature)

Input Impedance: ≈ Rb ∥ rπ (Medium, kΩ range)
Output Impedance: ≈ Rc

Common Collector / Emitter Follower

Voltage Gain ≈ 1 (Non-inverting)
High input impedance, low output impedance
Used as a buffer

Small-Signal Model (Hybrid-π)

      B ──┬── rπ ───┬── C
         │          │
         │   ┌──────┤
         │   │  ↑   │
         └───┘ gm·Vπ│
                 │   │
                 E   │
                     │
                    GND

rπ = β / gm
gm = Ic / VT
ro = VA / Ic  (Early Effect)

Biasing Circuits

Fixed Bias (Simplest, large thermal drift)

Vcc → Rb → Base
Unstable, not recommended

Voltage Divider Bias (Standard practice)

         Vcc
          │
          R1
          │
    ┌─────┼── Base
    │     │
    R1    R2
    │     │
    └─────┼── GND
          │
         Re (Emitter resistor — provides negative feedback, stabilizes operating point)
          │
         GND

Adding Re greatly improves temperature stability
Ce parallel to Re restores AC gain

Comparison with MOSFET

Feature	BJT	MOSFET
Control Variable	Current (Ib)	Voltage (Vgs)
Input Impedance	Low~Medium (kΩ)	Extremely High (pA leakage)
Transconductance gm	Ic/VT (Linear)	2Id/(Vgs-Vth) (Square)
Switching Speed	Slow (stored charge)	Fast
On-State Voltage Drop	Vce(sat)≈0.1V	Rds(on)×Id
Noise	Low 1/f noise	Higher 1/f noise
Cost	Low	Low
ESD Sensitivity	Robust	Extremely fragile!

Selection Advice:

Switching → MOSFET (High efficiency)
Low-noise amplification → BJT
High-current driving → MOSFET
Simple LED/relay driving → Either works fine

Darlington Pair

Two BJTs cascaded, total β = β₁ × β₂ (can reach 1000~10000+)

      C
      │
  B ──┤  Q1
      │  ├── E₁ → Q2 Base
      │
      E → Q2 Emitter

Disadvantages: Vbe doubles (≈1.4V), slow speed
Typical: TIP122 (NPN), ULN2003 (7-channel Darlington array)

Keywords: BJT, NPN, PNP, β, Common Emitter, Switch, Saturation, Transconductance, Bias, Darlington