andrew_h said:
Hi,
Just wanted to know if someone out there could explain, fairly simply,
exactly what transistor biasing is, and why it is needed. I.e. having a
resistor in series with the base, and also even a resistor in series
with the collector etc... what is the purpose of this?
I understand when a resistor would be used before the base of the
transistor, to limit the current to a known quantity (v=ir), but
biasing didnt make full sense.
Thanks,
Andrew
i'll try to explain in layman's terms.
think of Biased in english terms.
in transistors, you determine how much
current you need flowing between the base
and emitter to calculate the amount of
current that will flow between the Collector
and emitter.
this is called Beta (Hfe) etc. others call
it different things. the Hfe is like a gear
box on a mechanical device. with lets say 1 foot
pounds of torque on the input with a gear box of
1:10 for example, will give you 10 pounds of torque
on the output.
now transistor operate in similar fashion.
if you have lets say a beta of 100 and you apply
lets say 1 ma of current through the base and emitter
you will have aprox 100 times that flowing from the Collector
to the emitter. that of course saying there is no Resistor
from the Vcc..
so the results would be around 100 ma of current in the collector.
this is just rough details and are designed to help you better
understand. other things do swing the behavior of the final results
of course.
for the internal workings of this behavior, when current is present
in the Be (base emitter), electrons are emitted, i guess that is why
they call it a emitter!

these electrons kind of help fill the gap
causing the electrons from the emitter to push along to the other side.
its like a bridge that helps electrons get to the other side when the
electrons from the emitter is clouding(some would say boiling off) to
act as a path..
etc..
also remember that These junctions act like diodes, that is they have
a minimum voltage operating point and is usually starting around 0.6 on
the average. just one of those extra calculations you need to throw in
there when doing exact calculations.
hope that back yard explanation helped you some!