There are two major reasons for which we require biasing in BJT

1] To maintain the position of Q-point.

2] Variation of IC d.r.t. $\beta$ VBE & temperature.

1] Q pt position:

Ideally Q pt shall be maintained at the middle of DC load line. If Q pt shifts np or down there is a possibility of clipping off the o/p waveform

$\therefore$ Biasing is necessary.

2] Variation in IC:

• Q pt is a function of IC & VCE. Q (VCE, IC)

• For Q pt to become stable VCE and IC must have stable values.

• But IC = Function (temperature, $\beta$ and VBE) As temp/$\beta$/ VBE changes results change in IC so Q pt also changed.

1] IC against temperature.

pi (oc) = i/p DC power

$Vcc \times Ic$ - - - - (1)

PQ $\rightarrow$ o/p across transistor $VCE \times Ic$

$\because$ Q pt is @ middle of load line $VCE = \frac{Vcc}{2}$

$PQ = \frac{Vcc}{2} \times Ic$ = 50% of Pi dc - - -- (2)

$\therefore$ 50% of power is dropped across $T_r$. of taken power.

$\therefore$ Temp $\uparrow$

$\rightarrow$

Ic majority $\uparrow$

Ic = Ic minority + Ic majority

$\downarrow$

$Ic \uparrow$

$\downarrow$

$PQ \alpha Ic$

$\therefore$ $PQ \uparrow$

It may result into thermal runway.

2] Ic variation wrt r $\beta$

$Ic = \beta IB$

$\beta \rightarrow$ device parameter.

Change in device may change $’\beta’$ so Ic changes.

3] Ic variation against cut is vtg (VBE)

VBE $\uparrow$ if Temp $\uparrow$

By 2.5mv/˚c $\downarrow$

So IB $\downarrow$

$\downarrow$

$\because$ Ic = $\beta$ IB

$\downarrow$

Ic changes Q pt may change.