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

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Years: May 15

Biasing JFET for Zero Current Drift:

Transfer characteristic of JFET drifts with variation of temperature. Figure. 2.7 shows transfer characteristics of n-channel JFET as a function of temperature. It is seen from Fig. 2.7 that there is a value of $V_GS$ for which $I_D$ does not change with temperature. Hence it is possible to bias a JFET for zero drain current drift with respect to temperature.

The variation or drift of transfer characteristic can be explained with the help of two factors that affect drain current when temperature changes. The first factor is the decrease in majority-carrier mobility with temperature.

When temperature increases, the lattice ions in the crystal vibrate vigorously. Because of this, carriers cannot move freely in crystalline structure. This means for a given field strength existing in the crystal, the drift velocity of majority carriers is less. Since mobility µ = ν/ E, it decreases with rise in temperature.

Second factor is reduction in channel width with rise in temperature. The barrier potential across the channel reduces with increasing temperature.

Combining two effects that cause variation in $I_D$ in opposite directions, the condition for zero drain current drift is

$0.007|I_D|=0.0022g_m$

$\frac{|I_D|}{g_m}$=0.314V

Modulus sign in relation is used to cover both types of JFETs, i.e. n- and p-channel. It can be proved that condition for zero current drift in terms of V_P is |V_P| - |V_GS|= 0.63 V. The proof involves making use of relations

$I_D=I_DSS(1-\frac{V_GS}{V_P})^2$

$g_m=g_mo(1-\frac{V_GS}{V_P}) $g_mo=\frac{-2I_DSS}{V_P}$