Transistors circuit - Basic questions

[ehild]

A FET is a transistor! :p

FETis Field Effect Transistor, with high input resistance.
The one in the problem is bipolar transistor, and it has low input resistance.
A voltage divider is two resistors in series connected to a voltage source and using the voltage between one end of one resistor and the common point of the pair of resistors. A potentiometer can serve as voltage divider.

ehild

 

[NascentOxygen]

A potentiometer can serve as voltage divider.

Indeed it can. A potentiometer can serve as a variable voltage divider.

Potentiometer...not to be confused with a voltmeter...and definitely not to be confused with a voltameter. :shy:

 

[ehild]

Indeed it can. A potentiometer can serve as a variable voltage divider.

Potentiometer...not to be confused with a voltmeter...

which works on the principle of voltage divider.

and definitely not to be confused with a voltameter. :shy:

which is definitely not a voltage divider.

ehild

 

[Femme_physics]

So I talked to my teacher. He told me that I'm wrong and he doesn't know what I'm talking about and that voltage divider would give me the same result for Ib as Kirchhoff laws. I showed him all my papers trying to prove him wrong but he said I'm not using voltage divider correctly, that I should use the formula

R1 x R2 / R1 + R2

To create a new resistor, RBB, that Ib goes through, and then I can just use this resistor while ignoring R1 and R2. The result that turned up is indeed the same result as Ib with Kirchhoff's...

So I don't see how anything you said make sense if we simply didn't use the right formula.

 

[NascentOxygen]

So I talked to my teacher. He told me that I'm wrong and he doesn't know what I'm talking about

What a nasty fellow!

and that voltage divider would give me the same result for Ib as Kirchhoff laws.

That's because, as I explained, you have been using the phrase "voltage divider" to mean something completely different from the usual meaning. (I think we needn't go over all that again.)

I showed him all my papers trying to prove him wrong but he said I'm not using voltage divider correctly, that I should use the formula

R1 x R2 / R1 + R2

To create a new resistor, RBB, that Ib goes through, and then I can just use this resistor while ignoring R1 and R2.

That's what I showed as the Thevenin equivalent, but because you haven't studied that yet, there is no necessity to analyze the circuit that way. Whatever way the circuit is analyzed, if it is valid and is done correctly it will arrive at the same answer. It is all basic mathematics; there is no room for magic or skullduggery.

The result that turned up is indeed the same result as Ib with Kirchhoff's...

Well, it couldn't be called the Thevenin equivalent if it didn't give equivalent results.

So I don't see how anything you said make sense if we simply didn't use the right formula.

I can't see myself in this picture; I'm sure this must be a reference to some other "you".

So, is it all sorted now? Or (I hesitate to ask) still some confusion? :shy:

 

[Femme_physics]

What a nasty fellow!

Well, he said that HE doesn't know what I'm talking about not that I don't know what I'm talking about. He's ok :)

I can't see myself in this picture; I'm sure this must be a reference to some other "you".

So, is it all sorted now? Or (I hesitate to ask) still some confusion?

LOL. All I can do is thank you, you keep me engaged, asking, and curious,and with a supply of intelligent answers. What we did is not called voltage divider then, it's called "Thévenin's theorem", that's the only difference?

 

[NascentOxygen]

LOL. All I can do is thank you, you keep me engaged, asking, and curious,and with a supply of intelligent answers.

Sure don't see myself here, either. Really must be some other "you" this time.

What we did is not called voltage divider then, it's called "Thévenin's theorem", that's the only difference?

Back in this post I stated that it is possible to view the two resistors between Vcc and ground as equivalent to a voltage source in series with a single resistor. When applied to transistor biasing, as here, that Thevenin voltage source is referred to as V_BB. But as someone else had introduced a weird way to name voltage from base to emitter as V_BB I didn't compound the confusion by pointing out that's not right and the symbol V_BB should refer to the Thevenin base voltage (in precisely the same way that V_CC truly does refer to the Thevenin voltage of the collector supply).

I think you should give this topic a rest, and put your energies towards other topics. Come back to this in a few weeks, and with a fresh start you'll wonder how you managed to make such heavy work of it first time around.

You know enough now to be able to sort it out yourself, just pencil, paper, calculator and Ohms Law, and a quiet location away from distractions.