Vbe voltage in NPN transistor

Discussion in 'Homework Help' started by edouarduk, Nov 14, 2008.

  1. edouarduk

    Thread Starter New Member

    Nov 14, 2008
    Hi there,

    I've been reading quite a lot lately as to master enough basics to work on my tube amps.

    Bought Paynter's introduction to electronic devices circuits and Morgan Jones's valve amplifiers that I've been seriously reading.

    There's something that I find really disturbing in Jones's is that he defines gm (transconductance, mutual conductance) as gm = Δ Ic / Δ Vbe !!! (page 48).

    I thought Vbe was only due to the structure of the NP layers and was always 0,7v.

    Could someone help me with that ?


  2. mik3

    Senior Member

    Feb 4, 2008
    The book is correct! Vbe is assumed to be constant (0.7V approximately) if the input voltage to the transistor is much larger than this. If the input voltage swings around 0.7V then the current will vary exponentially according to the diode equation (the base-emitter diode of the transistor).