# BJT

Discussion in 'Homework Help' started by dey_jayanta, Jan 7, 2014.

1. ### dey_jayanta Thread Starter New Member

Jan 7, 2014
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Can anyone solve me this problem please??

What is the largest value of Rc can be raised while the transistor remains in active mode?

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2. ### LvW Well-Known Member

Jun 13, 2013
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Question back: Do you know
* what "active mode" means, and
* which effect limits operation in active mode?

3. ### donpetru AAC Fanatic!

Nov 14, 2008
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I think "active mode" means "saturation". If so, then the resistor Rc can be 0 ohms because Re is 2kohm and thus transistor power dissipation shall not exceed 50mW. So, you can use any transistor that supports more than 50 mW and voltage Vce is greater than 20V.

LATER EDIT: If you ask which is highest value of the resistor Rc the transistor to remain in the saturation, then you have to decide on the transistor. Specifically, what type of transistor you use, because the situation varies from case to case. For example, Rc may have a different maximum value when the transistor hFE is 100 and another value when the transistor hFE is 500. You can devise a formula according to hFE value but I think it goes far beyond the subject of this topic or hobby side of this forum.

Last edited: Jan 7, 2014
4. ### LvW Well-Known Member

Jun 13, 2013
779
105
@donpetru
My question in post#2 was to the OP because we are in the "homework help" section - and the OP should show that - at least - he understands the task.

Nov 9, 2007
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6. ### WBahn Moderator

Mar 31, 2012
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For a BJT, the three primary regions are "cutoff", "active", "saturation". The "active" region is also often called the "linear" region.

For a FET, the "active" region and the "saturation" region are the same thing. An unfortunate choice of terminology decades ago that we are stuck with now.

7. ### LvW Well-Known Member

Jun 13, 2013
779
105
Yes - indeed, sometimes this terminology causes some confusion.
Quote from R.C. Jaeger, Microelectronic Circuit design:
"It is important to note that the saturation region of the BJT does not correspond to the saturation region of the FET. This unfortunate use of terms is historical in nature.."