Basic Electronics (3110016)

BE | Semester-1   Winter-2019 | 06-01-2020

Q4) (b)

Explain FET as an Amplifier.

  • When used in amplifier applications, the FET has an important advantage compared to the BJT due to the FET's extremely high input impedance.
  • Disadvantages, however, include higher distortion and lower gain.
  • The common-source (CS) amplifier is comparable to the common-emitter BJT amplifier
  • A common-source JFET amplifier is one in which the ac input signal is applied to the gate and the ac output signal is taken from the drain.
  • The source terminal is common to both the input and output signal. A common-source amplifier either has no source resistor or has a bypassed source resistor, so the source is connected to ac ground.
  • A self-biased common-source n-channel JFET amplifier with an ac source capacitively coupled to the gate.
  • The input signal voltage causes the gate-to-source voltage to swing above and below its Q-point value (VGSQ), causing a corresponding swing in drain current. As the drain current increases, the voltage drop across RD also increases, causing the drain voltage to decrease.
  • The drain current swings above and below its Q-point value in phase with the gate-to-source voltage. The drain-to-source voltage swings above and below its Q-point value (VDSQ) and is 180° out of phase with the gate-to-source voltage.