why do we need a resistor connected to the base of the transistor?
To set the base current to the required value for your application.
birkomester:
why do we need a resistor connected to the base of the transistor?
For a silicon BJT, the base-emitter voltage is about 0.7 volts. If you try to drive it higher, it will fight you and the base current will rapidly rise (and burn out the transistor).
If you want to drive the transistor with, say, an Arduino output port, the port produces approximately 5 volts.
Think of it this way... imagine you have a cable that you want to pull. The cable has a "stop collar" on it which limits the amount you can pull to 0.7 inches. But, you have a hydraulic cylinder that has a 5 inch stroke that you want to connect to the cable.
Obviously, you can see that either the cable or the cylinder will break.
The solution is to put a SPRING in between the two (this is the resistor). Now, the cylinder can stroke 5 inches, the cable can pull 0.7 inches up to it's stop and the spring "absorbs" the difference.
This is exactly what the base resistor does.
Lastly, imagine you want to control how hard you pull the cable. You simply use a different spring (stronger or weaker). This is the same as choosing a different resistor to give you the base current you need in your circuit.
Hope this helps.
You don't need one if the current source is regulated to a value within the range of the transistor.
jackrae:
You don't need one if the current source is regulated to a value within the range of the transistor.
A transistor is rarely driven by a current source. And, if the OP is asking such a question, he obviously will not know what you are even talking about (no offense to the OP).
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birkomester:
why do we need a resistor connected to the base of the transistor?
Simple: To prevent more than 20mA coming out of the Arduino pin (which might damage it).
(Usually you limit the current to a lot less than 20mA but that's the reason).
Krupski:
jackrae:
You don't need one if the current source is regulated to a value within the range of the transistor.A transistor is rarely driven by a current source. And, if the OP is asking such a question, he obviously will not know what you are even talking about (no offense to the OP).
And here was me thinking all these years that transistors (BJT) were current amplifying devices.
Gain or Beta is I Collector / I Base
Doesn't that mean that a transistor needs current to work and that can only come from a current source.
jackrae:
Doesn't that mean that a transistor needs current to work and that can only come from a current source.
A source of current == a "current source" ?
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jackrae:
Krupski:
jackrae:
You don't need one if the current source is regulated to a value within the range of the transistor.A transistor is rarely driven by a current source. And, if the OP is asking such a question, he obviously will not know what you are even talking about (no offense to the OP).
And here was me thinking all these years that transistors (BJT) were current amplifying devices.
Gain or Beta is I Collector / I Base
Doesn't that mean that a transistor needs current to work and that can only come from a current source.
Actually no, a BJT is a forward biased diode (emitter-base) with a second junction
to capture most of the current from the diode and inject it into the base-collector
circuit. The current in the emitter depends sensitively and exponentially on the
base-emitter voltage, if driven from a voltage source, and roughly proportional
to the base current if driven by a current source.
For linear amps you tend to want to feed the base a current to get the most linear
behaviour, but other circuits take advantage of the exponential relation of current
to base-emitter voltage (logarithmic amps, analog multipliers, etc).
A powerful way to think of non-linear circuit elements is that they maintain (within
limits) a mathematical relationship or relationships between the various parameters
at their "ports" - this directly leads to ways to calculate circuit behaviour and its
how circuit simulators do things.
jackrae:
And here was me thinking all these years that transistors (BJT) were current amplifying devices.
Gain or Beta is I Collector / I Base
Doesn't that mean that a transistor needs current to work and that can only come from a current source.
It's funny how a lot of people here try to one-up each other with semantics, technicalities and pointless details.
Yes of course the gain of a BJT is Ic/Ib and it's called "current gain". An analog voltage signal applied to the base of a BJT through a resistor creates a varying base CURRENT which is amplified by a factor of BETA and appears as varying collector current with a magnitude of Ib times BETA.
However, the term "current source" usually refers to a circuit that supplies a constant (or adjustable) current. For example, a current source set to 20 milliamps and with a compliance voltage of at least 2 to 3 volts would nicely drive an LED.
However, you don't build a complete "current source" circuit to drive an LED... you just use a resistor and call it "good enough".
Would you use a circuit like this:
...just to drive an LED?
IMHO, replies to OP's should be tailored to the question and to the perceived knowledge level of the poster. Rather than saying something like "you use a resistor to control the current" (which means NOTHING to a person who asks such a question), I think it's better to explain exactly what's going on, and if possible use a MECHANICAL example (which people can much more easily relate to) to describe what's going on.
I've been teaching for over 20 years and I have yet to encounter anyone (who is awake) that can't "get it" after I explain it.
I have to agree with Krupski. There are posters who try to use their knowledge and experience in a pedantic fashion.
