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Electronic Devices and Circuit Theory Boylestad FET Amplifiers Chapter 8 Ch.8 Summary Introduction FETs provide: • Excellent voltage gain • High input impedance • Low-power consumption • Good frequency response Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary FET Small-Signal Model Transconductance: The ratio of a change in ID to the corresponding change in VGS • Transconductance is denoted gm and given by: ΔID gm ΔV GS Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Geographical Determination of gm Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Mathematical Definitions of gm gm ΔID ΔVGS 2I g m DSS VP VGS 1 V P For VGS = 0 V g m0 2I DSS VP Electronic Devices and Circuit Theory Boylestad VGS ID g m g m0 1 g m0 V IDSS P © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary FET Impedence Input impedance: Zi Ω Output Impedance: 1 Zo rd y os where rd ΔVDS ΔID VGS constant yos= admittance parameter listed on FET spec sheets Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary FET AC Equivalent Circuit Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Common-Source (CS) Fixed-Bias The input is applied to the gate and the output is taken from the drain There is a 180 phase shift between the circuit input and output Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Calculations Input impedance: Zi RG Output impedance: Zo RD||rd Zo RD rd 10 RD Voltage gain: Av Av Vo g m (rd||R D ) Vi Vo g m RD Vi Electronic Devices and Circuit Theory Boylestad rd 10 RD © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Common-Source (CS) Self-Bias This is a common-source amplifier configuration, so the input is applied to the gate and the output is taken from the drain. There is a 180 phase shift between input and output. Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Calculations Input impedance: Zi RG Output impedance: Zo rd||RD Zo RD rd 10 RD Voltage gain: Av g m (rd||RD ) Av g mRD rd 10 RD Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Common-Source (CS) Self-Bias Removing Cs affects the gain of the circuit. Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Calculations Input impedance: Zi RG Output impedance: Zo RD rd 10 RD Voltage gain: Vo g m RD R RS Vi 1 g m RS D rd V g R Av o m D rd 10 ( RD RS ) Vi 1 g m RS Av Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Common-Source (CS) Voltage-Divider Bias This is a common-source amplifier configuration, so the input is applied to the gate and the output is taken from the drain. Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Impedances Input impedance: Zi R1 ||R2 Output impedance: Zo rd ||RD Zo RD rd 10 RD Voltage gain: Av g m (rd||RD ) Av g mRD rd 10 RD Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Source Follower (Common-Drain) In a common-drain amplifier configuration, the input is applied to the gate, but the output is taken from the source. There is no phase shift between input and output. Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Impedances Input impedance: Zi RG Output impedance: Zo rd ||RS || Zo RS || 1 gm 1 gm rd 10 RS Voltage gain: Av Vo g m (rd ||RS ) Vi 1 g m (rd ||RS ) Electronic Devices and Circuit Theory Boylestad Av Vo g mRS Vi 1 g mRS rd 10 © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Common-Gate (CG) Circuit The input is applied to the source and the output is taken from the drain. There is no phase shift between input and output. Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Calculations Input impedance: r RD Zi RS || d 1 g r m d 1 Zi RS || r 10 RD gm d Output impedance: Zo RD ||rd Zo RD rd 10 Voltage gain: Electronic Devices and Circuit Theory Boylestad RD g mRD r V d Av o Vi RD 1 r d Av g m RD rd 10 RD © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary D-Type MOSFET AC Equivalent Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary E-Type MOSFET AC Equivalent gm and rd can be found in the specification sheet for the FET. Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Common-Source Drain-Feedback There is a 180 phase shift between input and output. Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Calculations Input impedance: Zi Zi RF rd ||R D 1 g m (rd ||R D ) RF 1 g m RD RF rd ||R D ,rd 10 RD Output impedance: Zo RF||rd ||R D Zo RD RF rd ||R D ,rd 10 RD Voltage gain: Electronic Devices and Circuit Theory Boylestad Av g m (RF||rd ||RD ) Av g mRD RF rd ||RD, rd 10 RD © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Common-Source Voltage-Divider Bias The input is applied to the gate and the output is taken from the drain. There is a 180º voltage phase shift between input and output. Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Calculations Input impedance: Z i R1||R2 Output impedance: Zo rd ||RD Zo RD rd 10 Voltage gain: Electronic Devices and Circuit Theory Boylestad Av g m (rd ||RD ) Av g m RD rd 10 RD © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Summary Table Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Summary Table Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Summary Table Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary 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 Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.8 Summary Practical Applications • Three-Channel Audio Mixer • Silent Switching • Phase Shift Networks • Motion Detection System Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved
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