Mumbai University > Computer Engineering > Sem 3 > Electronic Circuits and Communication Fundamentals

Marks: 5 Marks

Year: Dec 2015

1. FET is called as a square law device

1.The input-output transfer characteristic of the JFET is not as straight forward as it is for the BJT.

2.In a BJT, β ($h_{FE}$) defined the relationship between $I_B$ (input current) and $I_C$ (output current).

3.In a JFET, the relationship (Shockley’s Equation) between $V_{GS}$ (input voltage) and $I_D$ (output current) is used to define the transfer characteristics, and a little more complicated (and not linear):

$$I_D=I_{DSS}\bigg(1-\dfrac{V_{GS}}{V_P}\bigg)$$

4.As a result, FET’s are referred to a square law devices

2. Barkhausens criteria should be satisfied to get oscillations

• In electronics, the Barkhausen stability criterion is a mathematical condition to determine when a linear electronic circuit will oscillate.
• There are two conditions of Barkhausen Criteria which states that:
  • The total phase shift around a loop, as the signal proceeds from input through amplifier, feedback network back to input again, completing a loop, is precisely $0^0$ or $360^0$, or of course an integral multiple of 2π radians.
  • The magnitude of the product of the open loop gain of the amplifier (A) and the feedback factor β is unity.
• Since, by satisfying these conditions, the circuit works as an oscillator producing sustainable oscillation of frequency.
• Hence, Barkhausen criteria should be satisfied to get oscillation.