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## Principles of Operation

Figure 5.15 shows an N-channel JFET with DC bias voltage applied. Just as for a simple diode, the depletion region grows as the reverse bias across the PN junction is increased, thereby constricting the cross section of the conducting N-channel material and increasing the resistance of the channel. The major current in the channel is caused by the applied voltage between the drain and source, , and is controlled by the applied voltage between the gate and source, . Figure 5.15:  An N-channel JFET with DC bias voltages applied.

The JFET has two distinct modes of operation: the variable-resistance mode, and the pinch-off mode. In the variable-resistance mode the JFET behaves like a resistor whose value is controlled by . In the pinch-off mode, the channel has been heavily constricted with most of the drain-source voltage drop occuring along the narrow and therefore high-resistance part of the channel near the depletion regions.

The characteristic curves of a typical JFET are shown in figure 5.16. At small values of (in the range of a few tenths of a volt), the curves of constant show a linear relationship between and . This is the variable-resistance region of the graph. As increases, each of the curves of constant enters a region of nearly constant . This is the pinch-off region, where the JFET can be used as a linear voltage and current amplifier. At the current through the JFET reaches a maximum known as , the current from Drain to Source with the gate Shorted to the source. If goes positive for this N-channel JFET, the PN junction becomes conducting and the JFET becomes just a forward-biased diode. Figure 5.16:  Characteristic curves of a typical N-channel JFET.    Next: Small-Signal AC Model Up: The Junction Field Effect Previous: The Junction Field Effect

Doug Gingrich
Tue Jul 13 16:55:15 EDT 1999