Martaine2005
- May 12, 2015
- 4,932
- Joined
- May 12, 2015
- Messages
- 4,932
Chelsea, Chelsea..
Oops wrong forum.
Martin
Oops wrong forum.
Martin
BJT = bi-polar (Not the disorder):- Bipolar Junction TransistorThen of course there is a few types of transistors all having different characteristics, current bias to voltage bias.
Bi-polar.
Mosfet.
BJT etc.
M.
Actually I am new to this forum and from general instructions I posed my question and I apologize of something went wrong
I'm glad I didn't have your lecturer when I was a student! I think Old Steve has it right, in post #13.my lecturer said that transistor transform itself when it is in that mode
I agree. How can a transistor 'transform itself' ???I'm glad I didn't have your lecturer when I was a student!
They do diet! This one shed a little weight.How can a transistor 'transform itself' ???
Well, I followed the link that Martin posted (I wish I hadn't) and found that I totally missed season 1 of this bloody thriller. What I found in that thread was reminiscent of another electronics forum that I left when I found EP, where members are typically friendly, helpful and above all,
do not use EP as a springboard to feed ones overblown ego
About the only positive thing I can say about that thread is that it's located in the Homework section; where it rightly belongs. This topic belongs there too.
Apparently I was bored today and started looking on this forum.
Thanks for the entertainment.
lets assume the NPN transistor in the schematic is a silicon transistor
a fully forward biased silicon B-E junction has approximately 0.7 V while a germanium device would be 0.3 V
most people don't understand that biasing a transistor usually refers to the DC operating point where the transistor is in the active region
interpretation - the transistor is not fully on or off but operating somewhere in between
You can argue all you want rather the current or the voltage at the base is controlling the C-E current flow.
lets apply a DC power supply voltage directly to the base at 0.2 V (VIN)
lets assume this power supply is capable of supplying infinite current
infinite current = 0.2 V /0 ohms
what will happen?
not much because the base voltage is not high enough to turn on the NP junction
interpretation - not enough potential difference across the P-N junction
lets adjust the power supply to 10 V
what will happen?
instant smoke
Ib = (10 -0.7) V / 1 ohm (assumes a forward biased diode as a very low resistance)
9.3/1 = 9.3 amps
not many diode junctions could take that kind of current thus smoke
so I do think the voltage at the base has something to do with turning on the transistor
next discussion
if you are just talking about biasing a transistor in the active region
the simplest way to do this without any calculations is throw out the feedback resistor and create a resistor voltage divider,
That is two resistors,
R1-1 connected to VCC
R1-2 connected to R2-1 and the base
R2-2 connected to ground.
Use a 1K ohm resistor for R1.
Use an adjustable resistor for R2.
vary R2 until VCE is about one-half of Vcc (if Vcc=12 then measure Vce=6)
Using ohm's law you can calculate exactly how much current is going through the biasing resistors and the base.
what is the gain?
looking at a transistor datasheet, a DC and AC hfe is specified
interpretation - current gain of the transistor in the common emitter configuration
The DC hfe is typically between 50 and 200 for a transistor.
the gain is somewhat fixed at a particular temperature (junction) and collector current
lets assume that Ib = 1ma so with a transistor gain of 100 the Ic would be 100 mA
or will it
it will only be 100ma only when the value of VCC and Rc are capable of delivering that much current. Make Rc infinite and there will be no Ic but the B-E junction will still be fully forward biased.
this scenario demonstrate that circuit gain is also independent of the transistor gain
on the other hand when Vcc (large enough) and Rc (small enough) to allow the current flow then the gain (hfe) of the transistor will determine the circuit gain Ic/Ib
DC Stability
unfortunately the hfe for a transistor is temperature dependent (junction) and as the transistor warms up the DC operating point will drift.
Thus there is usually some sort of feedback such as adding a small Re to stabilize the DC operating point or having resistor across the B to C terminals.
the schematic fragment shown above is only good for an academic discussion
although Rc could be the load but applying any voltage to Vin from a low impedance DC source will negate Rb.
I hope this has been useful.
I have a football game to watch.
Joe
Little knowledge is more dangerous than no knowledge.
From what I've seen, everybody likes beating up on everybody here and in the other transistor thread.
That circuit was posted in direct response to the OP's original query about self-biasing. It was never intended to be any more than that.
Anyway, this whole thread has become so messed up that the poor OP won't know if he's coming or going any more. He's just trying to get his head around the basics, to clarify what his confusing lecturer has been saying, not to learn all transistor theory in one big rush. He'll leave here even more confused than when he arrived.
Now, I’ve noticed a tendency for this thread to get rather silly!I don't think so. Some of the comments are gratuitous, non-technical, and silly,
Ratch
If its that boring Laplace then dont get involved, its that simple.There are no professional gladiators in this Colosseum, except, possibly for the moderators. Entertainment here is provided by spectators willing to jump down on the blood & sand to make the good kill. Also, the zombies will rise again! How many times do you need to kill the same zombie before it becomes just a boring chore?
If you asked somone how something worked would you expect them to lie to you? Or just read something from a book that might be wrong. If everyone thinks that what is writen in a book is right even though its wrong, is that ok?Now, I’ve noticed a tendency for this thread to get rather silly!
(M.P)
I assume being of UK origin you are familiar with Monty Python?If you asked somone how something worked would you expect them to lie to you? Or just read something from a book that might be wrong. If everyone thinks that what is writen in a book is right even though its wrong, is that ok?
Adam
I assume being of UK origin you are familiar with Monty Python?
The post was an attempt at levity and a response to a previous comment.?
M.
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