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**1.The V-I characteristic of a silicon diode is shown in the figure. Calculate the resistance of the diode at **

Considering the diode characteristics as a straight line between I = 10mA to I = 20 mA passing through the origin, we can calculate the resistance using Ohm's law.

(a) From the curve, at I = 20 mA, V = 0.8 V, I = 10 mA, V - 0.7V

(b) From the curve at

**2.In a Zener regulated power supply a Zener diode with V _{z} =6.0 V is used for regulation. The load current is to be 4.0mA and the unregulated input is 10.0V. What should be the value of series resistor R_{s}?**

The value of R_{s} should be such that the current through the Zener diode (I_{z} )is much larger than the load current (I_{L} ). This is to have good load regulation.

Let us choose I_{z} =5I_{L} , i.e., I_{z} = 20 mA. The total current through R_{s} is, therefore, 24 mA. The voltage drop across R_{s}. is 10.0-6.0 = 4.0 V. This gives R_{s} = 4.0 V / The nearest value of carbon resistor is 150 Ω is appropriate. Slight variation in the value of the resistor does not matter, what is important is that the current I_{z} should be sufficiently larger than I_{L.} **3. (a)The number of electron-hole pairs in an intrinsic semiconductor is Calculate the number of electron-hole pairs at 227 ^{o} C. Given that Boltzmann constant is (b) If this semiconductor is doped by a donor impurity such that the number of conduction electron becomes calculate the number of holes at 27^{o} C. Also approximately calculate the dopant concentration.**

(a)

**4. In half wave rectification, what is the output frequency if the input frequency is 50 Hz? What is the output frequency of a full wave rectifier for the same input frequency?**

Output frequency in Half Wave Rectifier : 50 Hz.

Output frequency in Full Wave Rectifier : 100 Hz.

**5.For a CE transistor amplifier, the audio signal voltage across the collector resistance of 2 k is 2 V. Current amplification factor of the transistor is 100. Find the input signal voltage and base current if the resistance is 1 k.**

**6.You are given two circuits as shown, which consist of NAND gates. Identify the logic operation carried out by the two circuits.**

The truth table for first circuit :

If we remove the intermediate output Y', the overall result stands for the AND gate.

Hence, the circuit is an equivalent of AND gate. The truth table for second circuit :

If we remove the intermediate output the overall result stands for the OR gate.

This can be represented as Y = A + B

Hence, the circuit is an equivalent of OR gate.

**7. A transistor has a current amplification factor (current gain) of 50. In a CE - amplifier circuit, the collector resistance is chosen as 50 k and the input resistance is 1 k. Calculate the output voltage if input voltage is 0.01 V.**

**8.The number of silicon atoms per . This is dipped simultaneously with atoms per m ^{3} of Arsenic and atoms of Indium. Calculate the number of electrons and holes. . Is the material n-type or p-type?**

**9.In an intrinsic semiconductor the energy gap E _{g} is 1.2 eV. Its hole mobility is much smaller than electron mobility and independent of temperature. What is the ratio between conductivity at 600 K and that at 300 K? Assume that the temperature dependence of intrinsic carrier concentration n_{i} is given by**

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Plus 2 Computer Science

ICSE/ISC