Explain CE configuration of BJT with the input and output characteristics.

Common-emitter (CE) configuration: emitter is common to input and output. Input is applied between base and emitter (V_BE, I_B); output is taken between collector and emitter (V_CE, I_C). For an NPN transistor, V_BB forward biases the B-E junction and V_CC reverse biases the C-B junction.

           I_C ->  Rc (mA)
        +----------C
  I_B   |          |  NPN
 V_BB +-RB---B ----|        V_CC
        |          E
        +----------+--------+
                 (common)

Input characteristics (I_B vs V_BE at constant V_CE): similar to a forward-biased diode; I_B is negligible until V_BE ≈ 0.7 V (Si) cut-in, then rises rapidly. Increasing V_CE shifts the curve slightly right (early effect / base-width modulation). Input resistance \( r_i = \Delta V_{BE}/\Delta I_B \) is about 1 kΩ (medium).

 I_B(uA)          VCE=1V  VCE=10V
   |              /     /
   |             /     /
   |           _/    _/
   +----------0.7-------- V_BE (V)

Output characteristics (I_C vs V_CE for different I_B):

 I_C(mA)      ______________ I_B = 60 uA
   |  /______________________ I_B = 40 uA
   | /_______________________ I_B = 20 uA
   |/________________________ I_B = 0  (cut-off)
   +--------------------------- V_CE (V)
   saturation | active region

• Saturation region: V_CE < ~0.2 V; both junctions forward biased; I_C rises steeply with V_CE.
• Active region: curves nearly flat; I_C ≈ βI_B, almost independent of V_CE (slight slope). Used for amplification.
• Cut-off region: below the I_B = 0 curve; I_C = I_CEO (small leakage).

Output resistance \( r_o = \Delta V_{CE}/\Delta I_C \) is high (≈ 10 to 50 kΩ). Current gain \( \beta = \Delta I_C/\Delta I_B \) (50 to 300). CE gives high current, voltage and power gain with 180° phase shift; most widely used (amplifiers, switches).