Download presentation

Presentation is loading. Please wait.

Published byJason Sharp Modified over 2 years ago

1
FET Small-Signal Analysis

2
FET Small-Signal Model Transconductance The relationship of V GS (input) to I D (output) is called transconductance. The transconductance is denoted g m. Transfer Curve

3
Graphical Determination of g m

4
Mathematical Definition of g m Using differential calculus Maximum g m at V GS =0V: Effect of I D on g m for

5
FET Impedance Input Impedance Z i : Very large to assume input terminal approximate an open circuit Output Impedance Z o : y os : admittance equivalent circuit parameter listed on FET specification sheets.

6
FET Specification

7
FET AC Equivalent Circuit

8
JFET Fixed-Bias Configuration The input is on the gate and the output is on the drain.

9
JFET Fixed-Bias Configuration Once again: same step as BJT to redraw the network to AC equivalent circuit. Capacitor – short circuit DC batteries V GG and V DD are set to zero volts by a short-circuit equivalent

10
AC Equivalent Circuit

11

12
Impedances Input Impedance:Output Impedance:

13
Voltage Gain

14
Phase Relationship A CS amplifier configuration has a 180-degree phase shift between input and output.

15
Example Fixed-bias configuration has an operating point defined by V GSQ = -2V and I DQ = mA, with I DSS = 10mA and V P = -8V. The value of y os is provided as 40 µS. Determine: a) g m b) Z i c) Z o d) A V e) A V ignoring the effects of r d

16
Solution

17
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.

18
AC Equivalent Circuit

19
Impedances Input Impedance: Output Impedance:

20
Voltage Gain

21
Phase Relationship A CS amplifier configuration has a 180-degree phase shift between input and output.

22
JFET CS Self-Bias Configuration – Unbypassed Rs If Cs is removed, it affects the gain of the circuit.

23
AC Equivalent Circuit

24
Impedances Input Impedance: Output Impedance:

25
Impedances

26
Voltage Gain

27

28
Example

29
Solution

30

31
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.

32
AC Equivalent Circuit

33
Impedances Input Impedance:Output Impedance:

34
Voltage Gain

35
JFET Source Follower (Common-Drain) Configuration In a CD amplifier configuration the input is on the gate, but the output is from the source.

36
AC Equivalent Circuit

37
Impedances Input Impedance:Output Impedance:

38
Voltage Gain

39
Phase Relationship A CD amplifier configuration has no phase shift between input and output.

40
JFET Common-Gate Configuration The input is on source and the output is on the drain.

41
AC Equivalent Circuit

42
Impedances Applying Kirchhoffs voltage law around the output perimeter and Kirchhoffs current law at node a ::

43
Impedances Input Impedance: Output Impedance:

44
Voltage Gain Applying Kirchhoffs current law at node b ::

45
Phase Relationship A CG amplifier configuration has no phase shift between input and output.

46
Depletion-Type MOSFETs 1.D-MOSFETs have similar AC equivalent models. 2.The only difference is that V SGQ can be positive for n- channel devices and negative for p-channel devices. 3.This means that g m can be greater than g m0.

47
D-MOSFET AC Equivalent Model

48
Example Find –V GSQ and I DQ –Determine g m and compare to g m0 –r d –Find Z i, Z o, A v

49
Enhancement-Type MOSFETs There are two types of E-MOSFETs: nMOS or n-channel MOSFETs pMOS or p-channel MOSFETs

50
E-MOSFET AC Equivalent Model g m and r d can be found in the specification sheet for the FET. Forward transfer admittance

51
E-MOSFET CS Drain-Feedback Configuration

52
AC Equivalent Circuit

53
Impedances Input Impedance: Output Impedance:

54
The calculation

55

56
The AC analysis of E-MOSFET Remember that, the biasing arrangement are limited for E-MOSFET

57
Voltage Gain

58
Phase Relationship This is a CS amplifier configuration therefore it has 180-degree phase shift between input and output.

59
Do it Determine input and output and also A V impedance for k=0.3X10 -3

60
E-MOSFET CS Voltage-Divider Configuration

61
AC Equivalent Circuit

62
Impedances Input Impedance: Output Impedance:

63
Voltage Gain

64
Phase Relationship This is a CS amplifier configuration therefore it has 180-degree phase shift between input and output.

65
Solution

66
E-MOSFET CS Voltage-Divider Configuration

67
AC Equivalent Circuit

68
Impedances Input Impedance: [Formula 9.52] Output Impedance:[Formula 9.53] [Formula 9.54]

69
Voltage Gain [Formula 9.55] [Formula 9.56]

70
Summary Table

71

72
Try yourself Design a self-bias network that have gain of 10. The device should be biased at V GSQ =1/3V P

73
Solution

74
To be continued……

Similar presentations

© 2016 SlidePlayer.com Inc.

All rights reserved.

Ads by Google