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

[cabraham]

BJT is ultimately charge controlled. The following terminal equations are used to describe bjt:
Ic = beta*Ib
Ic = Ies *exp ((Vbe/Vt) - 1)
Ic = alpha * Ie
A good simulation program or lab tests will affirm that changes in current happen before change in voltage. A p-n junction has some capacitance, as well as inductance. At the frequencies the bjt operates at, capacitance dominates. The *Vbe* cannot change unless *charge* is injected into the b-e junction in the form of base & emitter currents, Ib & Ie. As soon as Ie changes, Ic changes a moment later, then Vbe eventually "catches up".
The easiest way to see this is to observe a simulation of a signal diode, such as a 1N914B, or 1N4148.
A simulation is attached. Notice how current exhibits the stair case function, while voltage gradually rises to reach the new value. The diodes voltage is determined by its current, not vice-versa.
An i-v graph of the 1N914B diode is attached. The Vbe "barrier value" depends on forward current. At 10 uA current, Vbe is just under 0.40 volt at room temp.
The Shockley diode equation simply describes the relation between I & V. It can be written 2 ways:
1) Is = Is * exp ((Vd/Vt) - 1),
2) Vd = Vt * ln ((Id/Is) + 1).
Just as Id = f(Vd), Vd = f(Id).
For further study, please search using these terms:
Diode reverse recovery
Diode forward recovery.
Finally, we can discuss *Early effect.* it is true that collector current is *slightly influenced* by Vce, collector emitter voltage, but this is relatively small. A 20% increase in Ie results in about a 20% increase in Ic.
But a 20% increase in Vce results in an Ic change *much less* than 20%.
To claim that the bjt is slightly "voltage controlled by Early effect" is not wrong. The key word here is "slightly".

 

[LvW]

The Shockley diode equation simply describes the relation between I & V. It can be written 2 ways:
1) Is = Is * exp ((Vd/Vt) - 1),
2) Vd = Vt * ln ((Id/Is) + 1).

Yes , no doubt about it. So what?
Do you mean to say that when you rearrange an equation, cause and effect are also automatically swapped?

 

[cabraham]

Yes , no doubt about it. So what?
Do you mean to say that when you rearrange an equation, cause and effect are also automatically swapped?

No! I am saying that cause & effect cannot be determined from this equation. I attached simulation showing I-V response to a staircase generator function. The plot waveform gives insight into which quantity changes first, & which responds to said change later.

 

[LvW]

No! I am saying that cause & effect cannot be determined from this equation. I attached simulation showing I-V response to a staircase generator function. The plot waveform gives insight into which quantity changes first, & which responds to said change later.

I totally agree with you because cause& effect must not automatically derived from any equation.
This is the reason why I only have mentioned physical explanations, real observations and corresponding design techniques in my argumentation (see point 4.) in my post #25).

 

[Averagesupernova]

@LvW & @cabraham I hate to sound like a jerk but I think it's best to take your level of discussion out of this thread. Some guidance on basic circuit analysis is all that is necessary. At this point, whether the chicken or the egg came first just doesn't matter.

 

[LvW]

Averagesupernova - with all respect: What is wrong with my "level of discussion"?
The questioner wants to know how a transistor works and how to treat a simple gain stage.
And he was confused about the various views.
Please, can you show me in which contribution I did not try to clear his confusion?
For my opinion it is of fundamental importance to know that there are two different methods which describe the basic function of the transistor - and I have tried to explain which method is in accordance with all the observations and design rules.
And - I think - this has nothing to do with a "chicken-or-egg" problem.
In one of my last contribution (post#30) I have listed the most important steps for designing a gain stage.
Not in accordance with the subject of discussion? Did I not present some "guidance on basic circuit analysis" (your words) ?
So - what is the problem with my "level of discussion"?

 

[Averagesupernova]

So - what is the problem with my "level of discussion"?

Too much not yet relevant information. Why confuse the op even more than they already are?

 

[DaveE]

It's just that transistor behavies differently and wierdly for me. I can't predict anything what happens here

Which is why I suggested that you simply replace the transistor with a simple model with resistors, voltage source(s), and one very important dependent current source, as in my previous post. Those models are VERY predictable, they should be simple to understand by themselves. Then it is just a network that you can solve without regard to how transistors are weird. That is what everyone does, except you, I guess.

You're not getting anywhere saying your confused about transistors. At some level, we are all confused; they are complicated. Redraw the schematic (sorry I'm having a hard time following which one were on about now) with the transistor replaced. Solve it, think about the solution.

 

[DaveE]

What is wrong with my "level of discussion"?

