At 06:36 AM 6/6/2011, you wrote:
>Hi all, I made a simple LTSPICE simulation to try and understand how BJTs
>work.
>
>Screenshot: http://postimage.org/image/1dk9mvetg/full/
>
>On the plot (from left to right):
>
>* Power dissipation of the BJT
>* Vdrop across the resistor
>* Vdrop across the BJT
>* Current through the resistor
>* Impedance of the BJT
>
>Specifically, I was trying to see how the BJT behaves (it's impedance),
>under various collector voltages when 5mA flows through its base (hFE is
>100). I don't understand the impedance spike at the beginning of the cyan
>line in the region of x=3D0 to x=3D0.3ish Volts. Also, why is the graph sh=
owing
>negative resistance?

A transistor is a 3-terminal device. The base current can (and does, in you=
r
circuit) flow _out_ of the collector in saturation*. When V1 matches the
Vce(sat) you'd see for Ic=3D0 then the current drops to zero. Above that, i=
t goes
positive (current flows into the collector).

* saturation defined as Vce < Vbe.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.co=
m
Embedded software/hardware/analog  Info for designers:  http://www.speff.co=
m



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