1
MCQ (Single Correct Answer)

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.
2
MCQ (Single Correct Answer)

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.
3
MCQ (Single Correct Answer)

AIPMT 2012 Prelims

Two ideal diodes are connected to a battery as shown in the circuit. The current supplied by the battery is

A
0.75 A
B
zero
C
0.25 A
D
0.5 A

Explanation

Here D1 is in forward bias and D2 is in reverse bias so, D1 will conduct and D2 will not conduct.
So, the current supplied by the battery is

I = $${5 \over {10}}$$ = 0.5 A
4
MCQ (Single Correct Answer)

AIPMT 2012 Prelims

In a CE transistor amplifier, the audio signal voltage across the collector resistance of 2 k$$\Omega $$ is 2 V. If the base resistance is 1 k$$\Omega $$ and the current amplification of the transistor is 100, the input signal voltage is
A
0.1 V
B
1.0 V
C
1 mV
D
10 mV

Explanation

Here, RC = 2 k$$\Omega $$ = 2 × 103 $$\Omega $$

V0 = 2 V, RB = 1 k$$\Omega $$ = 1 × 103 $$\Omega $$, $$\beta $$ = 100

The output voltage, across the load RC

V0 = ICRC = 2

The collector current (IC)

IC = $${2 \over {2 \times {{10}^3}}}$$ = 10-3 A = 1 mA

Current gain($$\beta $$) = $${{{I_C}} \over {{I_B}}}$$ = 100

$$ \Rightarrow $$ IB = $${{{I_C}} \over {100}}$$ = $${{{{10}^{ - 3}}} \over {100}}$$ = 10-5 A

Input voltage, Vi

= IBRB = (10–5 A) (1 × 103 $$\Omega $$) = 10–2 V = 10 mV

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