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Chapter 8: FET Amplifiers
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Introduction FETs provide: Excellent voltage gain High input impedance Low-power consumption Good frequency range 2
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky FET Small-Signal Model Transconductance transconductance The relationship of a change in I D to the corresponding change in V GS is called transconductance Transconductance is denoted g m and given by: 3
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Graphical Determination of g m 4
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Mathematical Definitions of g m Where V GS =0V Where 5
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Input impedance: FET Impedance Output Impedance: where: y os = admittance parameter listed on FET specification sheets. 6
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky FET AC Equivalent Circuit 7
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Common-Source (CS) Fixed-Bias Circuit The input is on the gate and the output is on the drain 8 There is a 180 phase shift between input and output
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Calculations Input impedance: Output impedance: 9 Voltage gain:
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Common-Source (CS) Self-Bias Circuit This is a common-source amplifier configuration, so the input is on the gate and the output is on the drain 10 There is a 180 phase shift between input and output
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Calculations Input impedance: Output impedance: 11 Voltage gain:
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Common-Source (CS) Self-Bias Circuit Removing C s affects the gain of the circuit. 12
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Calculations Input impedance: Output impedance: 13 Voltage gain:
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Common-Source (CS) Voltage-Divider Bias This is a common-source amplifier configuration, so the input is on the gate and the output is on the drain. 14
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Impedances Input impedance: Output impedance: 15 Voltage gain:
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Source Follower (Common-Drain) Circuit In a common-drain amplifier configuration, the input is on the gate, but the output is from the source. 16 There is no phase shift between input and output.
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Impedances Input impedance: Output impedance: 17 Voltage gain:
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Common-Gate (CG) Circuit The input is on the source and the output is on the drain. 18 There is no phase shift between input and output.
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Calculations Input impedance: Output impedance: 19 Voltage gain:
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky D-Type MOSFET AC Equivalent 20
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky E-Type MOSFET AC Equivalent g m and r d can be found in the specification sheet for the FET. 21
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Common-Source Drain-Feedback 22 There is a 180 phase shift between input and output.
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Calculations Input impedance: Output impedance : 23 Voltage gain :
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Common-Source Voltage-Divider Bias 24
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Calculations Input impedance : Output impedance: 25 Voltage gain:
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Summary Table more… 26
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Summary Table 27
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Troubleshooting. Check the DC bias voltages: If not correct check power supply, resistors, FET. Also check to ensure that the coupling capacitor between amplifier stages is OK. Check the AC voltages: If not correct check FET, capacitors and the loading effect of the next stage 28
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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Practical Applications Three-Channel Audio Mixer Silent Switching Phase Shift Networks Motion Detection System 29
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