My own view , as the OP obviously has no knowledge of how a transistor operates, would be to direct them to Google - typing
"how to use a transistor" will provide all the info they need.
fungus:
birkomester:
why do we need a resistor connected to the base of the transistor?Simple: To prevent more than 20mA coming out of the Arduino pin (which might damage it).
(Usually you limit the current to a lot less than 20mA but that's the reason).
.
That's not why a resistor is needed on the base pin of the transistor, the resistor is there to limit the current flowing into the transistor via the base...
Krupski:
jackrae:
And here was me thinking all these years that transistors (BJT) were current amplifying devices.
Gain or Beta is I Collector / I Base
Doesn't that mean that a transistor needs current to work and that can only come from a current source.It's funny how a lot of people here try to one-up each other with semantics, technicalities and pointless details.
Yes of course the gain of a BJT is Ic/Ib and it's called "current gain". An analog voltage signal applied to the base of a BJT through a resistor creates a varying base CURRENT which is amplified by a factor of BETA and appears as varying collector current with a magnitude of Ib times BETA.
However, the term "current source" usually refers to a circuit that supplies a constant (or adjustable) current. For example, a current source set to 20 milliamps and with a compliance voltage of at least 2 to 3 volts would nicely drive an LED.
However, you don't build a complete "current source" circuit to drive an LED... you just use a resistor and call it "good enough".
Would you use a circuit like this:
...just to drive an LED?
IMHO, replies to OP's should be tailored to the question and to the perceived knowledge level of the poster. Rather than saying something like "you use a resistor to control the current" (which means NOTHING to a person who asks such a question), I think it's better to explain exactly what's going on, and if possible use a MECHANICAL example (which people can much more easily relate to) to describe what's going on.
I've been teaching for over 20 years and I have yet to encounter anyone (who is awake) that can't "get it" after I explain it.
I don't like op-amps... sure they are useful but I'll use transistors when possible, I use them mainly for comparitor circuits, checking voltage level (with a zener)
If you want current regulation a zener and a resistor/transistor would do nicely avoiding an op-amp
birkomester:
why do we need a resistor connected to the base of the transistor?
You only need a base resistor when you operate a transistor in the common emitter mode, and that, as has been said is to limit the base current to protect both the transistor and the arduino output.
However, you do not need a base resistor if you operate the transistor in the common collector configuration sometimes called an emitter follower. This is because any current flowing through the emitter load cause the voltage on the emitter to rise to a point where it is 0.7V below the voltage on the base and prevent any further current flowing. This is a sort of negative feedback on the base current.
batteryman:
I have to agree with Krupski. There are posters who try to use their knowledge and experience in a pedantic fashion.
My own view , as the OP obviously has no knowledge of how a transistor operates, would be to direct them to Google - typing
"how to use a transistor" will provide all the info they need.
What's the point of having this forum if people looking for help are going to be redirected to a search engine?
In my personal opinion, if a member here is going to reply to someone, they should do so with the intention of HELPING that person, not merely getting their post count higher.
So many times I see replies to questions that are not only not helpful, sometimes they are downright rude.
Some people here obviously can't tell that a foreigner is using an online translate to post their question. Instead, they ridicule the poster, tell them not to use "l33t text" and/or give explanations in complex English which usually won't translate back properly.
The end result is another disgusted member who won't come back.
If someone asks "how can I control a relay with an Arduino", a good and proper response is not "use a BJT".
If a person is asking such an obviously basic question, answering them at a level ABOVE where they are at doesn't help them at all, nor does it impress anyone else. All it does is rack up the person's post count and show them for the snob that they are.
IMHO, that kind of thing is pure BS.
OK, rant off.....
cjdelphi:
I don't like op-amps... sure they are useful but I'll use transistors when possible, I use them mainly for comparitor circuits, checking voltage level (with a zener)If you want current regulation a zener and a resistor/transistor would do nicely avoiding an op-amp
Did you honestly completely miss the point of my post?
Krupski:
cjdelphi:
I don't like op-amps... sure they are useful but I'll use transistors when possible, I use them mainly for comparitor circuits, checking voltage level (with a zener)If you want current regulation a zener and a resistor/transistor would do nicely avoiding an op-amp
Did you honestly completely miss the point of my post?
I have a habit of reading just the technical side and ignoring opinion. And well to answer your question, helping people here esp when the person is new here, I think it is better to tell them what they need rather than spell it out for a thankless task, otherwise they wont learn and you'd have done the work for them!
I started out with your exact same opinion, but in the long term it's not helping them improve their skills at all, telling them to google bjt/fet/whatever at least gives them a starting point.
Going back to the original question, it needs to be asked "what are you trying to do with the transistor" ?
Depending on whether you are trying to use the transistor for a linear purpose, or as a current amplifier, or as a switch for a higher-powered device, then the answer is different.
I notice that the OP seems to have gone away.
Yup... Last Active: January 28, 2014, 09:00:52 am