DE-MOSFET can be operated either with positive or negative gate, so its operating or quiescent point can be set at VGS = 0, as illustrated in figure. In figure the gate G is at ground potential for dc since ac signal source appears to be short-circuited for dc and sets Q-point at VGS = 0. The ac input signal causes the gate-source voltage V to vary above and below zero developing drain current Id and drain voltage Vout. The biasing circuit given in figure, has V = 0, Id = IDDS and dc drain voltage VDS = VDD – IDSS RD
The zero bias of figure is unique with depletion-mode MOSFETs. Although any of the JFET biasing method will work with a depletion-mode MOSFET, the zero-bias method shown in figure is preferred as it does the job simply and adequately.
After biasing the depletion-mode MOSFET to a Q-point, it can amplify small signals. The JFET formulas for voltage gain can be applied directly to a MOSFET amplifier. But like the JFET, the depletion-mode MOSFET has a relatively low voltage gain. MOSFETs have excellent low-noise properties, a definite advantage for any stage near the front end of a system where the signal is weak, such as a TV receiver. As with a JFET, the transconductance gm in case of MOSFET can be controlled by varying the gate-source voltage.
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