PRESENTED BY: LUXMI PRAKASH

01470104911 B.TECH(EEE)

Need of Biasing a).A transistor can work as amplifier,

only if the dc voltages and currents in the circuit are suitably fixed.Hence, dc biasing is a must.

b).The dc current and voltage decided the operating point (or quiescent point, or simply Q-pint).

OperatingOperating PointPointThe operating point of a device, also known as

bias point or quiescent point (or simply Q-point), is the DC voltage and/or current which, when applied to a device, causes it to operate in a certain desired fashion. The term is normally used in connection with devices such as transistors and diodes which are used in amplification or rectification.

BIAS STABILIZATION:-the maintaince of operating point or Q-point stable.Need for bias stabilization:-Temperature depandance of collector current

is due to 3 factors-1.Reverse saturation current which doubles for

every 10 degree rise in temperature.2.Transistor current gain,which increases with

increase in temperature.3.Base-emitter voltage,which decreases by 2.5

mV per degree centrigate.

Thermal runawayThermal runaway:the collector

current ,increases with increase in temprature.This leads to increased power dissipation with further increasin temperature.Being a cumulative process it can lead to to thermal runawayresulting in burnout of the transistor. The self destruction of an unstabilized transistor is called thermal runaway.

#INDIVIDUAL VARIATIONS

Types of bias circuitFixed-bias circuitEmitter-bias circuitSelf bias circuit

Fixed biasingApplying KVL through Base Applying KVL through Base Circuit we can write, ICircuit we can write, Ib b RRbb+ + VVbebe= V= VBBBB

IIb =b = V VBBBB – V – Vbe be / / RRbbIIc=c= β β IIb b

ApplyingApplying kirchoff’s 2 kirchoff’s 2ndnd law to the law to the branch circuit branch circuit CCCCCE RIVV

VCC = CC = IIB B RRBB+ V+ VBE BE + I + IEERREE = = = = IIB B RR BB+ V+ VBE BE + + (1+(1+ββ))

IIBBRREE

EB

BECCB 1)R(R

V-VI

Applying Kirchoff’s voltage law:Applying Kirchoff’s voltage law:

= = IIC C RRC C + V+ VCE CE + + IIEERRE=E== = VVCC CC

= = VVCECE == V VCCCC -I -ICCRRE-E- I ICCRRCC

SINCE SINCE

IIC=C= I IEE

VVCECE == V VCCCC -I -ICC (R (RE+E+RRC C ) )

Voltage-divider bias is the most widely used type of bias circuit. Only one power supply is needed and voltage-divider bias is more stable( independent)

VB = (R2/R1 + R2)VCC

The General Equation for Thermal The General Equation for Thermal Stability Factor,Stability Factor, S S ═ 1 + ═ 1 + ββ / /

1-1-ββ((∂∂IIbb//∂∂IICC))

Applying KVL through input base Applying KVL through input base circuit circuit we can write Iwe can write IbbRRThTh + I + IE E RREE+ V+ Vbebe= V= VThTh

Therefore, ITherefore, IbbRRThTh + (I + (ICC+ I+ Ibb) R) REE+ V+ VBEBE= =

VVThTh

Diff. w. r. t. IDiff. w. r. t. ICC & rearranging we get & rearranging we get

((∂∂IIbb / ∂I / ∂Icc) = - R) = - RE E / (R/ (RThTh + R + REE))