CHAPTER 8: MOSFETS.OBJECTIVES

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CHAPTER 8: MOSFETS.OBJECTIVESCHAPTER 8MOSFETS OBJECTIVESDescribe and Analyze:• Theory of MOSFETS• MOSFET Amplifiers• E-MOSFET Switches• Troubleshooting Introduction• MOSFET stand for Metal-Oxide-Semiconductor (or Metal-Oxide-Silicon) Field-Effect-Transistor• Like JFETs, MOSFETs come in N-channel and P-Channel types• Unlike JFETs, MOSFETs can be manufactured as enhancement-mode (E-MOSFETs) as well as depletion-mode (D-MOSFETs).• There is no PN junction. The metal gate of a MOSFET is isolated from the silicon channel by a thin layer of silicon oxide (Si O2, commonly known as glass)• MOSFETs can be damaged by static electricity D-MOSFETsSimilar to a JFET, but Zin of device is almost infinite D-MOSFETsUnlike JFETs, D-MOSFETS can work with zero bias D-MOSFETsThe same bias circuits used with JFETs can beused with E-MOSFETs. In addition, a class-AMOSFET amplifier can work with VGS = 0. D-MOSFET AmplifiersThe equations for Zin, Zout and Av developed for JFET amplifiers can be used with D-MOSFET amplifiers.Like JFETs, D-MOSFETs are used in the front ends of radio receivers because of their inherently low internal noise.D-MOSFET AGC AmplifiersSince gm depends on the Q-point, MOSFETs are used for Automatic Gain Control in radio receivers D-MOSFET MixersUnlike JFETs, D-MOSFETs can be built with two gates. That allows them to be used as radio “mixers” to multiply one signal by another. E-MOSFETsIn an enhancement-mode MOSFET, the drain isisolated from the source because the substrate isdoped opposite the source and drain. Voltage appliedto the gate causes the substrate under the gate to “flippolarity”. P-material becomes N-material as chargecarriers are attracted into the region by the gate. E-MOSFETsThe key parameter for an E-MOSFET is the threshold voltage (VGS(TH)) required to turn it on E-MOSFET SwitchesThe most common use for an E-MOSFET is switching E-MOSFET SwitchesThe IRF510 E-MOSFET is a typical power switch. Its key specifications are:• VGS(TH) = 4 Volts max• RDS(on) = 0.54 Ohms max• ID(MAX) = 5.6 Amps IDSS = 25 A (remember: it’s off)•• BVDSS = VDS(MAX) = 100 Volts• PD(MAX) = 43 Watts• Rise-time tR = 63 ns E-MOSFET Switches• The most common application of E-MOSFETs like the IRF510 is to drive the transformer in switch-mode power supplies and DC to DC converters. E-MOSFET SwitchesE-MOSFETs can also switch analog signals E-MOSFET SwitchesChoppers convert DC or low-frequency AC into higher-frequency AC suitable for processing Digital MOSFET SwitchBecause of their small size, low power, and speed, digital ICs such as microprocessors use MOSFET switches E-MOSFET AmplifiersWhen biased on, E-MOSFETs can have a high gm CMOS• CMOS stands for: Complementary Metal-Oxide-Semiconductor.• They combine N-channel and P-channel MOSFETs.• They are primarily used in low-power digital ICs.• Are sometimes used in “mixed signal” ICs which combine analog and digital signals on one chip. Troubleshooting• MOSFETs can not be checked with an Ohm-meter.• As usual, check the DC bias levels.• Check the input and output levels of signals to see if they are approximately what you expected.• If it’s necessary to replace a MOSFET, use the same part number. If that’s not an option, pick a device suitable for the application: switch, RF mixer, AGC amplifier, etc.