Lecture 17 Small AC Signal Model of FETeng.staff.alexu.edu.eg/.../Lecture_17_MOSFET.pdf · Small AC Signal Equivalent Model for MOSFET -The ac equivalent model for MOSFETs is exactly

Lecture 17

Small AC Signal Model of FET

FET Small AC Signal Model 1-1 Wednesday 6/12/2017

Outline Small AC Signal Equivalent Circuits for

FETs Amplifier Circuits Examples

Introduction to Power Electronics Power Semiconductor Devices

• Power MOSFET

FET Small AC Signal Model 1-2

Revisiting MOSFET

FET Small AC Signal Model 1-3

MOSFET Characteristics Curve

FET Small-Signal Model

Transconductance

The relationship of a change in ID to the corresponding

change in VGS is called transconductance or mutual

conductance

Transconductance is denoted gm and given by:

Device transconductance, gm, is provided on

specification sheets as forward transfer admittance, yfs

FET Small AC Signal Model 1-4

GS

Dm

VΔ

IΔg

Graphical Determination of gm

FET Small AC Signal Model 1-5

Mathematical Definitions of gm

FET Small AC Signal Model 1-6

GS

Dm

V

Ig

DSS GSm

P P

2I Vg 1 _ _ _

V Vdifferentiating the Shockley eq

P

DSSm0

V

2Ig

P

GSm0m

V

V1gg

DSS

D

P

GS

I

I

V

V1

DSS

Dm0

P

GSm0m

I

Ig

V

V1gg

When VGS =0V

Where

For JFET and Depletion Type MOSFET

Mathematical Definitions of gm

For Enhancement Type MOSFET

FET Small AC Signal Model 1-7

GS

Dm

V

Ig

K

I)VV( D

TGS

DD

m I2K

I2Kg K

Where

2

TGSD VVKI

TGSm VVK2g

FET AC Equivalent Circuit

FET Small AC Signal Model 1-8

Zi

osdo

y

1rZ

constant VD

DSd GSI

Vr

Input impedance:

Output Impedance:

where:

yos= output admittance parameter listed on FET specification sheets.

Common-Source (CS) Fixed-Bias Circuit

FET Small AC Signal Model 1-9

Gi RZ

dDo r||RZ

10RrDo

Dd

RZ

Input impedance:

Output impedance:

)R||(rgV

VA Ddm

i

ov

Dd 10RrDmi

ov Rg

V

VA

Voltage gain:

Coupling

capacitors

Common-Source (CS) Voltage-Divider Bias

FET Small AC Signal Model 1-10

21i R||RZ

Ddo R||rZ

10RrDo

Dd

RZ

Input impedance:

Output impedance:

)R||(rgA Ddmv

Dd 10RrDmv RgA

Voltage gain:

Example (1)

FET Small AC Signal Model 1-11

The fixed-bias configuration had an operating point defined by VGSQ = - 2 V and IDQ = 5.625 mA, with IDSS = 10 mA and VP = -8 V. The value of yos is provided as 40 μS.

(a) Determine gm

(b) Find rd

(c) Determine Zi

(d) Calculate Zo

(e) Determine the voltage gain Av

Example (1) – Solution

FET Small AC Signal Model 1-12

Small AC Signal Equivalent Model for MOSFET

- The ac equivalent model for MOSFETs is exactly the same as that employed for JFETs

- The only difference offered by D-MOSFETs is that VGSQ can be positive for n-channel devices and negative for p-channel units

- The result is that gm can be greater than gm0

FET Small AC Signal Model 1-13

Exercise

What is the function of the following MOSFET circuit ?

FET Small AC Signal Model 1-14

MOSFET related Application

Complementary MOSFET Motor Controller

FET Small AC Signal Model 1-15

Power Electronics Technology

Power electronics is the application of solid-state (e.g., crystalline semiconductor) electronics for the control and conversion of electric power

The potential for applications of power electronics become wider used in a great variety of high power product, including heat

controls, light controls, electric motor control, power supplies, vehicle propulsion system and high voltage direct current (HVDC) systems

Many power semiconductor devices (e.g., power MOSFET) are available and directed to the field of power electronics A power semiconductor device is an electronic device that can

be used as switches in power electronic circuits FET Small AC Signal Model 1-16

Power Electronics Application

FET Small AC Signal Model 1-17

Power Semiconductor Devices

Interesting parameters Breakdown voltage

On-resistance

Rise and fall times

Safe operating area

Thermal resistance

Different power transistors BJT (Bipolar Junction Transistor)

MOSFET (Metal Oxide Semiconductor Field effect transistor)

IGBT (Insulated Gate Bipolar Transistor) FET Small AC Signal Model 1-18

Power MOSFET

Vertical diffused MOS (VDMOS) or Double-Diffused MOS (or simply DMOS) Structure

designed to handle significant power levels

high current commutation speed and good efficiency at low voltages

found in most power supplies, DC to DC converters, and low voltage motor controllers

FET Small AC Signal Model 1-19

Lecture-related Question

Design the fixed-bias circuit to have an AC gain of 10

FET Small AC Signal Model 1-20

Solution

FET Small AC Signal Model 1-21

Lecture Summary

Covered material

Small AC Signal Equivalent Circuits for FETs Amplifier Circuits Examples

Introduction to Power Electronics Power Semiconductor Devices

• Power MOSFET

Material to be covered next lecture

Introduction to operational amplifiers and their applications