I CEO in common emitter configuration

Discussion in 'General Electronics Chat' started by YoGMan, Dec 10, 2017.

  1. YoGMan

    Thread Starter Member

    Sep 20, 2017
    67
    1
    Hello friends, What causes the reverse saturation current when Ib=0 A to be amplified by (B+1) in common emitter configuration ? I know the mathematical derivation but i want a clearer explanation like what really causes this amplification.
     
  2. neonstrobe

    Well-Known Member

    May 15, 2009
    73
    12
    The starting point in discussing how a transistor works is, to my mind, is to consider it's diode characteristics. Diodes have a built-in electric field, which opposes current flow which is trying to occur through simple diffusion (more electrons on one side try to get to fill the space in the p region on the other). The behaviour is controlled by voltage, not current. A slight reduction in the built-in field reduces the opposition to the current flow, so current flows. When current flows, electron and hole currents both flow, but in opposite directions. So when a transistor emitter-base junction is forward biased, current flows into the base, through and into the collector and at the same time holes (for an NPN) flow into the emitter. The ratio is set by the physical construction of the device (doping etc). What is perceived as gain is due to the hole current being optimised low compared to the collector current (emitter=> base=> collector) being optimised high. So leakage current from the collector biases the base and causes some (slight) forward bias, hence giving rise to a higher emitter current which goes through the base into the collector. It's not gain per se (gain is a result of optimsation of the design) but due to the electrostatic potentials that control these, primarily.
     
  3. Bordodynov

    Well-Known Member

    May 20, 2015
    2,240
    689
    And what do you say to this:
    Draft572_.png
     
  4. neonstrobe

    Well-Known Member

    May 15, 2009
    73
    12
    There is a difference between Iceo, which is mainly due to collector base leakage currents and avalanche currents, where electrons are stripped from the base-collector diode.
    Not sure what your point is.
    Have you measured the base voltage during your sweep?
     
  5. crutschow

    Expert

    Mar 14, 2008
    22,226
    6,481
    The collector-base reverse leakage current (Icbo) is from the collector -base reverse biased diode.
    In an Iceo configuration the base is open so this current has no place to go but into the base-emitter region where it is amplified the same as any base current would be.
    The emitter current is thus Icbo plus its component amplified by the transistor β giving a total emitter current of Icbo*(β+1).
    Make sense?
     
    YoGMan likes this.
  6. Bordodynov

    Well-Known Member

    May 20, 2015
    2,240
    689
    See
    Draft1.png
     
    YoGMan likes this.
  7. neonstrobe

    Well-Known Member

    May 15, 2009
    73
    12
    Succinctly put. The point I was making was that the transistor is primarily controlled by voltage, not current. Icbo, caused by generation current, means electron-hole pairs are formed. The holes create effective base current as you say. But that forward biases the base-emitter junction, so increasing the electron current.
    Measuring the base voltage in a Vceo sweep should show that it is slightly forward biased, thus adding to the Icbo.
     
    YoGMan likes this.
Loading...