Test: BJT Configuration - Electronics and Communication Engineering (ECE) MCQ

In a common base (CB) configuration of a Bipolar Junction Transistor (BJT), the input characteristics describe the relationship between the input current ($I_E$) and the input voltage ($V_{BE}$), with the output voltage ($V_{CB}$) held constant.

Explanation:

• The input side of a common base configuration involves the emitter-base junction.
• The input current is the emitter current ($I_E$).
• The input voltage is the emitter-base voltage ($V_{BE}$).
• The characteristics show how $I_E$ varies with $V_{BE}$, while $V_{CB}$ (or equivalently $V_{CE}$) is fixed.

Thus, the input characteristics are represented by a plot between $V_{BE}$ and $I_E$.

If the collector-base junction is reverse-biased and the base-emitter junction is forward-biased, then the BJT functions in the active region of the output characteristics.

If the collector-base junction and the base-emitter junction are both reverse-biased, then the BJT functions in the cutoff region of the output characteristics.

From the characteristics, the value of I_B at V_BE=0.7V is 20uA. Now, from I_B=20 uA, we get I_C = 2.5 mA.

The input signal is applied between the collector and the emitter terminals. Input current flowing is the collector current and hence characteristics are represented by a plot between V_CE and I_C.

If the collector-base junction and the base-emitter junction are both forward-biased, then the BJT functions in the saturation region of the output characteristics.

α and β are related as β = α/(1 - α).
In a BJT, β = I_C/I_B. and α = I_C/I_E
β = αI_E/I_B = (1+β)α
β = α + αβ
β = α/1 - α.

All the relations hold true i.e. I_C = βI_B and I_C = αI_E. As α<1, hence I_C < I_E.

On application of a forward bias voltage across E-B junction, the width of the depletion region decreases.

In a BJT, depending upon the biasing of the two junctions, the BJT behaves differently. The BJT may be in saturation, wherein it acts like a short circuit, or it may be in cut-off, i.e an open circuit. The BJT can be either in forward active or reverse active mode. Active mode is the common mode, used in BJTs and obtained by one forward biased and one reverse biased junction.