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1

### AIPMT 2011 Mains

A Zener diode, having breakdown voltage equal to 15 V, is used in a voltage regulator circuit shown in figure. The current through the diode is

A
5 mA
B
10 mA
C
15 mA
D
20 mA

## Explanation

Voltage across 250 Ω resistance = 20 V – 15 V = 5 V

Now current through 250 $$\Omega$$ resistance: 5/250 = 20 mA

If voltage across load resistance 1 k$$\Omega$$ is 15 V, then

current through 1 kΩ is 15/1000 = 15 mA

The current through the zener diode is

= Current through 250 $$\Omega$$ resistance – Current through 1 k$$\Omega$$ resistance.

= 20 - 15

= 5 mA
2

### AIPMT 2011 Prelims

If a small amount of antimony is added to germanium crystal
A
it becomes a p-type semiconductor
B
the antimony becomes an acceptor atom
C
there will be more free electrons than holes in the semiconductor
D
its resistance is increased

## Explanation

If a small amount of antimony is added to germanium crystal, crystal becomes n-type semiconductor. Hence, there will be more free electrons than holes.
3

### AIPMT 2010 Mains

The following figure shows a logic gate circuit with two inputs A and B and the output Y. The voltage waveforms of A, B and Y are as given.

The logic gate is
A
NOR gate
B
OR gate
C
AND gate
D
NAND gate

## Explanation

It is clear from given logic circuit, that out put Y is low when both the inputs are high, otherwise it is high. Thus logic circuit is NAND gate.
4

### AIPMT 2010 Mains

For transistor action
(1)  Base, emitter and collector regions should have similar size and doping concentrations.
(2)  The base region must be vety thin and lightly doped.
(3)  The emitter-base junction is forward biased and base-collector junction is reverse biased.
(4)  Both the emitter-base junction as well as the base-collector junction are forward biased.

Which one of the following pairs of statements is correct?
A
(4) and (1)
B
(1) and (2)
C
(2) and (3)
D
(3) and (4)

## Explanation

For transistor action, the base region must be very thin and lightly doped. Also, the emitter-base junction is forward biased and base-collector junction is reverse biased.

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