Group member :- Fahid Bin Tariq (15f-

8389) Hassan Tariq (14f-8355)

2

Transistors•Transistors is a semiconductor which is used to control the flow of voltage or current.

• The switch current can be controlled by either current or voltage

• Bipolar Junction Transistors (BJT) control current by voltage

• Field Effect Transistors (FET) control voltage by current

•They can be used either as switches or as amplifiers

3

NPN Bipolar Junction Transistor•One N-P (Base Collector) diode one P-N (Base Emitter) diode

4

PNP Bipolar Junction Transistor•One P-N (Base Collector) diode one N-P (Base Emitter) diode

5

NPN BJT Current flow

6

BJT and •From the previous figure iE = iB + iC

•Define = iC / iE

•Define = iC / iB

•Then = iC / (iE –iC) = /(1- )

•Then iC = iE ; iB = (1-) iE

•Typically 100 for small signal BJTs (BJTs thathandle low power) operating in active region (region where BJTs work as amplifiers)

7

BJT in Active Region

Common Emitter(CE) Connection• Called CE because emitter is common to both VBB and VCC

8

BJT in Active Region •Base Emitter junction is forward biased

•Base Collector junction is reverse biased

•For a particular iB, iC is independent of RCC

transistor is acting as current controlled voltage source (iC is controlled by iB, and iC = iB)

• Since the base emitter junction is forward biased, from Shockleyequation

1exp

T

BECSC V

VIi

9

BJT in Active Region

•Normally the above equation is never used to calculate iC, iB Since for all small signal transistors vBE 0.7. It is only useful for deriving the small signal characteristics of the BJT.

•For example, for the CE connection, iB can be simply calculated as,

BB

BEBBB R

VVi

or by drawing load line on the base –emitter side

10

BJT in Cutoff Region

•Under this condition iB= 0

•As a result iC becomes negligibly small

•Both base-emitter as well base-collector junctions may be reverse biased

•Under this condition the BJT can be treated as an off switch

11

BJT in Saturation Region

•Under this condition iC / iB in active region

•Both base emitter as well as base collector junctions are forward biased

•VCE 0.2 V

•Under this condition the BJT can be treated as an on switch

12

•A BJT can enter saturation in the following ways (refer to the CE circuit)

•For a particular value of iB, if we keep on increasing RCC

•For a particular value of RCC, if we keep on increasing iB

•For a particular value of iB, if we replace the transistor with one with higher

BJT in Saturation Region

Early Effect and Early VoltageAs reverse-bias across collector-base junction increases, width of the

collector-base depletion layer increases and width of the base decreases (base-width modulation).

In a practical BJT, output characteristics have a positive slope in forward-active region; collector current is not independent of vCE.

Early effect: When output characteristics are extrapolated back to point of zero iC, curves intersect (approximately) at a common point vCE = -VA which lies between 15 V and 150 V. (VA is named the Early voltage)

Simplified equations (including Early effect)

13

iC IS expv

BE

VT

1v

CE

VA

F FO 1v

CE

VA

iB I

S

FO

expv

BE

VT

14

BJT Operating Regions

15

BJT ‘Q’ Point (Bias Point)•Q point means Quiescent or Operating point

• Very important for amplifiers because wrong ‘Q’ point selection increases amplifier distortion

•Need to have a stable ‘Q’ point, meaning the the operating point should not be sensitive to variation to temperature or BJT , which can vary widely

. 16

Input Characteristics

• Plot IB as f(VBE, VCE)

• As VCE increases, more VBE required to turn the BE on so that IB>0.

• Looks like a pn junction volt-ampere characteristic.

. 17

Output Characteristics• Plot IC as f(VCE, IB)• Cutoff region (off)

both BE and BC reverse biased

• Active regionBE Forward biasedBC Reverse biased

• Saturation region (on)both BE and BC forward biased

VCC/RC

VCC

. 18

Transfer Characteristics

Thank You

Any Question?

19