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Chapter 11 Field effect Transistors: Operation, Circuit, Models, and Applications Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Context • 11.1 Classification of Field-Effect Transistor • 11.2 Overview of Enhancement-mode Mosfet • 11.3 Biasing Mosfet Circuit • 11.4 Mosfet Large-Signal Amplifiers • 11.5 Mosfet Switches 2 Classification of Field-Effect Transistors • This figure depicts the classification of fieldeffect transistors, as well as the more commonly used symbols for these devices. Classification of field-effect transistors 3 Overview of Enhancement-Mode Mosfets • This figure depicts the circuit symbol and the approximate construction of a typical n-channel enhancement-mode MOSFET. The n-channel enhancement MOSFET construction and circuit symbol 4 Channel formation in NMOS transistor: (a) With no external gate voltage, the source-substrate and substrate-drain junctions are both reverse-biased, and no conduction occurs; (b) when a gate voltage is applied, chargecarrying electrons are drawn between the source and drain regions to form a conducting channel. 5 Operation of the n-channel Enhancement-Mode Mosfet Saturation region Regions of operation of NMOS transistor 6 Drain characteristic curves for a typical NMOS transistor with VT = 2 V and K = 1.5 mA/V2 7 EXAMPLE 11.1 Determining the Operating State of a Mosfet Problem • Determine the operating state of the MOSFET shown in the circuit of figure 11.6 for the given value of VDD and VGG if the ammeter and voltmeter shown read the following value: 8 b. VGG = 4V; VDD = 10V ;νDS = 2.8V; ίD = 72mA; RD = 100Ώ c. VGG = 3V; VDD = 10V ;νDS = 1.5V; ίD = 13.5mA; RD = 630Ώ 9 CHECK YOUR UNDERSTANDING • What is the operating state of the MOSFET of the example 11.1 for the following conditions? VGG = 10/3V; VDD = 10V ;νDS = 3.6V;ίD = 32mA; RD = 200Ώ 10 11 EXAMPLE 11.2 Mosfet Q-Point Graphical Determination Problem • Determine the Q point for the MOSFET in the circuit of figure 11.7. The n-channel enhancement MOSFET circuit and drain characteristic for Example 11.2 12 13 CHECK YOUR UNDERSTANDING • Determine the operating region of the MOSFET of example 11.2 when νGS = 3.5V. 14 EXAMPLE 11.3 Mosfet Q-Point Calculation Problem • Determine the Q point for the MOSFET in the circuit of figure 11.7. 15 16 CHECK YOUR UNDERSTANDING • Find the lowest value of RD for the MOSFET of the example 11.3 that will place the MOSFET in the ohmic region. 17 EXAMPLE 11.4 Mosfet Self-Bias Circuit Problem • Figure 11.8(a) depicts a self-bias circuit for a MOSFET. Determine the Q point for the MOSFET by choosing Rs such that νDSQ =8V. 18 19 CHECK YOUR UNDERSTANDING • Determine the appropriate value of RS if we wish to move the operating point of the MOSFET of example 11.4 to νDSQ =12V. Also find the value of νGSQ and ίDQ. Are these values unique? 20 EXAMPLE 11.5 Analysis of Mosfet Amplifier Problem • Determine the gate and drain-source voltage and the drain current for the MOSFET amplifier of figure11.9. 21 22 Operation of the P-channel Enhacement-Mode Mosfet The p-channel enhancement-mode field-effect transistor (PMOS) 23 The Resulting equations for the three modes of operation of the PMOS 24 Mosfet Large-Signal Amplifiers Common-source MOSFET amplifier 25 • Thus, the load voltage, across the load resistance, is given by the expression. 26 • Where △υ= VG-VT. We can then solve for the load current from the quadratic equation 27 (a) Source-follower MOSFET amplifier. (b) Drain current response for a 100-Ω load when K = 0.018 and VT = 1.2 V 28 EXAMPLE 11.6 Using a Mosfet as a Current Source for Battery Charging • Analyze the two battery charging circuit shown in figure 11.14. use the transistor parameters to determine the range of require gate voltages, VG ,to provide a variable charging current up to a maximum of 0.1A. Assume that the terminal voltage of a fully dischanged battery is 9V, and of a fully charged battery 10.5V. 29 30 31 CHECK YOUR UNDERSTANDING • What is the maximum power dissipation of the MOSFET for each of the circuit in example 11.6? 32 EXAMPLE 11.7 Mosfet DC Motor Drive Circuit Problem DC motor drive circuit 33 34 CHECK YOUR UNDERSTANDING • What is the range of duty cycle needed to cover the current range of the Lego motor? 35 Digital Switches and Gates CMOS inverter CMOS inverter approximate by ideal switches: (a) When Vin is “high,” Vout is tied to ground; (b) when Vin is “low,” Vout is tied to VDD. 36 EXAMPLE 11.8 Mosfet Switch Problem • Determine the operating points of the MOSFET switch of figure 11.18 when the signal source output is equal to 0and 2.5V, respectively. 37 CHECK YOUR UNDERSTANDING • What value of RD would ensure a drain-tosource voltage νDS of 5V in the circuit of example 11.8? 38 EXAMPLE 11.9 COMS Gate Problem • Determine the logic function implemented by the CMOS gate of figure 11.20.use the table below to summarize the behavior of the circuit. 39 40 CHECK YOUR UNDERSTANDING • Analyze the CMOS gate of figure 11.23 and find the output voltage for the following conditions: (a) ν1 = 0v, ν2 =0V (b) ν1 = 5V, ν2 =0V (c) ν1 = 0V, ν2 =5V (d) ν1 = 5V, ν2 =5V.identify the logic function accomplished by the circuit. 41 Analog Switches 42 Symbol for bilateral FET analog gate MOSFET analog switch 43 44 Homework Problem 45