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.
why resistor
Dec 17, 2005
32 Replies
Biasing...
Do you have an electronics book? This is explained in detail in the most basic amplifier textbooks.
Tim
-- Deep Fryer: a very philosophical monk. Website:
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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.
Do yourself a favor and get a good book: Malvino's Electronic Principles is a particularly good one.
You don't need a resistor for a CC configuration. The load is in the emitter side so that the load voltage is always 0.7 volts below the base. No resistor needed.
-Bill
As Tim said - Biasing, for one.
Try this page and navigate your way
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( EDUCYPEDIA )
Cheers
PeteS
It needs to have a 'bias point' though. That requires a couple of resistors connected to the base.
Graham
An engineer I used to work with didn't use resistors at all. He used conductors instead. Wherever a circuit schematic had a resistor he would replace it with a conductor in the actual circuit, using a conducance equal to the reciprical of the resistance indicated by the diagram.
He claimed that any circuit built this way would work just fine.
Mark
Yes, if the input signal is not at the desired DC level. The bias resistors would shift the input level up or down as required, but would also attenuate the input signal.
But if the input signal is already at the desired DC level, no resistors are needed.
-Bill
He was a liar.
Been there, Done that, I\'ve got my DD214 to prove it.
Michael A. Terrell
Central Florida
Usually not a big deal - probably by less than a dB or better.
True enough but rarely the case IME.
Graham
No, he was absolutely correct. Read it again carefully.
Don
Right on.
How about a single bleeder resistor across the output of the power supply?
Been there, Done that, I\'ve got my DD214 to prove it.
Michael A. Terrell
Central Florida
"Michael A.
resistors
If most would use a 50,000 ohm resistor , he would use a 0.00002 mho conductor.
Yep. Besides, I was joking.
Mark
I wouldn't go that far. He was only calling resistors conductors.
Resistance and conductance are merely two different ways of expressing the same quantity.
Bye.
Jasen
so is that the same as a wire wound resistor ?
how would one go about determining the tolerance and power rating of a wire-sister ?
by "work just fine" did the Engy mean work and behave exactly like the circuit built with other types of resistors ?
are there any tyoes of circuits one would not want to use the wire-sister?
curious robb
metal is not the only conductive material. carbon, for instance has a low conductance that can make it handy in certain aplications. also a thin layer (or film) of metal caoul be used ... :)
If you missed the earlier posts conductance is the reciprocal of resistance. All conductors are resistors.
Bye.
Jasen
The original post suggesting the use of conductors in place of resistors was a sort of "play on words". It was a (fairly successful) attempt to confuse.
A 1000 ohm resistor _is_ a .001 mho conductor.
Peter Bennett, VE7CEI
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