All About Circuits Forum BJT basics
 User Name Remember Me? Password
 Register Blogs FAQ Members List Today's Posts Search Today's Posts Mark Forums Read

 Homework Help Stuck on a textbook question or coursework? Cramming for a test and need help understanding something? Post your questions and attempts here and let others help.

#1
05-22-2008, 03:49 AM
 Manmeet Singh Junior Member Join Date: May 2008 Posts: 37
BJT basics

Im a bit confused about the BJT explanation provided by the site, Ive attached the part that is confusing me

For the first diagram this is pretty bad but since 2 batteries are connected to that node lets say the one on the left is 5V and the right 12V then what would the voltage of the base be? Since both are connected which one sets the voltage of the base lead?

For the second diagram in the reading it said that the base-collector junction is reversed biased therefore it would only be conducting leakage current but then it shows that the forward bias of the base-emitter is allowing a current to flow through this area. How does this make sense?
Attached Images
 question.GIF (8.3 KB, 18 views)
#2
05-22-2008, 04:16 AM
 Bill_Marsden Super Moderator Join Date: Mar 2008 Location: Dallas, TX (GMT-5 w/ DST) Posts: 19,066 Blog Entries: 5

Bipolar transistors are current devices, the BE junction will forward drop .6 Volts, same as a diode and for similar reasons. This is why you always see resistors in the drawings, to limit the current.

Beta is the parameter name given to define the DC gain a transistor has. It is defined by the Collector to Emitter Current created by the Base to Emitter Current. The Collector to Emitter Current is similar to a variable resistor. B=Ice/Ibe

In both drawings the bias is identical. Care to point the specific paragraph in the book out?

***********************

Found it.

You mismatched what was said to the wrong drawing, which has confused the issue quite a bit. What they are talking about is a transistor in the off state, and are trying to explain why it is off. The drawing is showing a transistor that is biased on, where the BE junction has been forward biased, and the collector conducts current.

When the BE junction is reverse biased it will act like a diode, and start dropping voltage in exactly the same way a reverse biased diode will. Look below the discription for the correct drawing.
__________________
..
"Good enough is enemy of the best." An old engineering saying, Author unknown.

General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input. Thanks for the verbage Wookie.

Last edited by Bill_Marsden; 05-22-2008 at 04:26 AM.
#3
05-22-2008, 04:43 PM
 Manmeet Singh Junior Member Join Date: May 2008 Posts: 37

"It is customary to reverse bias the base-collector junction of a bipolar junction transistor"...."There is no current flow, except leakage current, in the collector circuit."....."Normally we forward bias the emitter-base junction, overcoming the 0.6 V potential barrier."

This is before the diagrams Ive posted and then below those diagrams it states

"1) forward bias the emitter-base junction, 2) reverse bias the base-collector junction. "

well if this is the case then how would any current be getting through the base collector junction? is there another path the current is taking? the way im seeing it is that the emitter base junction is forward biased but the collector base junction is revsered biased so theres no way you would have any emitter collector current

Last edited by Manmeet Singh; 05-22-2008 at 04:54 PM.

 Tags basics, bjt

 Related Site Pages Section Title Worksheet Bipolar junction transistors in active mode Worksheet Bipolar junction transistor theory Video Lecture Transistor Biasing - Transistors and Transistor Circuits Textbook Introduction : Junction Field-effect Transistors Textbook Transistor ratings and packages : Bipolar Junction Transistors Textbook Current mirrors : Bipolar Junction Transistors Textbook Meter check of a transistor : Bipolar Junction Transistors Textbook Introduction : Bipolar Junction Transistors Textbook Junction field-effect transistors : Solid-state Device Theory Textbook Bipolar junction transistors : Solid-state Device Theory

 Similar Threads Thread Thread Starter Forum Replies Last Post johnny7b The Projects Forum 2 07-01-2014 02:45 PM ActivePower General Electronics Chat 18 01-01-2014 05:29 PM TimNJ General Electronics Chat 16 01-01-2014 09:31 AM AD633 Homework Help 4 06-23-2013 04:49 PM WolfgangBS General Electronics Chat 13 05-10-2013 03:10 AM

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Electronics Forums     General Electronics Chat     The Projects Forum     Homework Help     Electronics Resources Software, Microcomputing, and Communications Forums     Programmer's Corner     Embedded Systems and Microcontrollers     Computing and Networks     Radio and Communications Circuits and Projects     The Completed Projects Collection Abstract Forums     Math     Physics     General Science All About Circuits Commmunity Forums     Off-Topic     The Flea Market     Feedback and Suggestions

All times are GMT. The time now is 06:56 PM.