Lecture 17 Sall AC Signal Model of FET Wednesday 6/12/2017 FET Sall AC Signal Model 1-1
Outline Sall AC Signal Equivalent Circuits for FETs Aplifier Circuits Exaples Introduction to Power Electronics Power Seiconductor evices Power MOSFET FET Sall AC Signal Model 1-2
Revisiting MOSFET MOSFET Characteristics Curve FET Sall AC Signal Model 1-3
FET Sall-Signal Model Transconductance The relationship of a change in I to the corresponding change in GS is called transconductance or utual conductance Transconductance is denoted g and given by: g ΔI Δ evice transconductance, g, is provided on specification sheets as forward transfer adittance, y fs GS FET Sall AC Signal Model 1-4
Graphical eterination of g FET Sall AC Signal Model 1-5
Matheatical efinitions of g For JFET and epletion Type MOSFET 2I g I GS SS GS g 1 _ P P differentiating the Shockley eq When GS =0 g g 0 g 2I 0 SS P 1 GS P Where GS 1 P I I SS g g 0 1 GS P I g0 ISS FET Sall AC Signal Model 1-6
Matheatical efinitions of g For Enhanceent Type MOSFET g I GS I 2 K GS T g 2K GS T Where I ( GS T ) K g 2K I K 2 KI FET Sall AC Signal Model 1-7
FET AC Equivalent Circuit Input ipedance: Output Ipedance: Z i Z o r d 1 y os where: r d I S GS constant y os = output adittance paraeter listed on FET specification sheets. FET Sall AC Signal Model 1-8
Coon-Source (CS) Fixed- Bias Circuit Input ipedance: Zi R G Output ipedance: Coupling capacitors Z o R r d Z o R r d 10R oltage gain: A A v v o i o i g g (r R d R ) r d 10R FET Sall AC Signal Model 1-9
Coon-Source (CS) oltage- ivider Bias Input ipedance: Z R i 1 R 2 Output ipedance: Z r o d R Z o R r d 10R oltage gain: A v g (r d R ) A v g R r d 10R FET Sall AC Signal Model 1-10
Exaple (1) The fixed-bias configuration had an operating point defined by GSQ = - 2 and I Q = 5.625 A, with I SS = 10 A and P = -8. The value of y os is provided as 40 μs. (a) eterine g (b) Find r d (c) eterine Z i (d) Calculate Z o (e) eterine the voltage gain Av FET Sall AC Signal Model 1-11
Exaple (1) Solution FET Sall AC Signal Model 1-12
Sall AC Signal Equivalent Model for MOSFET - The ac equivalent odel for MOSFETs is exactly the sae as that eployed for JFETs - The only difference offered by - MOSFETs is that GSQ can be positive for n-channel devices and negative for p- channel units - The result is that g can be greater than g 0 FET Sall AC Signal Model 1-13
Exercise What is the function of the following MOSFET circuit? FET Sall AC Signal Model 1-14
MOSFET related Application Copleentary MOSFET Motor Controller FET Sall AC Signal Model 1-15
Power Electronics Technology Power electronics is the application of solid-state (e.g., crystalline seiconductor) electronics for the control and conversion of electric power The potential for applications of power electronics becoe wider used in a great variety of high power product, including heat controls, light controls, electric otor control, power supplies, vehicle propulsion syste and high voltage direct current (HC) systes Many power seiconductor devices (e.g., power MOSFET) are available and directed to the field of power electronics A power seiconductor device is an electronic device that can be used as switches in power electronic circuits FET Sall AC Signal Model 1-16
Power Electronics Application FET Sall AC Signal Model 1-17
Power Seiconductor evices Interesting paraeters Breakdown voltage On-resistance Rise and fall ties Safe operating area Theral resistance ifferent power transistors BJT (Bipolar Junction Transistor) MOSFET (Metal Oxide Seiconductor Field effect transistor) IGBT (Insulated Gate Bipolar Transistor) FET Sall AC Signal Model 1-18
Power MOSFET ertical diffused MOS (MOS) or ouble- iffused MOS (or siply MOS) Structure designed to handle significant power levels high current coutation speed and good efficiency at low voltages found in ost power supplies, C to C converters, and low voltage otor controllers FET Sall AC Signal Model 1-19
Lecture-related Question esign the fixed-bias circuit to have an AC gain of 10 Solution FET Sall AC Signal Model 1-20
Lecture Suary Covered aterial Sall AC Signal Equivalent Circuits for FETs Aplifier Circuits Exaples Introduction to Power Electronics Power Seiconductor evices Power MOSFET Material to be covered next lecture Introduction to operational aplifiers and their applications FET Sall AC Signal Model 1-21