BJT, Emitter bias, Beta, base current

Discussion in 'Homework Help' started by sbixby, Nov 15, 2011.

  1. sbixby

    Thread Starter Active Member

    May 8, 2010
    I'm working my way through Malvino/Bates "Electronic Principles 7ed", and looking at the example of emitter fixed bias. I don't think the text is pointing me wrong or anything, but I'm a bit confused.

    Attached is a snip of an example, using Falstad's applet. The NPN has a beta of 100.

    My question is - what controls the base current in this context? As I understand it, the emitter (and thus collector) current will be fixed to that of Vbb - Vbe (4.3v in the example), divided by the emitter resistance, so it comes out to 1.95mA. The base current ends up being simulated at about 19.8uA.

    How does the base current get limited?

    I understand that if the emitter is grounded and the base is biased to 19.8uA, a Beta of 100 would mean 1.98mA through the collector/emitter circuit. But I haven't seen any mention that a fixed CE current will control the base, too - is this true?
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    It's a case of negative feedback. If for some reason, we assume Ib were to increase beyond its steady state value, then the collector and emitter currents would tend to increase - since Ic=βIb & Ie=(1+β)Ib. If the emitter current tends to increase then the emitter voltage [VE] will tend to go higher. If VE tends to go higher then the base-emitter voltage [VBE] tends to decrease since VBB=5V is fixed. If VBE tends to decrease then Ib tends to decrease - countering the original 'assumed' tendency to increase. So the circuit DC operating conditions are notionally self-regulating.

    Some further thoughts ...
    The value of VBE is also dependent upon the quiescent emitter current so one might expect some variation in the final values of the quiescent device currents [IB, IC and IE]. In addition the DC current gain [β] will have a reasonable spread in value from one device to another - even for the same type number. This will also effect the quiescent values. A further matter to keep in mind, is that VBE isn't 'rock stable' with junction temperature variation. For a silicon BJT the value of VBE decreases by about 2.5mV per degree C increase in temp. But the consequences of these facts aren't particularly significant in this situation.
    Last edited: Nov 16, 2011