# GTU Exam Paper Basic Electronics May 2011

GUJARAT TECHNOLOGICAL UNIVERSITY

B.E. Sem-III Examination May 2011

Subject code: 131101

Subject Name: Basic Electronics

Total Marks: 70

Instructions:

1. Attempt all questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.

Q.1 (a) Prove that the minority carrier concentration, in an n-type semiconductor bar which was momentarily illuminated, decreases exponentially with time.

(b) Derive continuity equation and explain its importance.

Q.2 (a) When a diode is driven from forward condition to reverse condition, draw and explain waveforms for (1) minority carrier concentration at the junction of the diode (2) current flowing through the diode circuit, and (3) voltage across the diode. Assume resistance (RL) is present in series with diode.

(b) A symmetrical 5-kHz square wave whose output varies between +10 V and -10 V is impressed upon the clipping circuit shown in Fig. 1. Assume diode forward resistance (R/ as zero, diode reverse resistance as (Rr) 2M, diode cut-in voltage (Vr) as zero. Sketch the steady-state output waveform, indicating numerical values of the maximum, minimum, and constant portions.

OR

(b) Design a Zener regulator (Fig. 2) for following specifications: load current IL = 20 mA, output voltage Vo = 5 V, Zener wattage Pz = 500 mW, Input voltage V = 12 ± 2 V, and Iz,(min) = 8 mA,

Q.3 (a) A silicon transistor with VBE, sat = 0.8 V, P = hFE = 100, VCE, sat = 0.2 V is used in the circuit shown in Fig. 3. Find the minimum value of Rc for which the transistor remains in saturation.

(b) Derive expressions for AI; Ri, AV, and Yo in terms of CE h-parameters for emitter- follower circuit.

OR

Q.3 (a) Represent/derive CC h-parameters (hic and h/c) in terms of CE h-parameters .

(b) Explain the base-width modulation and its effect on minority-carrier concentration in the base region of a transistor as well as on the common-base input characteristics of a typical p-n-p transistor.

Q.4 (a) Define stabilization factors: S, S’, and S”. Derive expressions for S and S’ for self-bias transistor circuit.

(b) Derive an expression for voltage gain (AV) for CS amplifier with an bypassed source resistance Rs.

OR

Q.4 (a) The fixed-bias circuit is given in Fig. 4 and it is subjected to an increase in temperature from 25 OC to 75 OC. If P = 100 at 25 OC and P = 125 at 75 OC, determine the percentage change in Q point values (Vce, Ic) over the temperature range. Neglect any change in VBE. Take VBE = 0.7 V.

(b) Draw a structure of p-channel MOSFET. Explain its working for enhancement 07 type. Also draw and explain drain characteristics and transfer curve for the same device.

Q.5 (a) Illustrate how the energy levels of isolated atoms in group IV A (e.g., C, Si, Ge, 07 Sn) are split into energy bands when these atoms are brought into close proximity to form a crystal. Draw necessary energy band diagrams.

(b) Show that the upper limit of the conversion efficiency (n) for the series-fed class 07 A amplifier is 25 %.

OR

Q.5 (a) Draw class B push-pull system and show that the maximum conversion efficiency 07 (n) is 78.5 % for this system.