[Homework] BJT transistor Active region

Status
Not open for further replies.
H

HiImNew

Newbie level 1
Joined
Jun 13, 2014
Messages
1
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Visit site
Activity points
11


Hi I'm quite new to electronic circuits, this is one of the question extracted from Sedra/Smith 5th edition.

Question:
The transistor in the circuit of Fig. P5.69(which is the one posted) has a very
high beta. Find the highest value of VB(base voltage) for which the transistor
still operates in the active mode.(Vbe = 0.7V)

I've searched for some solution online and the condition imposed was that voltage at collector - voltage at base = -0.4V

The solution completely confuses, as to my understanding, for a transistor to work in the active region, the Vc must be greater than Vb, which will lead to a condition where Vc - Vb = 0V. Is there information I'm missing from the circuit?
 

Vsat of a transistor would be .2V, and as Vbe = .7, You can amplify very small signals with the collector voltage only slightly above Vsat AND below the base voltage!!
Your homework. Going round the circuit:- 5V = Ic X 1K + Vsat + Ic X 1K. So Ic X 2k +.4V = 5V , So Ic = 4.6/2 mA = 2.3 mA, So Ve = 2.3V, so Vb = 2.3 + .7 = 3V
The base must be positive wrt the emitter, NOT negative wrt the collector.
Frank
 

Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…