1

### AIPMT 2011 Prelims

A transistor is operated in common emitter configuration at VC = 2 V such that a change in the base current from 100 $\mu$A to 300 $\mu$A produces a change in the collector current from 10 mA to 20 mA. The current gain is
A
50
B
75
C
100
D
25

## Explanation

Current gain, $\beta$ = ${{\Delta {I_C}} \over {\Delta {I_B}}}$

= ${{\left( {20 - 10} \right) \times {{10}^{ - 3}}} \over {\left( {300 - 100} \right) \times {{10}^{ - 6}}}}$ = 50
2

### 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.
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 Prelims

A common emitter amplifier has a voltage gain of 50, an input impedance of 100 $\Omega$ and an output impedance of 200 $\Omega$. The power gain of the amplifier is
A
500
B
1000
C
1250
D
50

## Explanation

Voltage gain= $\beta$ × Impedance gain

$\Rightarrow$50 = β × (200/100)

$\Rightarrow$ $\beta$ = 25

Power gain = $\beta$ × Voltage gain

= 25 × 50 = 1250