1

### JEE Main 2016 (Online) 10th April Morning Slot

A realistic graph depicting the variation of the reciprocal of input resistance in an input characteristics measurement in a commonemitter transistor configuration is :
A
B
C
D

## Explanation

For common emitter configuration, the input characteristic graph is shown above,

ri $=$ ${{\Delta {V_{BE}}} \over {\Delta {{\rm I}_B}}}$

$\Rightarrow$   ${1 \over {{r_i}}} = {{d{{\rm I}_B}} \over {d{V_{BE}}}}$ $=$ shope of this curve.

Upto knee voltage 0.7 V the shope is almost constant. Then it increases sharply.

So, option (c) is the correct choice.
2

### JEE Main 2017 (Offline)

In a common emitter amplifier circuit using an n-p-n transistor, the phase difference between the input and the output voltages will be:
A
180°
B
45°
C
90°
D
135°

## Explanation

In common emitter configuration for n-p-n transistor input and output signals are 180° out of phase i.e., phase difference between output and input voltage is 180°.
3

### JEE Main 2017 (Online) 8th April Morning Slot

What is the conductivity of a semiconductor sale having electron concentration of $5 \times {10^{18}}\,\,{m^{ - 3}},$ hole concentration of $5 \times {10^{19}}\,\,{m^{ - 3}},$ electron mobility of 2.0 m2 V$-$1 s-1 and hole mobility of 0.01 m2 V$-$1 s$-$1 ?

(Take charge of electronas 1.6 $\times$ 10 $-$19 c)
A
1.68 ($\Omega$-m)$-$1
B
1.83 ($\Omega$-m)$-$1
C
0.59 ($\Omega$-m)$-$1
D
1.20 ($\Omega$-m)$-$1

## Explanation

Conductivity of semiconductor,

$\sigma$ = e$\left( {{\eta _e}{\mu _e} + \eta '{\mu _h}} \right)$

= 1.6 $\times$ 10$-$19 (5 $\times$ 1018 $\times$ 2 + 5 $\times$ 1019 $\times$ 0.01)

= 1.6 $\times$ 1.05

= 1.68
4

### JEE Main 2017 (Online) 8th April Morning Slot

The V-I characteristic of a diode is shown in the figure. The ratio of forward to reverse bias resistance is :

A
10
B
10$-$6
C
106
D
100

## Explanation

Forward bias resistance

R1 = ${{\Delta V} \over {\Delta I}}$ = ${{0.8 - 0.7} \over {\left( {20 - 10} \right) \times {{10}^{ - 3}}}}$ = 10 $\Omega$

Reverse bias resistance,

R2 = ${{10} \over {1 \times {{10}^{ - 6}}}}$ = 107 $\Omega$

$\therefore\,\,\,$ Ratio of resistance

= ${{{R_1}} \over {{R_2}}}$ = ${{10} \over {{{10}^7}}}$ = 10$-$6