FET Amplifiers Chapter 8 Boylestad Electronic Devices and Circuit Theory

Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved 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 All Rights Reserved Ch.8 Summary FET Small-Signal Model Transconductance: The ratio of a change in I D to the corresponding change in V GS Transconductance is denoted g m and given by:

Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Ch.8 Summary Geographical Determination of g m

Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Ch.8 Summary Mathematical Definitions of g m For V GS = 0 V

Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Ch.8 Summary FET Impedence Input impedance: Output Impedance: where y os = admittance parameter listed on FET spec sheets

Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 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 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 All Rights Reserved Ch.8 Summary Calculations Input impedance: Output impedance: Voltage gain:

Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 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 All Rights Reserved Ch.8 Summary Calculations Input impedance: Output impedance: Voltage gain:

Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Ch.8 Summary Common-Source (CS) Self-Bias Removing C s affects the gain of the circuit.

Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Ch.8 Summary Calculations Output impedance: Voltage gain: Input impedance:

Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 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 All Rights Reserved Ch.8 Summary Impedances Input impedance: Output impedance: Voltage gain:

Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 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 All Rights Reserved Ch.8 Summary Impedances Input impedance: Output impedance: Voltage gain:

Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 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 All Rights Reserved Ch.8 Summary Calculations Input impedance: Output impedance: Voltage gain:

Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 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 All Rights Reserved Ch.8 Summary E-Type MOSFET AC Equivalent g m and r d 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 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 All Rights Reserved Ch.8 Summary Calculations Input impedance: Output impedance: Voltage gain:

Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 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 All Rights Reserved Ch.8 Summary Calculations Input impedance: Output impedance: Voltage gain:

Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 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 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 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 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 All Rights Reserved Ch.8 Summary Practical Applications Three-Channel Audio Mixer Silent Switching Phase Shift Networks Motion Detection System