Question

For a common emitter amplifier, the audio signal voltage across a 5 k Ω collector resistance is 5v.
Suppose the current amplification factor of the transter is 100, find the input signal voltage and base
current, if the base resistance is 1 k Ω .

Answer 1

IB = 10 uA ; Input signal = 0.73

Explanation:

Collector Resistance = Rc = 5K ohm  

Voltage across resistance Rc= 5  

Beta = 100

RB = 1k ohm  

The collector current will be:  

IC = 5/5K = 1 mA

Now,

IB = 1m/100 = 10 uA

VB = 1k × 10u

        = 0.03 V

Voltage across base and emitter = 0.7 V

Voltage of input signal =  0.03+ 0.7  = 0.73 V

Answer 2

Given:

Collector Resistance,

Rc = 5 k Ω

Voltage across Rc,

V = 5

Base Resistance,

RB = 1 k Ω

Amplification factor of current,

Beta = 100

To Find:

Base current & Input signal voltage = ?

Solution:

As we  know,

The collector current will be,

⇒  $IC=\frac{V}{Rc}$

⇒        $=\frac{5}{5 K} = 1 \ mA$

and,

⇒  $IB=\frac{IC}{Beta}$

⇒        $=\frac{1m}{100} =10 \ uA$

Now,

⇒  $VB=RB\times IB$

On putting the estimated values, we get

⇒         $=1k\times 10u$

⇒         $=0.03 \ V$

So that,

Voltage across base,

$=0.7 \ V$

Voltage of input signal,

= $0.03+0.7$

$=0.73 \ V$

Therefore, the base current will be "10 uA" & Input signal will be "0.73 V".