NPN Characteristics - ELCT 301 Lab Report #2 Current...

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ELCT 301 Lab Report #2 Current Sources for Integrated Circuits ___________________ Jared Tucker Department of Electrical Engineering University of South Carolina Columbia, SC 29208 1
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Laboratory Grade: Pre-lab computation ____ of 10 Technical Content ____ of 60 Format/Presentation Clarity ____ of 20 Other ____ of 10 Late Deductions ____ ____ of 100 Student Comments: Grader Comments: 2
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Current Sources for Integrated Circuits I. Introduction The goal of this lab was to compare different types of current sources based on BJT configurations. II. Background An ideal current source is a model used to simplify circuit theory. In theory, the ideal current source sets the value of the current running through it, having a known current and unknown voltage; always delivering a constant amount of current no matter the demands of the rest of the system. This works only in theory, in real world cases, there are several other factors involved that could cause the “ideal” source to become nonideal (nonlinear). If the current source were to become overloaded, it would tend towards nonlinear operation. Overloading is an effect occurring due to physical limitations of the system that represents a current source. In this lab, I was to investigate how different transistor configurations measured up to the ideal current source model. It is here necessary to consider the physics of the transistor so the limitations will be apparent. Ebers-Moll, a distinguished physicist accurately defines the properties of the transistor through Figure (1) equations (1,2,3). Figure 1. Ebers-Moll circuit model for an npn transistor. - - - = 1 1 T BC T BE V V CS R V V ES E e I e I I α (1) - - - = 1 1 T BC T BE V V CS V V ES F C e I e I I (2) C E B I I I - = (3) where 1 + = β R (4) and 3
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q KT V T = (5) β representing the gain factor of the transistor, K the Boltsman’s constant, q the electron charge and T the temperature of the junction of the transistor. By inspection of the equations, one can see that the K and q will remain constant while a change in the temperature could cause the gain factor and current to change. Also one can see the limitations in the parenthetical portions of (1). So now that some dependencies and limitations are understood, we can now turn to the application and analysis of transistors as current sources. In Figure (2) Q1 and R1 are the Class A current source; Q2, Q3 , R2 is the current mirror; while components Q4,Q5, R4, R3 are the Widlar current source. This is how they shall be referred to throughout the duration of this report. Figure 2. From left to right, Class A, Current mirror and Widlar sources
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NPN Characteristics - ELCT 301 Lab Report #2 Current...

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