The OP is having trouble with the simplest of linear transistor models, he isn't ready for exponentials. The first step isn't accuracy, it's basic understanding.

 

[Xenon02]

So do you get confused if you draw in a resistor from base to emitter in the second schematic in post #13? Think carefully here.

I've triad to Understand the Re connected to transistor. But I had some problems with it because when i have also RB connected to transistor then Vb and Ve isn't easly determined the only know factor is that Vbe=0, 7V. But what is Vb and Ve value? I don't know.

Because if I know the Vb the i know the current Ib without it I don't know the Ic and I don:t know the current Ie which is in Ve. Or Rather it flowes through Re this current Ie.

Too much not yet relevant information. Why confuse the op even more than they already are?

The OP is having trouble with the simplest of linear transistor models, he isn't ready for exponentials. The first step isn't accuracy, it's basic understanding.

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.
In tutorials or in the websites they say that resistor limitu the current, follow the basic Rules such as 0,7V and done.
Usually what I hear from others is that it is easy only Basics are Requied to Understand every Circuits and from that I usually feel stupid.

So maybe somebody can tell me the basics? Use my pictures to show me that those basics Apple there as well. I need guidance because those transistors are Tricky for me. I want to use them in audio or in LED Circuits but I don't get it.

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

Which is why I suggested that you simply replace the transistor with a simple model with resistors, voltage source(s), and one very important dependent current source, as in my previous post. Those models are VERY predictable, they should be simple to understand by themselves. Then it is just a network that you can solve without regard to how transistors are weird. That is what everyone does, except you, I guess.

You're not getting anywhere saying your confused about transistors. At some level, we are all confused; they are complicated. Redraw the schematic (sorry I'm having a hard time following which one were on about now) with the transistor replaced. Solve it, think about the solution.

I tried to use this simple model woth divider and with Darlington but i Don't get it.

Can someone please show me how to Approach it with pictures? I learn better with them. Thank you very much and sorry.

Also

Quote Xenon:
Voltage is also involved as I can understand ?
That's why many things are mixing up in my head ...
......
So yea from the quotes I have used here maybe you can understand why I am confused ?
I can't understand them from the pictures of transistors tutorials.
___________________________________________________________________________________________________________________
Xenon 02, I can fully understand that you feel confused.
Let me explain:
1.) It can be shown, prooved and verified that the emitter- and (with it) the collector current is controlled solely by the voltage Vbe. This is according to the well-known Shockley equation Ie=f(Vbe) with Ie=Ib+Ic.
The base current is nothing else than a very small part of Ic (and must be seen as a kind of non-ideality or an unwanted side effect).

2.) The well-known equation Ib=Ic/B is, of course, correct but if used in the form Ic=B*Ib it must not be interpreted as a relation between cause and effect.

3.) The problem now is two-fold:
(a) In some books and other contributions (papers and internet forums) the BJT unfortunately is described as current controlled (just a claim without any verification).
This is really a phenomenon - and it is simply false. This results from a false interpretation of the relation Ic=B*Ib.

(b) There are two mostly used small-signal equivalent diagrams for the BJT and one of the two contains the current-controlled view using a current source controlled by B (resp. beta). For some calculations, this works of course and can be used (because the formula Ic=B*Ib is correct) - however, this diagram must not be used as a basis for explaining how the BJT physically works.
In contrast, the other small-signal representation with a transconductance gm in the output circuitry mimics the physical truth.
Remember: Transconductance gm is the slope of the Ic=f(Vbe) characteristic gm=d(Ic)/d(Vbe). This results in gm=Ic/Vt and - surprisingly - for gain calculations, also the current-control defenders are using this expression (which results from the voltage-control function!)

4.) Resume: For some calculations you can use the current-controlled view (sometimes simpler) - however, if you want to understand the working principle of transistor-based circuits you will have no success with current-control (Why emitter resistor? Why low-resistive base divider? How works the current mirror? What is the Early effect? What means the tempco of -2mV/K ? Why Vbe<0.6V for class B-operation?...).
There are many effects and observations (and circuit principles) that can be only explained with the voltage-control view.

Sorry LvW it is very complex i thought that transistors are more simple ;)

I don't mean anything bad from that. It is just that those Rules makes things more complicated for me.

Maybe a different Request from everybody. Sorry again for being a stubborn person. Can we start from the beggining? Like Understand the basics and progress a bit to the pictures from my 1st post? Again I'm very sorry I want to Understand transistors as a whole to have less problem in the future. Thank you everybody and again sorry @DaveE @LvW @Averagesupernova .

