On 25/05/2011 22:24, V G wrote:
> So what does the base control exactly? If the current through the base
> controls the current through the collector, and I =3D V/R, is it also
> controlling the resistance?

The base current does control the C-E current , but it acts more like a=20
current source as opposed to a resistance (over a region of compliance)
To get an idea, you could run a test in LTSpice - bias an NPN transistor=20
with say 1uA into the base (just use a current source), emitter to=20
ground and vary a voltage from e.g. 1 to 10 volts on the collector.
Do the same with a (say 100 or 1K) resistor (from voltage to ground) and=20
note the difference in current behaviour.
Use the Pulse setting for the voltage source and select a slow rise/fall=20
time, then monitor the results with transient simulation.
You can press add trace and use the d() function to check the dynamic=20
impedance - enter e.g. d(V(V+)/d(Ic(Q1)) assuming your voltage source is=20
called V+ and NPN is Q1.


--=20
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