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Ebers-Moll terminal currents; BJT configuration applications

Short answer

IE = -IF + αR IR, IC = αF IF - IR, IB = (1-αF)IF + (1-αR)IR, where IF = IES[e^(VBE/VT)-1], IR = ICS[e^(VBC/VT)-1]. Common emitter provides high voltage gain and phase inversion; common base is used for high-frequency amplification; common collector serves as a buffer with high input and low output impedance.

Find terminal currents of BJT using Ebers-Moll Model. Write applications of different BJT configurations.

Terminal currents from the Ebers-Moll model

The model has two diodes: the emitter-base (DE) with current IF and the collector-base (DC) with IR, and two controlled sources αF IF and αR IR.

  C o---+-----------+
        |           |
       ( ) αF·IF   DC | IR
        |           |
  B o---+-----------+
        |           |
       ( ) αR·IR   DE | IF
        |           |
  E o---+-----------+

IF = IES [exp(VBE/VT) - 1]
IR = ICS [exp(VBC/VT) - 1]

Applying KCL at each terminal (currents into the transistor):

IE = -IF + αR IR
IC =  αF IF - IR
IB = -(IE + IC) = (1 - αF) IF + (1 - αR) IR

with αF IES = αR ICS = IS. In the active region (VBC reverse, IR ≈ -ICS): IC ≈ αF IF = IS exp(VBE/VT), IE ≈ -IF, and IB ≈ IC/βF.

Applications of the BJT configurations

ConfigurationPropertiesApplications
Common emitterhigh Av and Ai, 180° phase, medium Rin and Routgeneral voltage amplifiers, audio preamps, switching circuits
Common baselow Rin, high Rout, Av high, Ai ≈ 1, no phase shiftRF/high-frequency amplifiers, current buffers, cascode top stage
Common collectorAv ≈ 1, high Rin, low Rout, high Aibuffer/impedance matching, output stage (power) driving low impedance loads, voltage regulators

✓ Answer checked by Claude Opus 5.5 (high effort)

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