COMSATS Institute of Information Technology Virtual campus Islamabad
COMSATS Institute of Information Technology Virtual campus Islamabad Dr. Nasim Zafar Electronics 1 EEE 231 – BS Electrical Engineering Fall Semester – 2012
BJT-Transistor Characteristics and Parameters: Lecture No: 15 Contents: Ø Transistor Characteristics and Parameters. Ø The Gain Factors: DC Beta( ) and DC Alpha ( ). Ø Relationship of and . Ø Early Effect. Ø Maximum Transistor Ratings. Nasim Zafar 2
References: Ø Microelectronic Circuits: Adel S. Sedra and Kenneth C. Smith. Ø Electronic Devices : Thomas L. Floyd ( Prentice Hall ). Ø Integrated Electronics: Jacob Millman and Christos Halkias (Mc. Graw-Hill). Ø Electronic Devices and Circuit Theory: Robert Boylestad & Louis Nashelsky ( Prentice Hall ). Ø Introductory Electronic Devices and Circuits: Robert T. Paynter.
Reference: Chapter 4 – Bipolar Junction Transistors: Figures are redrawn (with some modifications) from Electronic Devices By Thomas L. Floyd Nasim Zafar 4
Bipolar Junction Transistors BJTs-Circuits C B E Nasim Zafar 5
Transistor Types: v MOS - Metal Oxide Semiconductor v FET - Field Effect Transistor v BJT - Bipolar Junction Transistor Nasim Zafar 6 ◄
Transistor Characteristics and Hybrid Parameters Nasim Zafar 7
An Overview of Bipolar Transistors: Ø While control in an FET is due to an electric field. Ø 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’ Nasim Zafar 8
Transistor Characteristics: Ø Transistor Geometry. Ø Carrier motion (mobility). Ø Collector “collection efficiency” (Alpha). Ø Asymmetry: Efficiency / Breakdown voltages. Ø NPN transistors are normally better than PNP since electron mobility is better than hole mobility. Nasim Zafar 9
Transistor Biasing Configurations and Operation Modes: Nasim Zafar 10
Transistor Biasing Configurations: 1. Common-Base Configuration (CB) : input = VEB & IE ; output = VCB & IC 2. Common-Emitter Configuration (CE): input = VBE & IB ; output = VCE & IC 3. Common-Collector Configuration (CC): input = VBC & IB ; output = VEC & IE Nasim Zafar 11
Modes of BJT Operation: Ø Active: BJT acts like an amplifier (most common use). Ø Saturation: BJT acts like a short circuit. ØCutoff: BJT acts like an open circuit. Nasim Zafar 12
Modes of BJT Operation: Ø Active Region: Region where current curves are practically flat. ● In Active Region, the transistor is on. The collector current is proportional to and controlled by the base current IC (IC = βIB) and relatively insensitive to VCE. ● In this region the transistor can be used as an amplifier. Ø Cutoff Region: Current reduced to zero. – The transistor is off. There is no conduction between the collector and the emitter. (IB = 0 therefore IC = 0). – Equivalent to an off-state and the transistor behaves like an open switch. Low current flow, High Voltage. Nasim Zafar 13
Modes of BJT Operation: Ø Saturation Region: – In Saturation region: The transistor is on. The collector current varies very little with a change in the base current in the saturation region. – The output voltage VCE is small, a few tenths of a volt. – The collector current is strongly dependent on VCE unlike in the active region. – Ideal transistor behaves like a closed switch. Nasim Zafar 14
Modes of BJT Operation: Nasim Zafar 15
Transistor Characteristics and Hybrid Parameters Nasim Zafar 16
1. DC-Current Gain Parameters: DC Beta ( dc) and DC Alpha ( dc ): Two quantities of great importance in the characterization of the transistors are: Ø common-base current gain . Ø common-emitter current gain . = Common-emitter current gain = Common-base current gain Nasim Zafar 17
DC Common-Emitter Current Gain : Ø Current gain β, usually designated as an equivalent hybrid (h) parameter h. FE, is defined by: h. FE = DC Ø The ratio of the dc collector current IC to the dc base current IB is defined as the dc gain factor Beta ( dc) of a transistor. Thus: = IC/IB Nasim Zafar 18
DC Common-Emitter Current Gain : = Common-emitter-current gain (typical 50 -200) Nasim Zafar 19
