1. Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Chapter 9: FET Small-Signal Analysis

2. Slide 1 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Introduction FETs provide excellent voltage gain high input impedance low-power consumption good frequency range

3. Slide 2 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. FET Small-Signal Model Transconductance The relationship of VGS (input) to ID (output) is called transconductance. The transconductance is denoted gm. [Formula 9.2]

4. Slide 3 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Graphical Determination of gm

5. Slide 4 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Mathematical Definition of gm [Formula 9.2] [Formula 9.4] gm for VGS =0V: [Formula 9.5] [Formula 9.6] for [Formula 9.8] [Formula 9.9]

6. Slide 5 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. FET Impedance Input Impedance Zi:[Formula 9.10] Output Impedance Zo:[Formula 9.11] [Formula 9.12] yos: admittance equivalent circuit parameter listed on FET specification sheets.

7. Slide 6 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. FET AC Equivalent Circuit

8. Slide 7 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. JFET Common-Source (CS) Fixed-Bias Configuration The input is on the gate and the output is on the drain.

9. Slide 8 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. AC Equivalent Circuit

10. Slide 9 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Impedances Input Impedance: [Formula 9.13] Output Impedance:[Formula 9.14] [Formula 9.15]

11. Slide 10 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Voltage Gain [Formula 9.16] [Formula 9.17]

12. Slide 11 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Phase Relationship A CS amplifier configuration has a 180-degree phase shift between input and output.

13. Slide 12 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. JFET CS Self-Bias Configuration This is a CS amplifier configuration therefore the input is on the gate and the output is on the drain.

14. Slide 13 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. AC Equivalent Circuit

15. Slide 14 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Impedances Input Impedance: [Formula 9.18] Output Impedance:[Formula 9.19] [Formula 9.20]

16. Slide 15 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Voltage Gain [Formula 9.21] [Formula 9.22]

17. Slide 16 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Phase Relationship A CS amplifier configuration has a 180-degree phase shift between input and output.

18. Slide 17 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. JFET CS Self-Bias Configuration – Unbypassed Rs If Cs is removed, it affects the gain of the circuit.

19. Slide 18 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. AC Equivalent Circuit

20. Slide 19 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Impedances Input Impedance: [Formula 9.23] Output Impedance: [Formula 9.25(b)]

21. Slide 20 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Voltage Gain [Formula 9.26] [Formula 9.27]

22. Slide 21 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. JFET CS Voltage-Divider Configuration This is a CS amplifier configuration therefore the input is on the gate and the output is on the drain.

23. Slide 22 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. AC Equivalent Circuit

24. Slide 23 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Impedances Input Impedance: [Formula 9.28] Output Impedance:[Formula 9.29] [Formula 9.30]

25. Slide 24 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Voltage Gain [Formula 9.31] [Formula 9.32]

26. Slide 25 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. JFET Source Follower (Common-Drain) Configuration In a CD amplifier configuration the input is on the gate, but the output is from the source.

27. Slide 26 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. AC Equivalent Circuit

28. Slide 27 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Impedances Input Impedance: [Formula 9.33] Output Impedance:[Formula 9.34] [Formula 9.35]

29. Slide 28 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Voltage Gain [Formula 9.36] [Formula 9.37]

30. Slide 29 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Phase Relationship A CD amplifier configuration has no phase shift between input and output.

31. Slide 30 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. JFET Common-Gate Configuration The input is on source and the output is on the drain.

32. Slide 31 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. AC Equivalent Circuit

33. Slide 32 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Impedances Input Impedance: [Formula 9.39] [Formula 9.40] Output Impedance:[Formula 9.41] [Formula 9.42]

34. Slide 33 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Voltage Gain [Formula 9.43] [Formula 9.44]

35. Slide 34 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Phase Relationship A CG amplifier configuration has no phase shift between input and output.

36. Slide 35 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Depletion-Type MOSFETs D-MOSFETs have similar AC equivalent models. The only difference is that V SGQ can be positive for n-channel devices and negative for p-channel devices. This means that gm can be greater than gm0.

37. Slide 36 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. D-MOSFET AC Equivalent Model [Fig. 9.34]

38. Slide 37 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Enhancement-Type MOSFETs There are two types of E-MOSFETs: nMOS or n-channel MOSFETs pMOS or p-channel MOSFETs

39. Slide 38 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. E-MOSFET AC Equivalent Model gm and rd can be found in the specification sheet for the FET.

40. Slide 39 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. E-MOSFET CS Drain-Feedback Configuration

41. Slide 40 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. AC Equivalent Circuit

42. Slide 41 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Impedances Input Impedance: [Formula 9.46] [Formula 9.47] Output Impedance:[Formula 9.48] [Formula 9.49]

43. Slide 42 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Voltage Gain [Formula 9.50] [Formula 9.51]

44. Slide 43 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Phase Relationship This is a CS amplifier configuration therefore it has 180-degree phase shift between input and output.

45. Slide 44 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. E-MOSFET CS Voltage-Divider Configuration

46. Slide 45 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. AC Equivalent Circuit

47. Slide 46 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Impedances Input Impedance: [Formula 9.52] Output Impedance:[Formula 9.53] [Formula 9.54]

48. Slide 47 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Voltage Gain [Formula 9.55] [Formula 9.56]

49. Slide 48a Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Summary Table

50. Slide 48b Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Summary Table

51. Slide 49 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Troubleshooting 1. 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. 2. Check the AC voltages – if not correct check FET, capacitors and the loading effect of the next stage.

52. Slide 50 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Practical Applications Three-Channel Audio Mixer Silent Switching Phase Shift Networks Motion Detection System