I don't understand transistors in combinations .... Or maybe basics ....

[LvW]

That's right I have problem with basic Understanding of transistors.
I know only the thing I have showed from the tutorials. Like the Picture nr. 1 from post#1.

I just don't get it why transistor works from more complex situations such as Darlington or Voltage divider.
.....
So maybe somebody can tell me the basics?
......
This whole VC Ve Vb and currents such as Ib Ic makes me confused I don't know how to Approach it or how to Understand someones Circuit that has transistors. Or how to create my own Circuit wity transistors
......
Sorry LvW it is very complex i thought that transistors are more simple ;)

Xenon02 - no, it is not "very complex".
Although I have been accused here of giving irrelevant information ("too much not yet relevant information. Why confuse the op even more than they already are" ), I am writing another post.
I have tried to clear your confusion, because it is really a problem for the newcomer when there are two different explanations of how the transistor works.
I have more than 30 years of teaching experience in analog electronics, so I know where the problems in understanding are.
Before you think about dimensioning of external components or even more complicated circuits (Darlington etc) you have to understand the basic operation of the transistor - it is not that complicated.

In a nutshell:
* As with the classical pn diode, a voltage Vbe (about 0.6...0.7 volts) applied externally to the base-emitter diode "opens" the pn junction between B and E and produces an electron current Ie from the emitter toward the base.

* However, since the base layer is extremely thin, most of the electrons - under the influence of the positive collector voltage (several volts) - are attracted from the collector and move through the base region. They form the collector current Ic.

* Only a very small fraction (1% or less, depending on the technology) flows out through the base terminal and forms the base current Ib (Ib=Ie-Ic)

* Any change in the voltage Vbe (signal input voltage) changes the current Ie (as in any pn-diode) and thus also Ic.
At the same time, the proportion of the base current remains practically constant (1% or less).
This base current is a kind of "byproduct" and has no controlling function. This current is responsible for the transistors input resistance (relatively small if compared with the FET).

* The fluctuations of Ic are converted into the signal output voltage at a collector resistor Rc.
__________________________
This is my brief explanation of the principle of operation of the transistor.
Perhaps it helps a bit.

These properties of the transistor are used to design a simple gain stage (as shown in your first post, figure 2, with a base voltage divider) - in my post#30 I have listed the most important steps.

 

[Xenon02]

Yes it affects base current. If you fully had your head wrapped around Kirchoff and basic circuit analysis you would know that. There's no shame in having trouble with Kirchoff, etc. but please don't act like it doesn't apply to you or you've got it handled because you don't.

I won't deny that Kirchoff can mix up with many things :D I was thinking more ideology Like Vb certain value so Ve must have also a certain value following the rule of Vbe = 0,7V :)
But yea this is a diode. I guess.

You seem concerned about R2 changing the base current a little bit. Well, it does. In the case of transistors being used as switches, we design it so it doesn't matter. We can suck a fair amount of current away from the base and the selected collector load is such that it doesn't matter.

Sorry for being so obsessed with those resistors etc. I was taught that adding any new elements will cause big changes to the circuit. That's what at least my teacher said.

R2 and R3 (connected between base and emitter), are NOT there to be part of a base-bias voltage divider. When S1 or T1 is off, R2 and R3 will conduct leakage current away from the base without dropping sufficient voltage to turn the BJT base on to any extent.
0.1 V / 10 k = 10 uA maximum leakage when off.
0.6 V / 10 k = 60 uA wasted when on.
Since the supply voltage is many times VBE, the 60 uA will be insignificant when on.

Even though they look like voltage dividers ?

I highly recommend that you draw the schematic as you progress thru this post AND DO THE ACTUAL CALCULATIONS FOR EACH STEP.Please post your results. That way we are all on the same page of the problem, rather than jumping around trying to answer multiple questions that keep changing.

This note refers to the diagram with R1, the push button, R2.

IGNORE THE REST OF THE CIRCUIT FOR NOW!

Okej. But can I also use picture nr.1 from Post#1 as a reference to why I am confused ? Because all my confusions came actually from this 1st picture.

Those are results, I wasn't sure with Part A because I didn't understand if I was supposed to calculate getting rid of R2 or with R2.
But okey I can see that using this equivelant will make something like this :

But I do have some questions though.
If I can refere to the 1st picture of the Post#1.
I was taught there that Rb controlls or maybe better it limits the current that transistor takes from voltage supply and makes it into Ic.
So in this example I've used the equivelant but in reality the transistor doesn't have any RB or Should I consider R1 as a RB ? Sorry for saying dumb things I am stuck a bit with the 1st picture from Post#1. That there was always Rb.

