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| SEC. 14-1 THE DEPLETIONMODE
MOSFET The depletion-mode MOSFET, abbreviated D-MOSFET, has a source, gate, and drain. The gate is insulated from the channel. Because of this, the input resistance is very high. The D-MOSFET has limited use, mainly in RF circuits. SEC. 14-2 D-MOSFET CURVES The drain curves for a D-MOSFET are similar to those of a JFET when the MOS device is operating in the depletion mode. Unlike JFETs, D-MOSFETs can also operate in the enhancement mode. When operating in the enhancement mode, the drain current is greater than IDSS. SEC. 14-3 DEPLETION-MODE MOSFET AMPLIFIERS D-MOSFETs are mainly used as RF amplifiers. D-MOSFETs have good highfrequency response, generate low levels of electrical noise, and maintain high input impedance values when VGS is negative or positive. Dual-gate D-MOSFETs can be used with automatic gain control (AGC) circuits. SEC. 14-4 THE ENHANCEMENTMODE MOSFET The E-MOSFET is normally off. When the gate voltage equals the threshold voltage, an n-type inversion layer connects the source to the drain. When the gate voltage is much greater than the threshold voltage, the device conducts heavily. Because of the thin insulating layer, MOSFETs are easily destroyed unless you take precautions in handling them. SEC. 14-5 THE OHMIC REGION Since the E-MOSFET is primarily a switching device, it usually operates between cutoff and saturation. When it is biased in the ohmic region, it acts like a small resistance. If ID(sat) is less than ID(on) when VGS = VGS(on), the E-MOSFET is operating in the ohmic region. SEC. 14-6 DIGITAL SWITCHING Analog means that the signal changes continuously, that is, with no sudden jumps. Digital means that the signal jumps between two distinct voltage levels. Switching includes high-power circuits as well as small-signal digital circuits. Activeload switching means that one of the MOSFETs acts like a large resistor and the other like a switch. SEC. 14-7 CMOS CMOS uses two complementary MOSFETs, in which one conducts and the other shuts off. The CMOS inverter is a basic digital circuit. CMOS devices have the advantage of very low power consumption. SEC. 14-8 POWER FETS Discrete E-MOSFETs can be manufactured to switch very large currents. Known as power FETS, these devices are useful in automotive controls, disk drives, converters, printers, heating, lighting, motors, and other heavy-duty applications. SEC. 14-9 E-MOSFET AMPLIFIERS Besides their main use as power switches, E-MOSFETs find applications as amplifiers. The normally off characteristics of E-MOSFETs dictate that VGS be greater than VGS(th) when used as an amplifier. Drain-feedback bias is similar to collector feedback bias. SEC. 14-10 MOSFET TESTING It is difficult to safely test MOSFET devices using an ohmmeter. If a semiconductor curve tracer is not available, MOSFETs can be tested in test circuits or by simple substitution. |