Q1.(a) December 2015 of ECCF computer Engineering 3rd semester. B.E.

Q. Give reasons for the following:

  1. FET is called as a square law device**
  2. Barkhausens criteria should be satisfied to get oscillations.
1 Answer

1.We know that the expression for drain current of a JFET is as follows:

$\hspace {1cm} I_D = I_{DSS} \bigg[\frac{1-V_{GS}}{V_P}\bigg]^2$

This shows that $I_D$ ids proportional to square of VGS which is the controlling input parameters due to this square relationship between ID and VGS, the FET is called as a square law device.

2.An oscillator is basically an "amplifier" which does not have any ac input but it operates on the principle of positive feedback to generate an ac signal on its own at its output. Thus it is clear that an amplifier can work as an oscillator if positive feedback is made to exist. However positive feedback not always guarantees oscillations. An amplifier will work as an oscillator if and only if it satisfies a set of conditions called the "Barkhausen Criterion".

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