 

[Xenon02]

Also @DaveE @LvW @Averagesupernova @cabraham
I would like to apologize if my comments were rude in any aspect. I can be unsensitive sometimes.
But in overall, maybe I just don't understand the basics at all about transistors and it's usage in the circuits.
That's why I would be very gratefull if you could tell me the exact basics how it works etc (active, saturation,cutoff) for example like explaining the circuits I have posted or use pictures to tell me the basics and how to use them in more complex circuits ? Because as I have mentioned I don't know what to focus on. Because there are so many things to focus on. Like Ib, Ic, Vb,Ve,Vc because in circuit from post#13 Ve and Vb can change all the time they are not static like in post#10.

Sorry and thank you.

 

[Averagesupernova]

@Xenon02, your confusion is understandable. A bipolar transistor can be a puzzle. You need to realize that the schematic with the push button and LED is NOT anything more than two switches. I know the voltage divider on the left is throwing you. The bottom resistor R2 really doesn't need to be there. It is common practice to the include it because with the push button in the open position the base would be left floating. Bad practice. I think you are able to answer the question of what is the base to emitter voltage when the push button switch is closed. You know about the .7 volts across the base emitter junction, so tell us what is happening with T1.

 

[Xenon02]

@Xenon02, your confusion is understandable. A bipolar transistor can be a puzzle. You need to realize that the schematic with the push button and LED is NOT anything more than two switches. I know the voltage divider on the left is throwing you. The bottom resistor R2 really doesn't need to be there. It is common practice to the include it because with the push button in the open position the base would be left floating. Bad practice. I think you are able to answer the question of what is the base to emitter voltage when the push button switch is closed. You know about the .7 volts across the base emitter junction, so tell us what is happening with T1.

About the circuit with LED, I heard that pushing button is like a switch but only when the transistor is in saturation. But what if it's in active :D
I mean that those transistors are a real puzzle for me when it comes to how the transistors behaves and why it behaves that way. Because when I tried to predict it with divider or in many different circuits I failed :D Because many factors were overhelming me. But this is only DC, if I added there also AC ... My god ...
About your question. The funny fact is that for T1 the emiter doesn't have any resistor so Ve must be equal 0V. So Vb is equal 0,7V. Even if it's a divider then on the 10k ohm it has 0,7V. Which is really weird I guess ? Why do I say it like that because what is this resistor really doing ? does it have effect on how much Ib the transistor receives ? But okey comming back to it. All I know now is that the T1 is working. It is probably working in saturation but it's hard to say because if we want saturation then Vce = 0,2V. There is current comming from PNP and current from R3. So yea, also in the collector there is another voltage divider ... Which changes the Vb of R4 and R3. Ahhhh. Also I know that there is something like Ic_max the maximal value that makes transistor saturated, but since PNP can provide any value for collector of T1 because the base of T2 isn't limited with any Rb_2.
Well I would also add that there is voltage divider on T1, usually there should be on 10k ohm Vcc/2 right ? Well at the end the 10k ohm won't have Vcc/2 but will have 0,7V. Why do we do that ? This resistor needs also current it takes some current from T1 to make this 10k resistor have 0,7V. I mean what is the purpose ? If I will make this resistor bigger or smaller will it affect the T1 ? If for example I changed the resistor from 10k to 500k then ? or from 10k to 10 ohm. Just giving random numbers.

 

[Averagesupernova]

Even if it's a divider then on the 10k ohm it has 0,7V.

You're starting to see the light.
-
Hint: T2 is the exactly the same as T1 only upsidedown and a PNP. Imagine T1 being a push button switch and controlling T2. Simplify the circuits into smaller chunks and you will understand them. BTW, I told you why the resistor R2 is there.

 

[Xenon02]

You're starting to see the light.
-
Hint: T2 is the exactly the same as T1 only upsidedown and a PNP. Imagine T1 being a push button switch and controlling T2. Simplify the circuits into smaller chunks and you will understand them. BTW, I told you why the resistor R2 is there.

I mean in T2 it looks similat but it is connected to the collector of T1.
About the resistor R2 I know you mentioned that it must be there so that the transistor is not "floating" ? I don't understand this meaning but okey. But notice that it is still a voltage divider at least it looks like that.
So yea, basically R2 should have Vcc/2 but it has 0,7V. Even though Vcc/2 is connected directly to the base of the transistor it drops weird ...
Also what happens when I reduce the value of R2 or increase ? Like R2 = 100 and R2 = 100k.
Because even if this resistor must be because it must be it takes some current from transistor or something like that. Because what if this R2 had I don't know 0,2V then it won't rise to 0,7V ? As a divider of course. So many questions for transistor ...

