TC 9-62
increased the resistance of the JFET (from 500 ohms to 1 kilohm). In other words, a 1 volt
change in gate voltage has doubled the resistance of the device and cut current flow in half.
3-81. However, these measurements only show that a JFET operates in a manner similar
to a bipolar transistor, even though the two are constructed differently. Remember, the
main advantage of an FET is that its input impedance is significantly higher than that of a
bipolar transistor. The higher input impedance of the JFET under reverse gate bias
conditions can be seen by connecting a microammeter in series with the gate-source
voltage (VGG) (see Figure 3-49).
Figure 3-49. JFET Input Impedance
3-82. With a VGG of 1 volt, the microammeter reads .5 microamps. Applying Ohm's law
(1V .5A) shows that this very small amount of current flow results in a very high input
impedance (about 2 megohms). By contrast, a bipolar transistor in similar circumstances
would require higher current flow (for example, .1 to -1 mA), resulting in a much lower
input impedance (about 1000 ohms or less). The higher input impedance of the JFET is
possible because of the way reverse-bias gate voltage affects the cross-sectional area of the
channel.
3-83. The preceding example of JFET operation uses an N-channel JFET. However, a
P-channel JFET operates on identical principles. Figure 3-50 shows the differences
between the two types.
3-84. Since the materials used to make the bar and the gate is reversed, source voltage
potentials must also be reversed. The P-channel JFET therefore requires a positive gate
voltage in order to be reverse biased, and current flows through it from drain to source.
3-85. Figure 3-51 shows a basic common-source amplifier circuit containing an N-
channel JFET. The characteristics of this circuit include high input impedance and a high
voltage gain. The function of the circuit components in Figure 3-51 is very similar to those
in a triode vacuum tube common-cathode amplifier circuit. C1 and C3 are the input and
3-30
TC 9-62
23 June 2005
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