Electrical Engineering ⇒ Topic : Drain characteristics
When VDS = 0, ID = O. Let us take the drain characteristics for VGS = O. If VDS is increased by a small amount, a small drain current ID flows and causes the voltage drop along the channel. This voltage drop reverse biases the gate-channel junction by a small amount. So very thin depletion region will be formed. It has negligible effect on the channel resistance. ID will increase gradually with increase of VDS and the channel continues to behave like an almost constant resistance.When the voltage drop along it becomes large enough, sufficient penetration of depletion region occurs. Therefore, smaller increase in VDS causes smaller increase in ID which in turn causes more penetration of depletion region. So there is rapid increase of channel resistance. A saturation level of ID will reach. Any further increase of VDS changes ID only slightly. The drain current at this point is called drain source saturation current IDSS and corresponding VDS is called pinch off voltage VP. The pinch off voltages corresponding to VGS = 1 V, 2 V and 3 V are shown in Figure (a).
Figure (a) Drain characteristics of an N-channel JFET with increasing negative gate to source voltage.
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