Common emitter BJT characteristics

In the common emitter (CE) configuration the emitter is common to input and output: input is between base and emitter (V_BE, I_B) and output is between collector and emitter (V_CE, I_C).

          RC
   +-----/\/\---+--- +VCC
   |            |C
 I_B   RB     |/
 o--/\/\----|      npn
            |\
              |E
             GND

Input characteristics (I_B vs V_BE, at constant V_CE)

 I_B |        /  /  V_CE = 1 V, 10 V
 (µA)|       /  /
     |      /  /
     |_____/__/________ V_BE
     0    0.7 V

• Similar to a forward-biased diode curve: almost no I_B until V_BE ≈ 0.7 V (Si), then I_B rises sharply.
• Increasing V_CE shifts the curve slightly to the right (less I_B for the same V_BE) due to base-width modulation (Early effect).
• Input resistance r_i = ΔV_BE/ΔI_B is low (about 1 kΩ).

Output characteristics (I_C vs V_CE, at constant I_B)

 I_C |  sat  _______________ I_B=40µA
 (mA)|   |  /_______________ I_B=30µA
     |   | /________________ I_B=20µA
     |   |/_________________ I_B=10µA
     |___/__________________ I_B=0 (cutoff)
     0  0.2V           V_CE

• For each I_B, I_C rises quickly for small V_CE and then becomes nearly flat: I_C ≈ β I_B.
• The slight upward slope is again due to the Early effect.

Regions of operation

The active region is the flat part of the curves, the saturation region is the steep part near the I_C axis (V_CE < about 0.2–0.3 V), and the cutoff region lies below the I_B = 0 curve. CE gives both current and voltage gain, so it is the most widely used amplifier configuration.