why resistor

Discussion in 'Electronic Basics' started by novice, Dec 17, 2005.

1. noviceGuest

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.

2. Tim WilliamsGuest

Biasing...

Do you have an electronics book? This is explained in detail in the most
basic amplifier textbooks.

Tim

3. Bob MonsenGuest

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.

4. Charles SchulerGuest

Do yourself a favor and get a good book: Malvino's Electronic Principles is
a particularly good one.

5. Bill BowdenGuest

Why the resistors are connected at the input of any transistor
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

6. Pooh BearGuest

It needs to have a 'bias point' though. That requires a couple of resistors
connected to the base.

Graham

7. PeteSGuest

As Tim said - Biasing, for one.

http://users.pandora.be/educypedia/electronics/analogmis.htm

( EDUCYPEDIA )

Cheers

PeteS

8. redbellyGuest

An engineer I used to work with didn't use resistors at all. He used
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

9. Michael A. TerrellGuest

He was a liar.

10. Bill BowdenGuest

It needs to have a 'bias point' though. That requires a couple of resistors
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

11. Pooh BearGuest

Usually not a big deal - probably by less than a dB or better.

True enough but rarely the case IME.

Graham

12. Don BoweyGuest

No, he was absolutely correct. Read it again carefully.

Don

13. Michael A. TerrellGuest

How about a single bleeder resistor across the output of the power
supply?

14. Ralph MoweryGuest

If most would use a 50,000 ohm resistor , he would use a 0.00002 mho
conductor.

Right on.

16. Jasen BettsGuest

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.

17. redbellyGuest

Yep. Besides, I was joking.

Mark

18. Rob BGuest

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

19. Jasen BettsGuest

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.

20. Rob BGuest

I did't miss earlier posts i am trying to learn more, i have always found
electronics fascinating, want to learn, and was trying to relate my obvious
limited knowledge of standard resistor attributes/properties to the concept
of wires (i.e. conductors) as resistors.

i read about resistance of wires , that most all wires ( conductors and not
super-conductor) have some resistance depending on several factors,
composition , size ( length , cross sectional area) , shape ( for inductance
issue)

I=VR

but i could not relate tolerance and power rating to resistor wires ?
because the example given of the (0.00002 mho conductor ) as 50k ohm
resistor made me wonder where does one figure the power rating as it seems
like such a tiny wire that would easily melt at least i have have melted
bigger wires in what i thought were prett mild simple circuit

sorry to sound like giving an amateur lesson but i wanted to show that i did
put some time in to researching before wasting your time with a question

robb