# Electronic Circuits July 2007

Note : 1. Answer any FIVE full questions.

2. Assume missing data suitably.

1 a. What is synchronization? With a neat circuit diagram and waveform explain the working of a UJT time base generator.

b.  Derive an expression for the junction current of a semiconductor diode.

2 a. Draw the circuit diagram of a full wave bridge rectifier with capacitor filter and explain its operation with wave forms.

b.  Show that the maximum rectification efficiency of a full wave rectifier is 81.2%.

3 a. Sketch the CE I/O characteristics for a Si n-p-n BJT. Indicate the various regions of operation. Explain the shapes of the curves.

b.  Given p – 50 for the transistor circuit shown in Fig.3(b), find the transistor currents Ic, Ie and Ib. In which region is the transistor operating? Justify. Fig. 3(b)

4 a. Derive expression for current gain, voltage gain and input impedance of a Darlington pair emitter follower.

b.  Compare different methods of couplings used in amplifiers with application.

5 a. Write the hybrid-7t model of BJT and briefly explain the various parameters of the model.

b.  Draw the circuit of a bootstrapped emitter follower and explain how it gives higher input impedance than emitter follower.

c.  The cutoff frequencies of a transistor amplifier are fj = 500 Hz and f? = 400 kHz. The midband voltage gain is 80. Find the voltage gain at 250 Hz.

6 a. For the emitter follower network of Fig. 6(a), determine

i)  re               ii) Zi          iii) ZG              iv) Av                      v) A{

Fig.6(a)

Also derive the equations for Zj, Z0, Av and Aj

b. Explain with relevant sketches how power amplifiers are classified. C. Derive the expression for the conversion efficiency of a class B push pull amplifier.

7 a. Show that the maximum conversion efficiency of a transformer coupled class A power amplifier is 0.5.

b.  In a class A transformer coupled power amplifier Yqc = 12 V, Rl = 1 kQ, efficiency of a transformer is 75%. Power delivered to load is 1 Watt.

Find :

i)   Turns ratio of transistor.

ii) Collector dissipation.

iii) D.C. power drawn from supply.

c.  What is cross over distortion and how is it minimized?

8. Write short notes on :

a.  Varactor diode.

b.  Clipping circuits.

c.  Shunt voltage regulator.

d.  Significance of operating point in a transistor.