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CE configuration input/output characteristics

Short answer

In common-emitter configuration, input is applied between base and emitter, output taken between collector and emitter. Input characteristic shows negligible IB until VBE ≈ 0.7 V, then rises sharply; output shows saturation (VCE < 0.2 V), active region where IC ≈ βIB, and cut-off. β typically 50–300, output resistance ~40 kΩ.

Explain the common emitter configuration circuit of npn transistor with the help of input and output characteristics (draw the circuit diagram and I-V characteristics graph).

In the common emitter (CE) configuration, the emitter terminal is common to both input and output. The input signal is applied between base and emitter and the output is taken between collector and emitter. It gives both current gain and voltage gain, with a 180° phase shift.

Circuit

                            +VCC
                             |
                            [RC]
                             |  IC
                             +--------o C  (output VCE)
                           |/
 VBB (+)--[RB]---->--------|   NPN
               IB       B  |\
                             v E
                             |
 VBB (-), GND o--------------+--------o E  (common)

VBB forward biases the base-emitter junction and VCC reverse biases the collector-base junction. Measure IB and VBE (input), and IC and VCE (output).

Input characteristics

Plot of IB versus VBE at constant VCE.

  • Similar to a forward-biased diode: IB is very small until VBE exceeds the cut-in voltage (about 0.7 V for Si), then it rises sharply.
  • Increasing VCE shifts the curve slightly to the right (a little less IB for the same VBE).
  • Input resistance: ri = ΔVBE/ΔIB at constant VCE (about 1 to 2 kΩ, higher than in CB).
 IB
  |           /  VCE=0
  |          / / VCE>0
  |         / /
  +-------/-/------ VBE
         0.7V

Output characteristics

Plot of IC versus VCE at constant IB.

 IC
  |      ______ IB3
  |     /______ IB2
  |    /_______ IB1
  |   /________ IB=0 (ICEO)
  +----------------- VCE
   sat  active    breakdown
  • Saturation region: VCE is small (below about 0.2 V); both junctions are forward biased; IC rises steeply.
  • Active region: the base-emitter junction is forward biased and the collector-base junction is reverse biased; IC ≈ β IB, almost independent of VCE. The transistor is used as an amplifier here.
  • Cut-off region: IB = 0, IC = ICEO (very small leakage); both junctions are reverse biased.
  • Output resistance: ro = ΔVCE/ΔIC at constant IB (about 40 kΩ).

Current gain

β = IC/IB (typically 50 to 300); ICEO = (1 + β) ICBO.

✓ Answer checked by Claude Opus 5.5 (high effort)

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