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DC load line and CE characteristics

Short answer

DC load line is the straight line IC = (VCC−VCE)/RC on output characteristics, showing all possible DC operating points. Saturation point (VCE≈0, IC(sat)=VCC/RC) and cut-off (IC=0, VCE=VCC). Q point is intersection with IB curve defining operating IC and VCE. CE input characteristic is IB vs VBE like forward diode; output shows saturation, active, cut-off regions, β.

Define DC load line. Explain the common emitter configuration circuit with the help of input and output characteristics.

DC load line

The DC load line is the straight line on the output characteristics (IC versus VCE) that represents all possible dc operating points of a transistor for a given VCC and load resistance. It is obtained from KVL in the collector circuit: VCC = IC RC + VCE (for a circuit with the emitter grounded).

  • IC = (VCC − VCE)/RC
  • Point 1 (saturation, VCE ≈ 0): IC(sat) = VCC/RC
  • Point 2 (cut-off, IC = 0): VCE = VCC

The Q point (operating point) is the intersection of the load line with the output characteristic for the chosen base current IB.

 IC
 VCC/RC |\
        | \ Q
        |  *\   IB
        |    \
        +------\--- VCE
               VCC

Common emitter (CE) circuit

The input is applied between base and emitter and the output is taken between collector and emitter; the emitter is common to input and output. VBB forward biases the base-emitter junction and VCC reverse biases the base-collector junction.

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

Input characteristics

IB versus VBE at constant VCE. It looks like a forward diode curve: negligible IB below about 0.7 V (Si), then a steep rise. A higher VCE shifts the curve slightly to the right. Input resistance ri = ΔVBE/ΔIB.

 IB |        / /
    |       / /
    |      / /
    +-----/-/------ VBE
         0.7V

Output characteristics

IC versus VCE at constant IB.

 IC
  |     ____ IB3
  |    /____ IB2
  |   /_____ IB1
  |  /______ IB=0
  +--------------- VCE
  sat  active
  • Saturation: VCE below about 0.2 V; IC rises steeply.
  • Active: IC ≈ β IB, nearly flat; used for amplification.
  • Cut-off: IB = 0, IC = ICEO (small leakage).
  • β = IC/IB (50 to 300); the output resistance is ro = ΔVCE/ΔIC.

✓ Answer checked by Claude Opus 5.5 (high effort)

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