Skip to main content

Short note: CB output characteristics

Short answer

CB output characteristics plot IC vs VCB for constant IE. In active region, IC ≈ αIE (α < 1), curves are nearly flat with high output resistance. Saturation occurs for small forward VCB; cutoff with IE = 0 yields ICBO. Input impedance is low, no current gain, but voltage gain exists with no phase shift.

Write a short note on output characteristics of common base configuration.

In the common base (CB) configuration, the base is common to the input (emitter-base) and output (collector-base). The output characteristics are the plots of collector current IC versus collector-base voltage VCB for various fixed values of emitter current IE.

 IC (mA)
  |         ______ IE = 6 mA
  |        |______ IE = 4 mA
  |        |______ IE = 2 mA
  |        |______ IE = 0   (ICBO)
  +--------+----------------- VCB
  saturation  active
  (VCB < 0)

Regions

  • Active region: the emitter-base junction is forward biased and the collector-base junction is reverse biased. IC ≈ α IE, almost constant and independent of VCB (the curves are nearly flat; α = 0.95 to 0.99). A small slope exists due to the Early effect (base width modulation).
  • Saturation region: both junctions are forward biased (VCB negative by a few tenths of a volt). IC falls rapidly to zero as VCB decreases.
  • Cut-off region: IE = 0; both junctions are reverse biased. IC = ICBO, a very small leakage current.

Key points

  • Current gain α = ΔIC/ΔIE (at constant VCB), a little below 1.
  • Very high output resistance (about 1 MΩ), low input resistance.
  • No current gain but voltage gain exists; there is no phase shift.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Basic Electronics Engineering chapter 3 questions →