Electrical Engineering Answers

A series RL circuit consists of resistor with a resistance of 50.0 Ω and an inductor with an

inductance of 200 µH. The instantaneous voltage across the resistor (VR) is given as

VR = ( 10.5 V) cos (2000π t)

a) Derive an equation for the instantaneous current that flows in the circuit. (2 marks)

b) Calculate the reactance of the circuit. (1 mark)

c) Derive an equation for the instantaneous voltage across the inductor, VL. (3 marks)

d) Sketch a phasor diagram of the circuit. (1 mark)

e) Determine the equation for the instantaneous voltage of the ac source, Vs. (3 marks)

(3) What would be considered good values for the common base current gain α and 

the common emitter current gain β, respectively, in a BJT? ( ). 

A. 0.1 and 0.11 B. 0.5 and 1 

C. 0.99 and 99 D. 3 and 1.5 

(4) What is the order of highest doping, next highest doping, and lightest doping in a 

good BJT? ( ). 

A. Emitter, base, collector B. Emitter, collector, base 

C. Base, collector, emitter D. Collector, base, emitter

(1) There are two PN junctions in a bipolar transistor. Each one can be either forward 

biased (FB) or reverse biased (RB). In the reverse active mode of operation, how 

are these two junctions biased? ( ). 

A. Emitter-base is FB and base-collector is FB. 

B. Emitter-base is FB and base-collector is RB. 

C. Emitter-base is RB and base-collector is FB. 

D. Emitter-base is RB and base-collector is RB. 

(2) In a good bipolar transistor operating in the active region, what is the spatial profile 

of minority carriers in the base? ( ). 

A. Exponentially increasing from emitter to collector. 

B. Exponentially decreasing from emitter to collector. 

C. Linearly increasing from emitter to collector. 

D. Linearly decreasing from emitter to collector. 

2. Consider an NPN BJT with the following parameters: 

IEn = 1 mA, IEp = 0.01 mA, ICn = 0.98 mA, ICp = 0.0001 mA 

(1) Determine the currents, IE, IC, IB. 

(2) Determine the emitter injection efficiency, γ. 

(3) Determine the base transport factor, β*

. 

(4) Determine αDC, βDC.

3. Consider the NPN BJT made of silicon is in the forward active region, with IC = 10 

μA, doping concentrations NE = 1018 cm-3, NB = 1017 cm-3, and NC = 1016 cm-3; 

thicknesses WE = 0.5 μm, WB = 0.25 μm, and WC = 2 μm; and diffusion constants 

DpE = 2 cm2/s, DnB = 20 cm2/s, and DpC = 12 cm2/s, and Lnb = 1 μm. The 

cross-sectional area A = 1 μm2. Assume that recombination within the BJT is small 

(i.e., diffusion lengths are much larger than the thicknesses of each layer). 

Calculate the emitter injection efficiency γ, the base transport factor β*

, the common 

emitter current gain βdc, the common base current gain αdc, respectively.

4. Why is the collector less heavily doped than the base in a typical BJT? Give 3 

reasons.