DC Common-Base Current Gain : Ø Current gain , is also referred to as h. FB and is defined by: h. FB = DC Ø The ratio of the dc collector current IC to IE, due to the majority carriers, are related by a quantity called dc Alpha ( dc ): = IC / I E Also: Nasim Zafar 20
DC Common-Base Current Gain : = Common-Base Current Gain (typical 0. 99) Nasim Zafar 21
Beta ( ) or Amplification Factor: Ø IC and IB are determined at a particular operating point, Q-point (quiescent point). Ø Typical values of dc range from: 30 < dc < 200 2 N 3904 Ø On data sheet, dc= h. FE with h is derived from ac hybrid equivalent circuit. h. FE are derived from forward-current amplification and common-emitter configuration respectively. Nasim Zafar 22
AC Common-Base Current Gain : Ø For ac situations, where the point of operation moves on the characteristics curve, an ac alpha is defined by: Ø Alpha, a common base current gain factor, gives the efficiency of the transistor for a current flow from the emitter to the collector. Ø The value of is typical from 0. 95 ~ 0. 99. Nasim Zafar 23
2. Relationship of DC and DC:
2. Relationship of DC and DC: = Common-base current gain (0. 95 -0. 99) = Common-emitter current gain (typical 50 -200) Ø The relationship between the two parameters are: Nasim Zafar 25
3. 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: Nasim Zafar 26
The Early Effect (Early Voltage) Nasim Zafar 27
Early Effect (base width modulation): Ø In a Common Emitter Configuration, IC depends on VCE. Ø An increase in VCE means that the CB junction becomes more reverse biased. Ø The depletion layer width increases into the base, reducing the effective base width. Ø Hence the base transport efficiency (α) and β increase with increasing VCE. Ø This effect is known as base width modulation or the Early Effect. Nasim Zafar 28
The Early Effect (Early Voltage) IC IB -VA Common-Emitter Configuration VCE Green = Ideal IC Orange = Actual IC (IC’) Nasim Zafar 29
Actual Output Characteristics Ø Salient features are: Ø The finite slope of the plots (IC depends on VCE). Ø A limit on the power that can be dissipated. Ø The curves are not equally spaced (i. e β varies with base current, IB). Note: The finite slope of the (IC-VCE) plot would manifest itself as an output resistance. This would appear in a more detailed a. c. equivalent circuit of the transistor than the one we shall derive from the ideal curve. Nasim Zafar 30
Output Characteristics: Ideal C-E Output Characteristics: Actual C-E Output Characteristics: IB = Nasim Zafar 31
an Example-The Early Effect: • Given: • Find: The common-emitter circuit below with IB = 25 A, VCC = 15 V, = 100 and VA = 80. a) The ideal collector current b) The actual collector current Nasim Zafar 32
Power Across BJT: Ø PBJT = VCE * i. CE Ø Should be below the rated transistor power. Ø Should be kept in mind when considering heat dissipation. Ø Reducing power increases efficiency. Nasim Zafar 33
Derating PDmax Ø PDmax is usually specified at 25°C. Ø The higher temperature goes, the less is Pdmax Ø Example: – A derating factor of 2 m. W/°C indicates the power dissipation is reduced 2 m. W each degree centigrade increase of temperature. Nasim Zafar 34
Summary of Bipolar Transistors: Ø Bipolar transistors have three terminals: collector, base and emitter. Ø The base is the control input. Ø Two polarities of device: npn and pnp Ø The collector current is controlled by the base voltage/current IC = h. FEIB. Nasim Zafar 35
Summary of Bipolar Transistors: Ø Bipolar transistors are widely used in both analogue and digital circuits. Ø They can be considered as either voltage-controlled or currentcontrolled devices. Ø Their characteristics may be described by their gain or by their transconductance. Ø The majority of circuits use transistors in a common-emitter configuration where the input is applied to the base and the output is taken from the collector Ø Common-collector circuits make good buffer amplifiers Ø Bipolar transistors are used in a wide range of applications Nasim Zafar 36
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