Second question.
I can undestand that the diode is a priority and the R2 will have 0,7V because of the diode but if there was RE then the voltage on R2 would be more difficult to predict ? I ask it out of making circuits. I'll show later after some analizing a circuit that consist Re and other stuff.

Third question
Okej so the current is calculated for Ic and Ib. But adding new elements to collector won't change anything ?
Because for now I only understood the resistors examples (maybe except the one with Rb and Re, where I couldn't define the Vb and Ve) Adding now transistor won't make things a bit worse ? Now PNP can provide a lot of current. I am not confident when it comes to Adding transistor to another transistor. It is just weird because transistors are usually controlled by resistors but adding transistor to another transistor. I don't get it.

Before you think about dimensioning of external components or even more complicated circuits (Darlington etc) you have to understand the basic operation of the transistor - it is not that complicated.

In a nutshell:
* As with the classical pn diode, a voltage Vbe (about 0.6...0.7 volts) applied externally to the base-emitter diode "opens" the pn junction between B and E and produces an electron current Ie from the emitter toward the base.

* However, since the base layer is extremely thin, most of the electrons - under the influence of the positive collector voltage (several volts) - are attracted from the collector and move through the base region. They form the collector current Ic.

* Only a very small fraction (1% or less, depending on the technology) flows out through the base terminal and forms the base current Ib (Ib=Ie-Ic)

* Any change in the voltage Vbe (signal input voltage) changes the current Ie (as in any pn-diode) and thus also Ic.
At the same time, the proportion of the base current remains practically constant (1% or less).
This base current is a kind of "byproduct" and has no controlling function. This current is responsible for the transistors input resistance (relatively small if compared with the FET).

* The fluctuations of Ic are converted into the signal output voltage at a collector resistor Rc.
__________________________
This is my brief explanation of the principle of operation of the transistor.
Perhaps it helps a bit.

The first star, got it. Vbe is a diode.
Second star I guess I get it ?
Third star I guess that Ib is really small.
4th star I don't know ?
5th star I think I get it ?

Those are the basic principles ? Isn't it like I said in my post#1 ? That we need to limit the Ib or something like that I said there.

 

[Averagesupernova]

You're saying the voltage across R2 is 5 volts and the base-emitter voltage is .7 volts. Am I misinterpreting this?

 

[LvW]

4th star I don't know ?

As I am from Germany (english is not my mother tongue) - what means "dunno"?

 

[Xenon02]

You're saying the voltage across R2 is 5 volts and the base-emitter voltage is .7 volts. Am I misinterpreting this?

I understood from Tom that I was supposed to calculate the voltage across R2 when there is no transistor connected to it.

As I am from Germany (english is not my mother tongue) - what means "dunno"?

I'm sorry it means : I don't know.

 

[LvW]

OK - thank you.
Another question: When you have some problems with transistors (working principle and analyses of simple circuits) why do you want to start with circuits containing 2 transistors ? The same applies to the transistor in the handwritten figure (in green). This is an artificial arrangement without any practical relevance.
Recommendation: Let´s discuss the basic amplifier stage (your first post, second figure)

 

[Baluncore]

Even though they look like voltage dividers ?

They are not voltage dividers because the base voltage is fixed without reference to their ratio. They are independent devices performing different tasks. One half, R1 or R4, is limiting base current, the other half, R2 or R3, is mopping up any stray leakage.

 

[Xenon02]

OK - thank you.
Another question: When you have some problems with transistors (working principle and analyses of simple circuits) why do you want to start with circuits containing 2 transistors ? The same applies to the transistor in the handwritten figure (in green). This is an artificial arrangement without any practical relevance.
Recommendation: Let´s discuss the basic amplifier stage (your first post, second figure)

I fully agree.
I want to feel those basics in my veins !
Oh and to answer your question about two transistors, I thought that I understand the basics so I tried with something harder.
So let's say that I have analized first picture from the first post in which were RB, RC. Pretty basic. I think you have seen this analize of mine if not then post#10, you can agree or disagree with things I've said there.

So I can follow the reccomendation.
I need to get rid of the bad habbits I have abotu circuits. For now my mindset it : One transistor so there must be Rb to controll it and there is Rc or Re to define the max current, but adding other elements such as transistors then I get confused. Transistor is not like resistor that has current limits.

They are not voltage dividers because the base voltage is fixed without reference to their ratio. They are independent devices performing different tasks. One half, R1 or R4, is limiting base current, the other half, R2 or R3, is mopping up any stray leakage.

