# common emitter

#### muni

Joined Jul 29, 2008
45
sir, i've completed my AMIE degree through INSTITUTION OF ENGINEERS. Now i'm preparing for competitive exams.i request your help and guidance on my way.

i request to clear my doubt with reasonable explanation.

a common emitter transistor amplifier has a collector current of 1.0 mA when its base current is 25 micro ampere at the room temperature. it's input resistance is approximately equal to------------?

the answer given inthe book is 1000 ohms

#### hgmjr

Joined Jan 28, 2005
9,029
The problem as you have stated it is missing a key parameter. That parameter is beta.

hgmjr

#### hgmjr

Joined Jan 28, 2005
9,029

#### S_lannan

Joined Jun 20, 2007
246
give that Ic = 1ma and Ib = 25ua
beta is roughly ic/ib so that makes beta 40

input resistance is = 're * beta

're is 25mv/IE = 25

soooo input resistance = 1000

checks out to me...

#### muni

Joined Jul 29, 2008
45
give that Ic = 1ma and Ib = 25ua
beta is roughly ic/ib so that makes beta 40

input resistance is = 're * beta

're is 25mv/IE = 25

soooo input resistance = 1000

checks out to me...
sir, thank you. the book has also given me same solution.but in the book after calculating beta value=40, it is given that

for CE, Rm = (Vt(millivolts)(b+1))/1.025 milli amps

here at this point i was unable to undersatnd.

thank you for your solution sir.

i,m preparing for INDIAN ENGINEERING SERVICES. I've so many such unanswered problems.

sir i'm very new to this society. i don't know how to attach a file along with this quiery . i request you to suggest the way how i can add my files.

my e mail address is psmuni@gmail.com

#### studiot

Joined Nov 9, 2007
4,998
Your book solution was just being more exact (pedantic) than S Lannan.

The equations are the same.
The 1.025 arises because Ie =Ic+Ib , Vt = 25 millivolts is an assumed standard value. the factor Beta+1 is more exact but most normally use just beta since beta >>1.

For most purposes 'rule of thumb methods' are good enough.

#### ruchimittal1986

Joined Aug 23, 2008
2
Hey Muni,
Gr8, i am also very new to this site. Me also wants to prepare for IES. but though I am in job, i dont get much time for study..

#### Audioguru

Joined Dec 20, 2007
11,249
Doesn't the INSTITUTION OF INDIAN ENGINEERS teach basic electronics??
No text book??