# Electronic Devices and Circuits Questions and Answers – BJTs Current-Voltage Characteristics

This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “BJTs Current-Voltage Characteristics”.

1. The curve between the collector current versus the potential difference between the base and emitter is
a) A straight line inclined to the axes
b) A straight line parallel to the x-axis
c) An exponentially varying curve
d) A parabolic curve

Explanation: The natural logarithm of the collector current depends directly on the the potential difference between the base and the emitter.

2. The curve between the collector current and the saturation is
a) A straight line inclined to the axes
b) A straight line parallel to the x-axis
c) A straight line parallel to the y-axis
d) An exponential curve

Explanation: The collector current depends directly on the saturation current.

3. The magnitude of the thermal voltage is given by
a) k/Tq
b) kT/q
c) q/Kt
d) Tk/q

Explanation: kT/q is the correct mathematical expression for the thermal voltage.

4. The correct relation between the transistor parameters α and ß are related by
a) ß = 1 – α/α
b) ß = 1 + α/α
c) α = ß + 1/ß
d) α = ß/ß + 1

Explanation: Only expression α = ß/ß + 1 is the correct expression that relates α and ß.

5. The correct expression relating the emitter current Ie to the collector current Ic is
a) Ie = α Ic
b) Ic = α Ic
c) Ie = ß Ic
d) Ic = ß Ic

Explanation: Ie = Ic/α or Ic = α Ie
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6. The value of the thermal voltage at room temperature can be approximated as
a) 25 mV
b) 30 mV
c) 35 mV
d) 40 mV

Explanation: Thermal voltage is given by kT/q which at T = 25 degrees Celsius is approximately 25 mV.

7. The correct relation between the emitter current Ie and the base current Ib is given by
a) Ib = (1 + α) Ie
b) Ib = (α – 1) Ie
c) Ie = (1 – ß) Ib
d) Ie = (1 + ß) Ib

Explanation: The correct mathematical expression are Ie = (1 – ß) Ib and Ib = (1 – α) Ie respectively.

8. The Early Effect is also called as
a) Base-width modulation effect
b) Base-width amplification effect
c) Both of the mentioned
d) None of the mentioned

Explanation: At a given value of vBE, increasing vCE increases the reverse-bias voltage on the collector–base junction, and thus increases the width of the depletion region of this junction. This in turn results in a decrease in the effective base width W. Also the saturation current is inversely proportional to the width, the saturation current will increase and also makes collector current increases proportionally. This is the Early Effect. For the reasons mentioned above, it is also known as the base-width modulation effect.

9. For the BJT to operate in active mode Collector-Base junction must be
a) Heavily doped
b) Must reversed bias
c) Must be forward bias
d) Lightly doped

Explanation: The BJT operates in active mode when the collector-Base junction is reversed bias. Also doping cannot prevent saturation of the transistor.

10. Collector current (Ic) reaches zero when
a) Vce = Vt ln (Isc/I)
b) Vt = Vce ln (Isc/I)
c) Vce = Vt ln (I/Isc)
d) Vce = Vt ln (Isc + I/I)

Explanation: Ic = Is exp (Vbe/Vt) – Isc exp(Vbc/Vt). In this expression put ic = 0 and simplify.

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