I mean this is for more practical purpose thore R2 and R3, but I'm looking at it like in the picture nr.2 in Post#1.
But maybe let's start like @LvW mentioned, I need to understand the basics. And see how it works. From simple one, and adding new stuff sich as dividers and etc and analyzing it using the basics from simple circuits.
For new I just see it as a voltage divider, and some changes in it. Because adding new elements to the circuit can change a lot in the whole circuit.
Also my confusion comes also when there is no RB for the transistor like in the dividers or connecting transistor to another transister. Those things confusese me so far.
@Baluncore
As far I think my analyzis from Picture nr1 post#1 (my analizys Post#10) I for now took for granted that all the voltages and current is controlled through Rb and Rc, in which Rb controlls Ib and Rc controlles the Ic_max. Adding transistor makes me think it is not a resistor that has some limits also second transistos is also dependant of resistors. And The divider that doesn't have Rb.
So far my conclusions.

 

[Averagesupernova]

Also my confusion comes also when there is no RB for the transistor like in the dividers or connecting transistor to another transister.

If the current going into the base has to go through a resistor then you can consider that a base resistor insofar as it is limiting base current.

 

[Xenon02]

If the current going into the base has to go through a resistor then you can consider that a base resistor insofar as it is limiting base current.

Yea I tried to think of the upper resistor as an Rb. The thing is that the current is splitting into two pieces.
One piece is going into base which doesn't have Rb and the other piece is going into R2. So not limited Base can drain as much current it wants

 

[Averagesupernova]

So not limited Base can drain as much current it wants

Please explain your thinking here so maybe we can get you on track. What you've said is quite simply wrong.

 

[LvW]

Yea I tried to think of the upper resistor as an Rb. The thing is that the current is splitting into two pieces.
One piece is going into base which doesn't have Rb and the other piece is going into R2. So not limited Base can drain as much current it wants

It would help us if you could say about which circuit you are speaking (drawing).
Otherwise, misunderstandings are unavoidable.

 

[Xenon02]

Please explain your thinking here so maybe we can get you on track. What you've said is quite simply wrong.

Sure thing I'll use pictures to visualize my vision of it.

Basically I see it like that. There is Rb that limits the current from voltage source that is going into transistor because he himself cannot.
When I see the divider that has this current flow :

And the current is splitted into transistor that doesn't have Rb and it splits also to R2.

Transistor without Rb look like that in my eyes :

Of course the part only with transistor that doesn't have Rb. Because here in this picture the transistor can take any amount of current.

 

[Xenon02]

It would help us if you could say about which circuit you are speaking (drawing).
Otherwise, misunderstandings are unavoidable.

I'm sorry I was talking about the circuit with LEDs.
But I would like to stick with your recomendation and start with basics and using these basics understand the further. For example I've made an analize in Post#10 you can check if it's correct.
Because my mind set is stuck in the analizys in post#10 which transistor needs to have Rb and struggles to understand why the divider works when transistor doesn't have this Rb. Or combinations with many transistors.
Let's try from the very beggining and progress.

 

[Averagesupernova]

I've looked at your diagrams in post #63. Equating the second and third diagrams would be flat out wrong.
-
Take the first diagram and add a resistor tied to the base and emitter. Why is it you think that somehow by doing that it is the equivalent of having no resistors at all?

 

[Xenon02]

Take the first diagram and add a resistor tied to the base and emitter. Why is it you think that somehow by doing that it is the equivalent of having no resistors at all?

Add resistor you mean the add it to the 1st picture in post#63 ?
If yes then it would look like that :

Which there is a limit and the transistor that has no limit and another resistor.
Why is it like that ? I don't know because it wasn't covered in any tutorial nor in basics from websites. I just know that Rb there is direct limit for transistor.

 

[LvW]

I'm sorry I was talking about the circuit with LEDs.
But I would like to stick with your recomendation and start with basics and using these basics understand the further. For example I've made an analize in Post#10 you can check if it's correct.
Because my mind set is stuck in the analizys in post#10 which transistor needs to have Rb and struggles to understand why the divider works when transistor doesn't have this Rb. Or combinations with many transistors.
Let's try from the very beggining and progress.

Xenon02 - sorry to say, but it is not easy to discuss with you because you are not very "systematic".
You have agreed to analyze one particular circuit - and what is the result?
Again three different circuits - two of them without any practical relevance.
More than that, you continue to speak very often about a resistor Rb - and we only can guess what you mean. I cannot see any Rb in your drawings.

 

[Averagesupernova]

Add resistor you mean the add it to the 1st picture in post#63 ?
If yes then it would look like that :

View attachment 314904
Which there is a limit and the transistor that has no limit and another resistor.
Why is it like that ? I don't know because it wasn't covered in any tutorial nor in basics from websites. I just know that Rb there is direct limit for transistor.

Rc limits current no matter what. Are you able to see the schematic that you drew on a resistor is the same as the original with the push button as far as base associated components?

 

[berkeman]

Thread closed for Moderation...

UPDATE -- I'm in a PM conversation with the OP about learning resources right now...