transistor biasing???

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

 

The exact function of any particular resistor an a particular circuit
is hard to discuss in general, because there are lots of
possibilities. But a transistor is a device that passes collector
current and the amount of that current is dependent on both the
emitter to base situation, and also the emitter to collector
situation. Bias circuits are intended to set up both these situations
so that usable gain is available from the transistor, at the same time
that signal swing is available.

Another way to say that is that bias circuits are intended to get the
transistor conducting a middle amount of current, so that when an
input signal is added to the bias, the the output signal is a larger
(in some sense) copy of the input signal riding on some bias produced
average voltage, with acceptable distortion (neither hitting
completely on or completely off at the peaks of the signal).

 

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!

 

Biasing is relevant only to linear operation of transitors. Switching
circuits don't need bias.

For an ac amplifer for example, the transistor collecter-emitter curent is
modulated by the signal both positively and negatively. If the transistor
started with zero current ( no bias ) only half the signal woud get
amplified since the current can only increase from zero - not decrease.

Hence amplifers use bias to set an inital ( no signal ) operating point.
The best way of doinfgt his varies with application, circuit topology and
performance issues.

I can't think of an instance where a collector resistor is required solely
by bias considerations btw. The collector R usually forms a voltage ouput
node.

Graham

 

Here is a fairly simple approach to it, but it should help. Take a look at
http://www.fncwired.com/TransistorExample/

Brian