1

### NEET 2013

In a common emitter (CE) amplifier having a voltage gain G, the transistor used has transconductance 0.03 mho and current gain 25. If the above transistor is replaced with another one with transconductance 0.02 mho and current gain 20, the voltage gain will be
A
${1 \over 3}G$
B
${5 \over 4}G$
C
${2 \over 3}G$
D
1.5G

## Explanation

Formula for trans conductance $g$m is

$g$m = ${\beta \over {{r_i}}}$

Formula for voltage gain A,

A = $\beta$${{{R_L}} \over {{r_i}}}$

$\Rightarrow$ A = $g$mRL

Given A = G

$\therefore$ G = $g$mRL

$\Rightarrow$ G $\propto$ $g$m

$\Rightarrow$ ${{{G_2}} \over {{G_1}}} = {{{g_{{m_2}}}} \over {{g_{{m_1}}}}}$

$\Rightarrow$ G2 = ${{0.02} \over {0.03}}$ $\times$ G = ${2 \over 3}G$
2

### AIPMT 2012 Mains

To get an output Y= 1 in given circuit which of the following input will be correct ? A
A $\to$ 1,  B $\to$ 0,  C $\to$ 0
B
A $\to$ 1,  B $\to$ 0,  C $\to$ 1
C
A $\to$ 1,  B $\to$ 1,  C $\to$ 0
D
A $\to$ 0,  B $\to$ 1,  C $\to$ 0

## Explanation The Boolen expression for the given combination is

output Y = (A + B) · C

So it is clear that Y = 1, when A = 1, B = 0 and C = 1
3

### 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
4

### 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.