Dr. Nasim Zafar Electronics 1 EEE 231 – BS Electrical Engineering Fall Semester – 2012 COMSATS Institute of Information Technology Virtual campus Islamabad

Bipolar Junction Transistors - BJTs Lecture No: 14 Contents:  Introduction  Bipolar Transistor Currents  Bipolar Transistor Characteristics and Parameter  Early 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 By Thomas L. Floyd 4Nasim Zafar.

Bipolar Junction Transistors BJTs-Circuits B C E 5Nasim Zafar.

Transistor Types  MOS - Metal Oxide Semiconductor  FET - Field Effect Transistor  BJT - Bipolar Junction Transistor ◄◄◄◄ 6Nasim Zafar.

Transistor Current Characteristics 7Nasim 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 terminal determines the current between two others – as with an FET, a bipolar transistor can be used as a ‘control device’ 8Nasim Zafar.

Transistor Biasing Configurations: 1.Common-Base Configuration (CB) : input = V EB & I E ; output = V CB & I C 2. Common-Emitter Configuration (CE): input = V BE & I B ; output = V CE & I C 3.Common-Collector Configuration (CC): & I E input = V BC & I B ; output = V EC & I E 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 zero – Ideal 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 Characteristics V CE ICIC Active Region IBIBIBIB Saturation Region Cutoff Region I B = 0 Region of Operation Description ActiveSmall base current controls a large collector current SaturationV CE(sat) ~ 0.2V, V CE increases with I C CutoffAchieved by reducing I B to 0, Ideally, I C will also equal 0. Output Characteristic Curves - (V c - I c 13Nasim Zafar.

Common-Base-Configuration (CBC) NPN Transistor Circuit Diagram: NPN Transistor 14Nasim 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 BJT Output Characteristic Curves - (V c - I c Saturation Region IEIEIEIE ICIC V CB Active Region Cutoff I E = 0 0.8V2V4V6V8V mA I E =1mA I E =2mA Breakdown Region ) 15Nasim Zafar.

16 Transistor Currents - Output characteristics: Nasim Zafar.

Common-Collector Output Characteristics: Emitter-Current Curves V CE IEIE Active Region IBIB Saturation Region Cutoff Region I B = 0 17Nasim Zafar.

Bipolar Transistor Characteristics Behaviour can be described by the current gain, h fe or by the transconductance, g m of the device Nasim Zafar.

Conventional View & Current Components: NPN Transistor-CEC 19Nasim Zafar.

Current Components: NPN Transistor-CEC 20Nasim Zafar.

BJT Characteristics and Parameters 21Nasim 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 h FB and is defined by:  = h FB = I C / I E  Common-emitter current gain β, is also referred as h FE and is defined by:  = I C /I B 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).  It’s define by the following equation: 30 <  dc < 300  2N3904  dc =h FE h  On data sheet,  dc =h FE 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 I C and I E due to the majority carriers are related by a quantity called alpha:  = I C =  I E + I CBO  It can then be summarize to I C =  I E (ignore I CBO due to small value)  For a.c situations where the point of operation moves on the characteristics curve, an a.c alpha defined by common base current gain factor  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 ~ Nasim Zafar.

BJT-Current Gain Parameters:  = Common-Base Current Gain (typical 0.99) 26Nasim Zafar.

BJT-Current Gain Parameters:  = Common-emitter current gain ( ; typical ) 27Nasim Zafar.

DC  and DC   = Common-emitter current gain ( ; typical )  = Common-base current gain ( ; typical 0.99)  The relationship between the two parameters are: 28Nasim Zafar.

Performance 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: I B = 50  A, I C = 1 mA Find: I E, , and  Solution: I E = I B + I C = 0.05 mA + 1 mA = 1.05 mA b = I C / I B = 1 mA / 0.05 mA = 20  = I C / I E = 1 mA / 1.05 mA = ICIC IEIE IBIB V CB V BE E C B 31Nasim Zafar.

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