Question: Explain the i/p and o/p characteristics of CE configuration of BJT. (5 marks)
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• It is graph of i/p (VBE) versus i/p current ($I_B$) at constant o/p voltage (VCE). The i/p characteristics resembles the f/w characteristics of PN junction diode.

• The base current increases rapidly as the base – emitter voltage crosses the cut in voltage of E, PN junction.

• The dynamic i/p resistance ri can be obtained from i/p characteristics viz.

• $ri = \frac{\triangle VBE}{ \triangle I_B}$ VCE is constant.

• The value of i/p resistance is low for CE conf.

O/p characteristics:

• This is a graph between o/p voltage VCE vs o/p current Ic at constant i/p current (In)

• There are three regions of operations in o/p characteristics of BJT namely.

1] Cut-off region: Both BE and CB junction are reverse biased to operate Tr in cutoff region. The base current IB = 0 and collector current is equal to reverse leakage current ICEO. The region below the chartless below $I_B = 0$ is cut-off region.

2] Active region: The BE junction is f/w biased and CB junction is reverse biased. The collector current increases slightly with increase in VCE. However if is large dependent on base current. $I_B$ As $I_S \uparrow$ $I_C$ also $\uparrow$ it is because $I_C = \beta I_S$ this relation is valid only for active region operation.

3] Saturation region: The BE and CB both the junction when get forward Tr enters into saturation region. The collector current $\uparrow$ rapidly with increase in VCE. Note that $I_C$ and VCE has linear relationship in this region.

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