Why the resistors are connected at the input of any transistor
configurations(CB,CE,CC).What happens if the resistors are not
connected at the signal input? how do you decide the value of resistors
at i/p and o/p? Thanks a lot in advance.
The bipolar NPN transistor is basically a couple of diodes wedged
together on a piece of silicon. These diodes point from base to emitter
and base to collector. So, if you put more than the diode forward voltage
(usually about 0.7 volts) across the base to emitter (or base
to collector), it will pass lots of current. Thus, people often to put
resistors in series with the base, in order to prevent the diode from
overheating and failing.
Deciding the values of the resistors is a matter of deciding how much
current you want, and using the resistors to limit that current. If you
use the simple 'beta' model of a transistor, in which a transistor is a
current amplifier with a gain of beta, it is obvious that you want to
limit base current to the desired collector current divided by this gain,
which is often near 100 (sadly, sometimes much more, sometimes much
less...
This simple model really breaks down for precision analog
amplifiers, but you'll find out about that later.
The gain of the various amplifiers (your CB,CE,CC) is related to the
values of the attached resistors (and other circuit elements), so picking
them will determine the output.
--
Regards,
Bob Monsen
He was not in a hurry, "hurry" being one human concept he had failed
to grok at all. He was sensitively aware of the key importance of
correct timing in all acts but with the Martian approach: correct
timing was accomplished by waiting. He had noticed, of course, that
his human brothers lacked his own fine discrimination of time and
often were forced to wait a little faster than a Martian would but he
did not hold their innocent awkwardness against them; he simply
learned to wait faster himself to cover their lack.