Bipolar Junction Transisitors-II

Dr. Nasim ZafarElectronics 1EEE 231 BS Electrical EngineeringFall Semester 2012COMSATS Institute of Information TechnologyVirtual campusIslamabad

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Bipolar Junction Transistors-BJTs

Lecture No: 14Contents:Introduction Bipolar Transistor CurrentsBipolar Transistor Characteristics and ParameterEarly Effect

2Nasim Zafar.References:Microelectronic Circuits: Adel S. Sedra and Kenneth C. Smith.

Electronic Devices : Thomas L. Floyd ( Prentice Hall ).

Integrated Electronics Jacob Millman and Christos Halkias (McGraw-Hill).

Electronic Devices and Circuit Theory: Robert Boylestad & Louis Nashelsky ( Prentice Hall ).

Introductory Electronic Devices and Circuits: Robert T. Paynter.

3Nasim Zafar.Reference:Chapter 4 Bipolar Junction Transistors:

Figures are redrawn (with some modifications) from

Electronic Devices ByThomas L. Floyd4Nasim Zafar.

Bipolar Junction Transistors

BJTs-CircuitsBCE5Nasim Zafar.Transistor Types MOS - Metal Oxide Semiconductor FET - Field Effect Transistor BJT - Bipolar Junction Transistor6Nasim Zafar.

Transistor Current Characteristics7Nasim Zafar.

An Overview of Bipolar Transistors:While control in a FET is due to an electric field. Control in a bipolar transistor is generally considered to be due to an electric current.current into one terminaldetermines the currentbetween two othersas with an FET, abipolar transistorcan be used as acontrol device8Nasim Zafar.Transistor Biasing Configurations:

Common-Base Configuration (CB) : input = VEB & IE ; output = VCB & IC

2. Common-Emitter Configuration (CE): input = VBE & IB ; output = VCE & IC Common-Collector Configuration (CC): input = VBC & IB ; output = VEC & IE

9Nasim Zafar.Operation Modes:Active: Most importance mode, e.g. for amplifier operation.The region where current curves are practically flat.Saturation:Barrier potential of the junctions cancel each other out causing a virtual short.Ideal transistor behaves like a closed switch.Cutoff:Current reduced to zeroIdeal transistor behaves like an open switch.10Nasim Zafar.

Operation Modes: Active: BJT acts like an amplifier (most common use). Saturation: BJT acts like a short circuit. Cutoff: BJT acts like an open circuit.11Nasim Zafar.Common Emitter Characteristics:We consider DC behaviour and assume that we are working in the normal linear amplifier regime with the BE junction forward biased and the CB junction reverse biased.12Nasim Zafar.Common-Emitter Output CharacteristicsVCEICActive RegionIBSaturation RegionCutoff RegionIB = 0Region of OperationDescriptionActiveSmall base current controls a large collector currentSaturationVCE(sat) ~ 0.2V, VCE increases with ICCutoffAchieved by reducing IB to 0, Ideally, IC will also equal 0.Output Characteristic Curves - (Vc- Ic

13Nasim Zafar.Common-Base-Configuration (CBC) NPN Transistor

Circuit Diagram: NPN Transistor14Nasim Zafar.Common-Base Output Characteristics:Although the Common-Base configuration is not the most common configuration, it is often helpful in understanding the operation of BJTOutput Characteristic Curves - (Vc- Ic Saturation RegionIEIC VCBActive RegionCutoffIE = 00.8V2V4V6V8VmA246IE=1mAIE=2mABreakdown Region )15Nasim Zafar.16Transistor Currents - Output characteristics:

Nasim Zafar.Common-Collector Output Characteristics:Emitter-Current CurvesVCEIEActive RegionIBSaturation RegionCutoff Region IB = 0

17Nasim Zafar.Bipolar Transistor CharacteristicsBehaviour can be described by the current gain, hfe or by the transconductance, gm of the device

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18Nasim Zafar.Conventional View & Current Components:NPN Transistor-CEC

19Nasim Zafar.Current Components: NPN Transistor-CEC

20Nasim Zafar.BJT Characteristics and Parameters21Nasim Zafar.BJT-Current Gain Parameters:Two quantities of great importance in the characterization of transistors are the so-called common-base current gain .. and the so-called common-emitter gain . DC and DC = Common-emitter current gain = Common-base current gain

Note: and are sometimes referred to as dc and dc because the relationships being dealt within the BJT are DC.22Nasim Zafar.BJT-Current Gain Parameters:Common-base current gain , is also referred to as hFB and is defined by:

= hFB = IC / IE Common-emitter current gain , is also referred as hFE and is defined by: = IC/IB Thus:

23Nasim Zafar.Beta () or amplification factor:

The ratio of dc collector current (IC) to the dc base current (IB) is dc beta (dc ) which is dc current gain where IC and IB are determined at a particular operating point, Q-point (quiescent point). Its define by the following equation: 30 < dc < 300 2N3904

On data sheet, dc=hFE with h is derived from ac hybrid equivalent circuit. FE are derived from forward-current amplification and common-emitter configuration respectively.

24Nasim Zafar.In the dc mode the level of IC and IE due to the majority carriers are related by a quantity called alpha: =

IC = IE + ICBOIt can then be summarize to IC = IE (ignore ICBO due to small value)

For a.c situations where the point of operation moves on the characteristics curve, an a.c alpha defined by

Alpha a common base current gain factor that shows the efficiency by calculating the current percent from current flow from emitter to collector. The value of is typical from 0.9 ~ 0.998.

25Nasim Zafar.BJT-Current Gain Parameters:

= Common-Base Current Gain (typical 0.99)

26Nasim Zafar.26BJT-Current Gain Parameters:

= Common-emitter current gain (10-1000; typical 50-200)

27Nasim Zafar.27DC and DC

= Common-emitter current gain (10-1000; typical 50-200) = Common-base current gain (0.9-0.999; typical 0.99) The relationship between the two parameters are:28Nasim Zafar.28Performance Parameters for PNP:Common emitter dc current gain, dc:

But,

Note that is large (e.g. = 100)For NPN transistor, similar analysis can be carried out. However,the emitter current is mainly carried by electrons.Example:

29Nasim Zafar.Performance Parameters for PNP:Emitter efficiency:

Fraction of emitter current carried by holes.We want close to 1.Base transport factor:

Fraction of holes collected by the collector.We want T close to 1.Common base dc current gain:

30Nasim Zafar.Example: NPN Common-Base Configuration:

+_+_Given: IB = 50 A , IC = 1 mA

Find: IE , , and

Solution:IE = IB + IC = 0.05 mA + 1 mA = 1.05 mA = IC / IB = 1 mA / 0.05 mA = 20 = IC / IE = 1 mA / 1.05 mA = 0.95238ICIEIBVCBVBEECB31Nasim Zafar.Nasim Zafar.32