1

### AIPMT 2012 Mains

The input resistance of a silicon transistor is 100 $\Omega$. Base current is changed by 40 $\mu$A which results in a change in collector current by 2 mA. This transistor is used as a common emitter amplifier with a load resistance of 4 k$\Omega$. The voltage gain of the amplifier is
A
2000
B
3000
C
4000
D
1000

## Explanation

Current gain ($\beta$) :

$\beta$ = ${{\Delta {I_C}} \over {\Delta {I_B}}}$

= ${{2 \times {{10}^{ - 3}}} \over {40 \times {{10}^{ - 6}}}}$ = 50

Voltage gain of the amplifier is

AV = $\beta {{{R_L}} \over {{R_i}}}$

= 50 $\times$ ${{4 \times {{10}^3}} \over {100}}$ = 2000
2

### AIPMT 2012 Prelims

The figure shows a logic circuit with two inputs A and B and the output C. The voltage wave forms across A, B and C are as given. The logic circuit gate is
A
OR gate
B
NOR gate
C
AND gate
D
NAND gate

## Explanation

The truth table is
The logic circuit is OR gate.
3

### AIPMT 2012 Prelims

Transfer characteristics [output voltage (V0) vs input voltage (Vi)] for a base biased transistor in CE configuration is as shown in the figure. For using transistor as a switch, it is used

A
in region III
B
both in region (I) and (III)
C
in region II
D
in region I

## Explanation

In the given graph,

Region (I) – Cutoff region

Region (II) – Active region

Region (III) – Saturation region

Using transistor as a switch it is used in cutoff region or saturation region.

Using transistor as a amplifier it is used in active region.
4

### AIPMT 2012 Prelims

C and Si both have same lattice structure; having 4 bonding electrons in each. However, C is insulator where as Si is intrinsic semiconductor. This is because
A
In case of C the valence band is not completely filled at absolute zero temperature.
B
In case of C the conduction band is partly filled even at absolute zero temperature.
C
The four bonding electrons in the case of C lie in the second orbit, whereas in the case of Si they lie in the third.
D
The four bonding electrons in the case of C lie in the third orbit , whereas for Si they lie in the fourth orbit.

## Explanation

Electronic configuration of 6C

6C = 1s2, 2s2 2p2

The electronic configuration of 14Si

14Si = 1s2, 2s2 2p6, 3s2 3p2

As they are away from Nucleus, so effect of nucleus is low for Si even for Sn and Pb are almost mettalic.