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Mastering FET Amplifiers: Voltage Gain, Impedance, and Frequency Range

Dive into the world of Field-Effect Transistor amplifiers, exploring their key features, models, AC circuits, bias configurations, and practical applications. Learn to analyze impedance, voltage gain, troubleshoot, and implement in various systems.

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Mastering FET Amplifiers: Voltage Gain, Impedance, and Frequency Range

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  1. Chapter 8: FET Amplifiers

  2. Introduction • FETs provide: • Excellent voltage gain • High input impedance • Low-power consumption • Good frequency range 2

  3. FET Small-Signal Model • Transconductance • The relationship of a change in ID to the corresponding change in VGS is called transconductance • Transconductance is denoted gm and given by: 3

  4. Graphical Determination of gm 4

  5. Where VGS =0V Where Mathematical Definitions of gm 5

  6. FET Impedance Input impedance: Output Impedance: where: yos= admittance parameter listed on FET specification sheets. 6

  7. FET AC Equivalent Circuit 7

  8. Common-Source (CS) Fixed-Bias Circuit The input is on the gate and the output is on the drain There is a 180 phase shift between input and output 8

  9. Calculations Input impedance: Output impedance: Voltage gain: 9

  10. 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 There is a 180 phase shift between input and output 10

  11. Calculations Input impedance: Output impedance: Voltage gain: 11

  12. Common-Source (CS) Self-Bias Circuit Removing Cs affects the gain of the circuit. 12

  13. Calculations Input impedance: Output impedance: Voltage gain: 13

  14. 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

  15. Impedances Input impedance: Output impedance: Voltage gain: 15

  16. Source Follower (Common-Drain) Circuit In a common-drain amplifier configuration, the input is on the gate, but the output is from the source. There is no phase shift between input and output. 16

  17. Impedances Input impedance: Output impedance: Voltage gain: 17

  18. Common-Gate (CG) Circuit The input is on the source and the output is on the drain. There is no phase shift between input and output. 18

  19. Calculations Input impedance: Output impedance: Voltage gain: 19

  20. D-Type MOSFET AC Equivalent 20

  21. E-Type MOSFET AC Equivalent gm and rd can be found in the specification sheet for the FET. 21

  22. Common-Source Drain-Feedback There is a 180 phase shift between input and output. 22

  23. Calculations Input impedance: Output impedance: Voltage gain: 23

  24. Common-Source Voltage-Divider Bias 24

  25. Calculations Input impedance: Output impedance: Voltage gain: 25

  26. Summary Table more… 26

  27. Summary Table 27

  28. 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

  29. Practical Applications Three-Channel Audio Mixer Silent Switching Phase Shift Networks Motion Detection System 29

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