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which is the saturation current at. the V us =V line (see Figure,7-5(b)). Note in Figure 7-6 that the region to tHe right ol.the, parabola-is called the pinch-off region. This is the region in which 1he JFET is normally-operated when used -for small-signal amplification. It is also called the active region;or the saturation region. The to the -left of the parabola is called THC ooh age-controlled-resistance  region, the oh-mic region, or therefrom region. In this region, the resistance between drain and source-IS controlled by VG as we have previously discussed, and we can see that the lines become less -rec (implying larger resistance) as Vcs becomes more negative. The device acts like a voltage-controlled resistor in this region. and there are some practical application’> that exploit this characteristic. The line drawn along the horizontal axis in Figure 7- shows that I/J = 0 when V(;S = -4 V. regardless of the value of V{)s. When Us reverse biases the gate-to source junction by an amount equal to 1′,,, depletion regions meet along the entire length of the channel and the drain current is cut off. Since the value of V(;s at which the drain current is cut the same as V the pinch-off voltage is also called the gate-to-source cutout] u,I/.age. Thus, there are two ways to determine the value of V” from a set of rd .ii.t characteristics: It is the value of V ()S where If) saturates when C;s = O. and it j, the value of VS that causes all drain current to cease. i.c V One property or a field-effect transistor that makes it especially valuable as a voltage amplifier is the very high input resistance at its gate. Since the path from gate to source is a reverse-biased PN junction, the only current that flows into the gate is the very small leakage current associated with a reverse-biased junction. Therefore, very little current is drawn from a signal source driving the gate. and the FJ T input looks like a very large resistance. A de input resistance of several hundred McGowan is not unusual. Although the gate of an N-channel JFET can he driven slightly positive, this action causes the input junction to be forward biased and radically decreases the gate-to-source resistance. In most practical applications. the sudden and dramatic decrease in resistance when the gate is made positive would not be tolerable to a signal source driving a FET. figure 7-7 shows the structure and drain characteristics of a typical P-channel JFET. Since the channel is P material]. current is due to hole 110w, rather than electron now, between drain and source. The gate material is, of course, N type. Note that all voltage polarities arc opposite those in the N-channel JFET. Figure 7-7(b) shows that positive values of VGS control the amount of saturation current in the pinch-off region. Figure 7-‘1:; shows the schematic symbols used to represent N-channel and Channel J Fli’Fs. Note that the arrowhead on the gate points into an N-channel JFET and outward for a Channel device. The symbols showing the gate terminal drawn closest to the gate arrow. Some lFETs are manufactured so that the drain and source are interchangeable. and the symbols for these devices have the gate arrow drawn in the center. Figure 7-9 shows the breakdown characteristics of an N-channel JFET. Breakdown occurs at large values of Vos and is caused by the avalanche mechanism described in connection with 811’s. Note that the larger

Posted on November 18, 2015 in FIELD-EFFECT TRANSISTORS

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