Edit:
Maybe I am wrong but this R2 changes the Ib as I know right ? Even if it's for 0,7V or something like that but the value of R2 changes the Ib of the transistor ? So it somehow affects on the behavior of the transistor.

 

[Averagesupernova]

@Xenon02 you need to brush up on basic circuit analysis, Kirchoff, etc. The diode that is the base emitter junction will never under any circumstance drop more than about .7 volts until it is damaged from over current or excessive reverse voltage. Because it is hooked to a voltage divider does not change this. The diode is the dominant part. Hooking a resistor in parallel will not cause the voltage to rise. If anything it causes it to lower slightly. A small enough resistor such as zero ohms and yes obviously the voltage will drop to zero.

 

[Xenon02]

@Xenon02 you need to brush up on basic circuit analysis, Kirchoff, etc. The diode that is the base emitter junction will never under any circumstance drop more than about .7 volts until it is damaged from over current or excessive reverse voltage. Because it is hooked to a voltage divider does not change this. The diode is the dominant part. Hooking a resistor in parallel will not cause the voltage to rise. If anything it causes it to lower slightly. A small enough resistor such as zero ohms and yes obviously the voltage will drop to zero.

Okey so the voltage drops to 0,7V in the R2.
But R2 affects the current in Ib for the T1 as I know ? If I rised the value of R2 the current IB will change ? Or not ? I thought of it for a moment, because some current from R1 will go to transistor and some to R2.
So changing the value of R2 must do something right ?
Because you mentioned that it is there just to not make transistor float when the button is not pushed. But adding R2 changes a bit the Ib ? Or maybe it doesn't affect the Ib ?

Sorry if I am being to inquisitive, because it affects the value of Ic, and then even if there is a resistor for T2 that has 0,7V if there is not enought current Ic then the voltage will drop this 0,7V I guess ?

It is not typical for me yet :D That I see transistor without Rb just with voltage dividors.

 

[Averagesupernova]

But adding R2 changes a bit the Ib ? Or maybe it doesn't affect the Ib ?

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.
-
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.

 

[Baluncore]

But R2 affects the current in Ib for the T1 as I know ? If I rised the value of R2 the current IB will change ? Or not ? I thought of it for a moment, because some current from R1 will go to transistor and some to R2.
So changing the value of R2 must do something right ?

Re: The diagram in post #24.
R1 limits base current of T1 when S1 is pushed on.
R4 limits base current of T2 when T1 is on.
R5 limits LED1 current when T2 is on.

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 V_BE, the 60 uA will be insignificant when on.

 

[Tom.G]

@Xenon02 ,

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!

Since a supply voltage was not given, assume a 10V supply.

PART A:
1) With the button pushed, what is the voltage at the top of R2? (Ohms Law)

2) You stated that you knew Thevenins Theorem.

2A) Based on that statement, what is the equivalent source impedance for the voltage at the top of R2?​

PART B
Now connect the Base and the Emitter of T1 per the original diagram. (we don't care about the collector right now, so you can leave it not connected if you wish.)

3) What is the voltage at the Base of T1?

You now know the (nominal or unloaded) voltage at the top of R2 (Ohms Law from PART A above) and its equivalent source impedance (Thevenin from PART A above) AND the new voltage when the base of T1 is connected.​

4) You stated you knew Ohms Law.

With the equivalent impedance from #2 above, and the change in voltage (#1 & #3 above) at the top of R2.​

4A) How much Current is flowing from the bottom of R1?​

Discussion:
Because it is so low, in Almost all cases the current thru R2 can be ignored when the transistor is connected. For simplicity, we will therefor assume that all of the R1 current flows to the T1 Base.

You have now found the T1 Base current (#4A above). When you multiply that current by the β, Beta, or h_FE of T1, the result will be the Maximum current you can expect to flow thru the T1 Collector (providing you don't burn it up!).

Now for a change of subject:
Specifically the magic hidden inside a transistor (aka 'How The Darn Things Work".)

See post https://www.physicsforums.com/posts/6380238/ and the links therein. The GE Transistor Manual is still available as a free .PDF download, the hard copy RCA Transistor Manual (Technical Series SC-13) is listed for under USD $5 at both Amazon.com and Abebooks.com.

[possible rant]
There has been some discussion in this thread of a transistor being a Voltage-Controlled device versus a Current-Controlled device.

My take on the controversy is that both views are valid in that they give equivalent or identical results. Since transistors have many nonlinear and interacting characteristics , the calculations seem somewhat easier using the Current-Controlled approach for pencil-and-paper calculations; mostly because some of the non-linearities are hidden.

The GE Transistor Manual referenced above addresses both Current- and Voltage-Control approaches.
[/possible rant]

Cheers,
Tom

 

[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