Maker Pro
Maker Pro

Transistor bias configuration

Old Steve

Jul 23, 2015
734
Joined
Jul 23, 2015
Messages
734
Then of course there is a few types of transistors all having different characteristics, current bias to voltage bias.
Bi-polar.
Mosfet.
BJT etc.
M.
BJT = bi-polar (Not the disorder):- Bipolar Junction Transistor
 

M.Sridhar

Sep 16, 2015
10
Joined
Sep 16, 2015
Messages
10
Actually I am new to this forum and from general instructions I posed my question and I apologize of something went wrong
And when it comes to my question there is an experiment in my EDC Lab named Transistor in self bias condition.
I remember reading for effective working of transistor its i/p must be forward biased and o/p(collector- base) must be reverse biased.So I didn't understand what is transistor in self biased state.
In response to it my lecturer said that transistor transform itself when it is in that mode
 

davenn

Moderator
Sep 5, 2009
14,254
Joined
Sep 5, 2009
Messages
14,254
Actually I am new to this forum and from general instructions I posed my question and I apologize of something went wrong

hi ya, no, you didn't do anything wrong :)

of recent months there's been very intense discussions on how a transistor operates
Martin ( MArtaine2005) posted a link to that thread earlier in this thread

personally, I have no idea who is right and who is wrong in that other thread LOL


Dave
 

Martaine2005

May 12, 2015
4,932
Joined
May 12, 2015
Messages
4,932
How can a transistor 'transform itself' ???
They do diet! This one shed a little weight.

exploded_2sc1318.jpg


Martin
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Joined
May 8, 2012
Messages
4,960
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.

Chris
 

Ratch

Mar 10, 2013
1,099
Joined
Mar 10, 2013
Messages
1,099
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,

Yes, we do try to answer OP's questions thoroughly and completely, don't we?

do not use EP as a springboard to feed ones overblown ego

Where is that happening here? A passionate argument about technical details is not an ego trip. I don't see any disparagement, bragging, or any other forms of self-aggrandizement going on.

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.

The way a transistor works is not a typical homework assignment. It is a lecture topic. But surely, a topic's treatment should be judged on its veracity, not its classification.

Ratch
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Joined
May 8, 2012
Messages
4,960
Gee! Did I quote you but forgot?
"The gentleman doth protest too much, methinks"!

Chris
 

Joe21

Sep 20, 2015
1
Joined
Sep 20, 2015
Messages
1
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.



collector-feedback-biasing-jpg.22071



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.
 

Martaine2005

May 12, 2015
4,932
Joined
May 12, 2015
Messages
4,932
Nice first post Joe. Shame you was bored enough to register..
I am that blunt knife!!
But my blunt blade doesn't tell the sharp blade what to cut!!
Just a note..;)

Martin
 

Ratch

Mar 10, 2013
1,099
Joined
Mar 10, 2013
Messages
1,099
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.



collector-feedback-biasing-jpg.22071



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.

You are beating up on that abreviated circuit. Obviously, it only shows the DC bias components of its operation. To make it operational for AC, a capacitor will be placed between the voltage input and the base terminal. The feedback resistor Rb will be split and a capacitor inserted between the two resistors comprising Rb so that AC passing through the Rb will be grounded and not degenerate the signal. Not mentioned is that the Rc/Rb ratio should be as large as possible to avoid thermal runaway. Do you really want me to prove that? Add those components to the circuit and you have a practical bias method.

Ratch
 

Old Steve

Jul 23, 2015
734
Joined
Jul 23, 2015
Messages
734
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.
 

Ratch

Mar 10, 2013
1,099
Joined
Mar 10, 2013
Messages
1,099
From what I've seen, everybody likes beating up on everybody here and in the other transistor thread.

Correct, if we see something that we think is not correct, most of us take action to point it out ASAP.

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.

Then the OP's question was answered and he could just walk away if he wanted. However, these threads are also for folks other than the OP. They might enjoy a further discussion.

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.

I don't think so. Some of the comments are gratuitous, non-technical, and silly, but that should not mess up anybody's mind. The technical comments are basically correct if not necessarily complete. It is up to th OP to ask questions if he wants to understand something past his initial inquiry.

Ratch
 

Minder

Apr 24, 2015
3,478
Joined
Apr 24, 2015
Messages
3,478
I don't think so. Some of the comments are gratuitous, non-technical, and silly,
Ratch
Now, I’ve noticed a tendency for this thread to get rather silly!:(
(M.P)
 

Attachments

  • Colonel.jpg
    Colonel.jpg
    6.3 KB · Views: 73
Last edited:

Arouse1973

Adam
Dec 18, 2013
5,178
Joined
Dec 18, 2013
Messages
5,178
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 its that boring Laplace then dont get involved, its that simple.
Adam
 

Arouse1973

Adam
Dec 18, 2013
5,178
Joined
Dec 18, 2013
Messages
5,178
Now, I’ve noticed a tendency for this thread to get rather silly!:(
(M.P)
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
 

Minder

Apr 24, 2015
3,478
Joined
Apr 24, 2015
Messages
3,478
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.?:confused:
M.
.
 
Top