Mumbai University > Electronics ana telecommunication engineering > Sem 3 > Analog electronics 1

Marks: 10M

Years: May 16

JFET Operation and V-I Characteristics:

Operation with VGs = 0: Consider the operation of n-channel JFET when V_GS = 0. The reference terminal for measuring junction voltages is source (S). Hence $V_S$= 0. Thus when $V_GS$ = 0, $V_G$ is also 0. Now if we apply a small positive voltage at drain (D) terminal, $V_DS$ is positive. A small drain current flows through the n-channel. This situation is shown in Fig. 2.1. For smallV_DS, the channel acts essentially as a resistance. This region of operation of JFET is called as Ohmic Region.

Now if we increase $V_GS$ and make it more positive, the gate - channel p-n junction becomes reverse biased near drain terminal. Conventional drain current $I_D$, in flowing from drain-to-source through channel, produces a voltage that is highest near drain terminal and lowest near source terminal. Since the channel is n-type, depletion region is wider near the drain terminal than source terminal. This is shown in Fig 2.2.

If we further increase $V_DS$, the channel reaches a condition shown in Fig. 2.3. The channel is pinched-off near drain terminal. Any further increase in $V_DS$ will not increase the drain current $I_D$. The value of $V_DS$, under the condition of zero $V_GS$, where $I_D$ becomes essentially constant, is defined to be the Pinch-off voltage $V_P$.

The operation of JFET for different values of $V_DS$ can be separated into three distinct regions. They are:

(a) Ohmic or non-saturation region

(b) Saturation region

(c) Breakdown region

Operation under non-zero $V_GS$:

If we connect a negative gate-source voltage and vary $V_DS$, we get another characteristic curve as shown in Fig.2.4. For each value of $V_GS$, we obtain a set of readings that reflect variation of $I_D$ with $V_DS$. By plotting these values is known as drain characteristics of JFET. Figure 2.4 shows a family of drain characteristic curves.