All About Circuits Forum  

Go Back   All About Circuits Forum > Electronics Forums > Homework Help

Notices

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.

Reply   Post New Thread
 
Thread Tools Display Modes
  #1  
Old 05-17-2011, 04:19 AM
SanRath SanRath is offline
New Member
 
Join Date: Oct 2010
Posts: 8
Default NPN Transistor in Cut-Off & Saturation Function.

Dear All.

This is Sachin Rathod from Mumbai, India.

Can you explain me the basic idea of NPN Bipolar Junction Transistor in Cut-Off & Saturation Mode?

Waiting to have a help from you very soon.

Thank You,
Sachin Rathod.
Reply With Quote
  #2  
Old 05-17-2011, 05:06 AM
jegues's Avatar
jegues jegues is offline
Senior Member
 
Join Date: Sep 2010
Posts: 728
Default

Quote:
Originally Posted by SanRath View Post
Dear All.

This is Sachin Rathod from Mumbai, India.

Can you explain me the basic idea of NPN Bipolar Junction Transistor in Cut-Off & Saturation Mode?

Waiting to have a help from you very soon.

Thank You,
Sachin Rathod.
There is plenty information on these topics available on the web.

For example a quick wiki gives the following,

http://en.wikipedia.org/wiki/Bipolar...ion_transistor

Anyways I'll give you a quick idea of what I understand of the two modes of operation.

Saturation mode and cut off mode are commonly used amongst switching applications (e.g., logic circuits)

As the name implies, in the cutoff mode no current flows because both junctions are reverse biased.

In saturation mode, both junctions are forward biased.

In active mode, the collector current Ic is independent of the collector base voltage Vcb, this situation extends for Vcb going negative to approximately -0.4V.

Below this value of Vcb, the CBJ begins to conduct sufficiently (i.e. it is said to saturate) that the transistor leaves active mode and enter the saturation mode of operation, where Ic decreases.

This change in Vcb (or conversely, Vbc) and decrease in Ic causes the ratio for the DC current gain, β to decrease.

The of a saturated transistor can be tweaked to any desired value lower than β by adjusting Vbc.



To determine wheter the BJT is in saturation one of asks himself either of the two following questions.

Is the CBJ forward biased by more than 0.4V?

Is the ratio of lower than β?

The collector-emitter voltage of a saturated transistor is simply the difference between the forward-bias voltages of the EBJ and CBJ,



This is typically in the range of 0.1V to 0.3V.

I hope this helps!
Reply With Quote
  #3  
Old 05-17-2011, 09:45 AM
Jony130 Jony130 is online now
Senior Member
 
Join Date: Feb 2009
Location: Poland/Wroclaw
Posts: 2,279
Default

When the BJT is in Cut-Off mode, no current is flow through BJT.
Simply the BJT is in OFF state.
The saturation is opposite to Cut-off.
In saturation increase of the base current (or voltage) causes no further change in collector current (or voltage).
In saturation the BJT is in full open (ON), conduct the max. allowed current by the load.
And If NPN transistor is full ON (saturated) then he act just like a short (a switch). Short collector to GND. Or in general short collector to emitter.
And in real circuit the collector to emitter voltage in saturation will not gona be equal 0V. But will be equal Vce(sat) = 0.2V typical.





PS. jegues witch book prefer such a "strange" definition of saturation ?
I never check Vcb voltage to see if BJT is in saturation region.

Last edited by Jony130; 05-17-2011 at 09:54 AM.
Reply With Quote
The Following User Says Thank You to Jony130 For This Useful Post:
anhnha (08-22-2012)
  #4  
Old 05-17-2011, 12:00 PM
jegues's Avatar
jegues jegues is offline
Senior Member
 
Join Date: Sep 2010
Posts: 728
Default

Quote:
Originally Posted by Jony130 View Post
PS. jegues witch book prefer such a "strange" definition of saturation ?
I never check Vcb voltage to see if BJT is in saturation region.
My definition comes out of the Sedra/Smith text.

Usually I'd start off the DC analysis with the assumption the transistors in active mode, and verified this by looking at the voltage across the CBJ in order to reassure myself the transistor isn't in saturation.
Reply With Quote
Reply   Post New Thread

Tags
, , , ,


Related Site Pages
Section Title
Worksheet Class A BJT amplifiers
Worksheet Bipolar junction transistors as switches
Worksheet Bipolar junction transistor theory
Textbook IGBTs : Insulated-gate Field-effect Transistors
Textbook Transistor ratings and packages : Bipolar Junction Transistors
Textbook Current mirrors : Bipolar Junction Transistors
Textbook The common-collector amplifier : Bipolar Junction Transistors
Textbook The common-emitter amplifier : Bipolar Junction Transistors
Textbook Meter check of a transistor : Bipolar Junction Transistors
Textbook Bipolar junction transistors : Solid-state Device Theory


Similar Threads
Thread Thread Starter Forum Replies Last Post
Error in PNP and NPN direction syed_husain Feedback and Suggestions 1 04-19-2011 06:02 AM
Substitute NPN tranistor for pushbutton switch Circuit Breaker The Projects Forum 3 12-16-2010 12:20 PM
Help with bipolar NPN selection ke5nnt The Projects Forum 8 10-31-2009 05:17 AM
OPEN COLLECTOR -PNP or NPN GARYN The Projects Forum 2 09-30-2009 09:00 PM
How to implement an Upper Cut Off Frequency? Denny1234 Homework Help 4 04-18-2008 01:50 PM

Thread Tools
Display Modes

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 Jump


All times are GMT. The time now is 12:34 PM.


User-posted content, unless source quoted, is licensed under a Creative Commons Public Domain License.
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.