JJJuly09LessonSeven

JJJuly09LessonSeven - July 09 Edition Key Lecture Concepts...

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July 09 Edition Key Lecture Concepts for CoE225/EE 271 (Mostly Digital Electronics) LESSON SEVEN: Procedures for Finding Transfer Characteristics and Noise Margins of Inverters; Inverter Dynamic Power Dissipation; the MOSFET Threshold Voltage Shift (the Body Effect); Introduction to Memories; Comparison of the Different Types of NMOS Inverters. Lesson Overview and Objectives: One major learning objective of this lesson is to learn how to obtain pairs of input and output values by graphical load-line analysis for the purpose of constructing transfer characteristics for inverter circuits. Graphical load-line analysis will also be applied to visualize the factors that control how inverters switch from the low to the high level and from the high to low level. Another objective is to write equations for the different regions of the transfer characteristics and to identify the factors that control the noise margins. The lesson explains that the value of the threshold voltage of a MOST depends not only on the gate to source voltage but on the voltage between the source and substrate body, also called the back gate. This threshold voltage dependence (or body effect) significantly increases the low to high switching time of DMOST load inverters. The lesson explains how the body effect reduces current from the DMOS load needed to charge up capacitance. A key objective of the lesson is to learn how basic energy concepts can be used to derive the power dissipated in a logic circuit when charging load capacitance through the nonlinear load and also when discharging the load capacitance through the switch transistor. The derivation will show that the power dissipated is proportional to not only the clock frequency and capacitance and but also to the SQUARE of the POWER SUPPLY VOLTAGE! Dynamic power loss is usually greater than static power dissipation. It is a major concern as digital technology moves into the Gigahertz range. Each of you will have a chance to invent ways to alleviate this problem. A) Graphical Approach for Sketching the Transfer Characteristics of Inverters To obtain the transfer characteristics of the DMOS reference inverter, plotted in fig7.1b , the I/V characteristic of the DMOST (M L ) was plotted backwards from 3 [v], as shown in fig7.1a . [Recall that the threshold voltage for M L was minus 1.5 [v] and its saturation current for V GS = 0 was 80 μ A. Recall that the load characteristic is not a function of v I while the switch (M S ) characteristic is. For M S the threshold voltage is 0.5 [v], με /t 0X is 100 μ A/v 2 and W/L is 5/3.] The method for generating the v 0 /v I pairs to plot the transfer curve in fig7.1b can be learned by studying fig7.1a . The i D (SAT) for M S for a small value of v I just above the V T of 0.5 [v], is first calculated. For v I = 0.9[v], i D (SAT) equals 13.3 μ A, by equation one typed in the middle of fig7.1a . Observe that the only part of the M S curve for v I = 0.9[v] which needs to be plotted is the part that intersects the load curve.] The intersections of the M S curves with the M L curves
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by R.H.Cornely, July 09 Edition
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This note was uploaded on 02/18/2010 for the course ECET 271 taught by Professor Hu during the Spring '05 term at NJIT.

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JJJuly09LessonSeven - July 09 Edition Key Lecture Concepts...

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