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 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.
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.