# EMITTER-BASE and COLLECTOR-BASE resistance OF transistors???

Discussion in 'General Electronics Chat' started by dogar sahab, Sep 6, 2008.

1. ### dogar sahab Thread Starter Active Member

Mar 15, 2008
116
0
I want to ask the reason that
why emitter-base resistance of NPN TRANSISTOR is greater than collector-base reistance??/while in the case of PNP TRANSISTOR,emitter-base resistance is less than collector-base resistance????

2. ### hgmjr Moderator

Jan 28, 2005
9,030
214
It sounds like you are referring to a specific circuit. Can you post a diagram of the circuit you are studying?

hgmjr

3. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Have to agree with HJM on that one. I'm not sure the term resistance really applies in this case though, since we're talking about PN junctions.

4. ### Audioguru New Member

Dec 20, 2007
9,411
896
The amount of current in a transistor junction is determined by the amount of current in it. If it has a lot of current then its resistance is low. If it has hardly any current then its resistance is high.

An NPN transistor is the same as a PNP transistor except the polarity is opposite.

5. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
That's a tautology.

And, if forward biased, a PN junction has no resistance to current, so current has to be controlled by external resistors.

6. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
The thing to remember is a PN junction will conduct until it destructs without an external limiting resistor. You can measure an equivalent resistance, but it changes according to the current it is fed, it isn't a fixed value. This is why we're talking about the PN junction and resistance not being the same.

I know this has been stated in previous posts, just thought restating it slightly differently would make it clearer.

7. ### dogar sahab Thread Starter Active Member

Mar 15, 2008
116
0
actually,I have to perform an experiment to detect the pin configuration and type of the given transistor provided I dont have a facility of diode check through multimeter!!!!I am given a transistor and no other information is provided and I have to find that whether it is PNP or NPN transistor and also its pin configuration!!???
as we know emitter is heavily doped and it has the highest resistance as compared to collector,so we have to employ trial and error method to detect BASE,EMITTER and COLLECTOR TERMINALS!!!
once I found that,then I moved on to RESISTANCE CHECK of a transistor with the help of analog transistor!!!here,I found that in case of emitter-base resistance of NPN TRANSISTOR is greater than collector-base reistance while in the case of PNP TRANSISTOR,emitter-base resistance is less than collector-base resistance...
i want to ask the reason of it,why does this happen!!!???

8. ### dogar sahab Thread Starter Active Member

Mar 15, 2008
116
0
*analog multimeter

9. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
A check of a transistor with an ohm meter can be done as follows find a PN junction and measure it. You are touching either the base collector, or the base emitter. Then (while still touching the first two pins) short the 3rd lead to what you think is the collector. The conduction will increase substantually.

An emitter looks like a collector with an ohm meter, pretty much, using just two leads. If you have the base collector and emitter measured with the ohm meter it will show minimum resistance (as opposed to having the base emitter and collector across the meter leads) because you are taking advantage of the gain of the transistor.

10. ### dogar sahab Thread Starter Active Member

Mar 15, 2008
116
0
^u r right but If we dont have ohm-meter too!!!..i want to make it clear that it is my ANALOG-CIRCUITS course and this is the work of my FIRST lab session!!!so all your replies are fine but my problem still is unsolved..
so i want to make it clear and in simple words
Reb(EMITTER-BASE RESISTANCE)>Rcb(COLLECTOR-BASE RESISTANCE) in case of NPN
Reb(EMITTER-BASE RESISTANCE)<Rcb(COLLECTOR-BASE RESISTANCE) in case of PNP????
what is the reason behind that???
this has been checked by ANALOG MULTIMETER!!!!

11. ### dogar sahab Thread Starter Active Member

Mar 15, 2008
116
0
^u r right but If we dont have ohm-meter too!!!..i want to make it clear that it is my ANALOG-CIRCUITS course and this is the work of my FIRST lab session!!!so all your replies are fine but my problem still is unsolved..
so i want to make it clear and in simple words
Reb(EMITTER-BASE RESISTANCE)>Rcb(COLLECTOR-BASE RESISTANCE) in case of NPN
Reb(EMITTER-BASE RESISTANCE)<Rcb(COLLECTOR-BASE RESISTANCE) in case of PNP????
what is the reason behind that???
this has been checked by ANALOG MULTIMETER!!!!

12. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
The answer has been given, there is no set resistance number to give. It simply isn't clear cut. Different ranges will give different numbers, different make/models of meters will vary too.

Mar 15, 2008
116
0