JFET as A SwitchPublished on 20/11/2018 & updated on 20/11/2018
When a simple switch is in ON condition, that will behave as a short circuit and hence voltage drop across the switch is zero. Hence whatever may be the current through the switch there will be no power loss in the switch. On the other hand, when a switch is in OFF condition the switch behaves as an open circuit hence current through the switch is zero so power loss at this case is also zero.
Now if we use a JFET as a switch, we have to establish that there is also very low power loss across the JFET during both in conducting and none conducting mode.
This is only possible if the JFET is being operated in its ohmic and cut off region.
Before going to actual topic let us recall the basic idea of ohmic and cut off region of the JFET characteristic.
By keeping the gate terminal voltage at zero if we slowly increase the drain voltage the drain current gets increased linearly up to a certain drain voltage. After that certain drain voltage, the drain current becomes constant. This drain voltage at which the drain current gets constant is called pinch-off voltage. The physics behind pinch off is not discussed here as it is out of the scope of this article. Anyway, this is seen that before pinch off the JFET behaves as a simple resistance of constant value. The value of this resistance is quite a law as it is made of the semiconductor. The region of JFET characteristic before pinch-off is called ohmic region. As the value of the pinch-off voltage of a junction field effect transistor is small the power dissipating from the transistor during a condition in the ohmic region is very small and ideally, we can consider it as zero. So we can conclude that switching on a small signal through a JFET keeps the drain to source voltage below pinch-off voltage and hence limits the power loss.
Now if we decrease the gate potential from zero, the current flowing through the channel for a certain drain to source voltage also gets decreased. This is because reverse biasing of the gate to channel pn junction gets increased with the decrease in gate terminal voltage from its zero potential level. After a certain negative gate voltage, the drain current becomes zero. This negative gate voltage at which the drain current becomes zero is called cut off gate voltage and hence for any applied gate voltage below the cut off voltage, there is no current flowing through the device and the device will behave as an off switch. So in the cutoff region of operations, there is no significant power loss in the transistor.
Here another point to be noted that in JFET, the gate current is always zero irrespective of its conducting or none conducting mode. So there is no power loss caused by the gate signal.
So we can say that the junction field effect transistor behaves as a switch when it is operated in the ohmic and cutoff region of its characteristic.
Let us see some practical applications of JFET as a switch. There are two types of switching operation of a JFET. Shunt Switching Series Switching
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