[Homework] BJT transistor Active region

[HiImNew]

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?

 

[chuckey]

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

 

[BradtheRad]

If you continue to increase base V, it will reach a point where it forces current flow out C and E.

This happens due to the direction of the internal PN junctions.