• For a Bipolar Junction Transistor commonly called as the BJT Transistor the stability factor is defined as the change in the collector current of a Bipolar Junction Transistor (BJT) with respect to change in various transistor parameters like as Collector Leakage Current.
• The Stability factor for a BJT Transistor is denoted by S.
• The process of making operating point independent of temperature changes or variation in transistor parameters is known as stabilization.Ic is thecollector current and Icl is the collector leakage current.

$S=dIc/dIcl$

STABILITY FACTOR FOR VARIOUS BIAS:

FIXED BIAS:

• This form of biasing is also called base bias. The single power source is used for both collector and base of transistor, although separate batteries can also be used.

Using KVL we have:

$VCC – IBRB –VBE = 0 ; IB = (VCC-VBE)/RB$

                                                       $C = βIB= β(VCC-VBE)/RB$

Using KVL in the collector-emitter loop

$VCC – ICRC –VCE = 0; VCE = VCC - ICRC$

ADVNTAGES AND DISADVANTAGES OF FIXED BIAS :

ADVANTAGE:

• Operating point can be shifted easily anywhere in the active region by merely changing the base resistor (RB).
• A very small number of components are required.

DISADVANTAGE:

• Poor stabilization
• High stability factor(S=β+1 because IB is constant so dIB/dIC =0 ), hence prone to thermal runaway

APPLICATION OF FIXED BIAS

• Due to the above inherent drawbacks, fixed bias is rarely used in linear circuits (i.e., those circuits which use the transistor as a current source). Instead, it is often used in circuits where transistor is used as a switch.

COLLECTOR BIAS VOLTAGE:

• This configuration employs negative feedback to prevent thermal runaway and stabilize the operating point.
• In this form of biasing, the base resistor RB is connected to the collector instead of connecting it to the DC source Vcc.
• So any thermal runaway will induce a voltage drop across the RC resistor that will throttle the transistor's base current.

Applying KVL

$VCC = (IC+IB)RC + VCE$

$ VCE = IBRB + VBE $

Since, IC = βIB s

                                                      $IB=VCC-VBE/RB+ (1 + β)RC$

ADVANTAGES AND DISADVANTAGES OF COLLECTOR BIAS

ADVANTAGES:

• Better stabilization compared to fixed bias

DISADVANTAGES

• This circuit provides negative feedback which reduces the gain of the amplifier

APPLICATION:

• The feedback also decreases the input impedance of the amplifier as seen from the base, which can be advantageous.
• Due to the gain reduction from feedback, this biasing form is used only when the trade-off for stability is warranted.

VOLTAGE DIVIDER BIAS:

• This is the most widely used method to provide biasing and stabilization to a transistor.
• In this form of biasing, R1 and R2 divide the supply voltage VCC and voltage across R2 provide fixed bias voltage VB at the transistor base.
• Also a resistance RE is included in series with the emitter that provides the stabilization.

$VB=VTh=VCC=R2/R1+R2$

$Rth=R1R2/R1+R2$

Base Emitter Loop

$VTh-IBRth-VBE-(β +1)IBRE=0$

Collector Emitter Loop

$IC=βIB=β(VTh-VBE)/RTH+(β+1)RE$

For Bias stability 

$IC=VTh-VBE/RE$

The stability factor is given by

$S=(β +1)/1-β(dIB/dIC)$

Differentiating with respect to IC

$VTh = IB Rth − VBE − (IB +IC )RE$

$dIB/dIC=-RE/Rth+RE$

$S=β+1( Rth+RE)/(RE(1+β)+Rth $

$S=β+1( Rth/RE+1)/((1+β)+Rth/RE $

• For stability, S should be small which can be achieved by making RTh/R E small. For very small RTh/RE ; S = 1 (ideal case)
• For very small RTh/RE : R 2 RTh current drawan by R 2 will be large. R E Large Vcc required. Hence comproise is made in selection.
• The Stability of Voltage divider is better compared to the other two

ADVANTAGES AND DISADVANTAGES:

ADVANTAGES:

• Stability is more stable here as it is able to reach its maximum value

DISADVANTAGES:

• Circuit us a bit complex.
• Stability depends on β which is dependent upon the manufuacturer