TC 9-62
phase shift can easily be seen by observing the operation of the N-channel JFET. On the
positive alternation of the input signal, the amount of reverse bias on the P-type gate
material is reduced, thereby increasing the effective cross-sectional area of the channel and
decreasing source-to-drain resistance. When resistance decreases, current flow through the
JFET increases. This increase causes the voltage drop across R3 to increase, which in turn
causes the drain voltage to decrease. On the negative alternation of the cycle, the amount of
reverse bias on the gate of the JFET is increased and the action of the circuit is reversed.
The result is an output signal that is an amplified 180-degree-out-of-phase version of the
input signal.
3-87. A second type of FET has been introduced in recent years that have some
advantages over the JFET. This device is the metal oxide semiconductor field-effect
transistor. The MOSFET has an even higher input impedance (10 to 100 million megohms)
than the FET. Therefore, the MOSFET is even less of a load on preceding circuits. The
highly efficient input device for RF and IF amplifiers and mixers and for many types of
test equipment.
3-88. The MOSFET is normally constructed so that it operates in one of two basic modes
(the depletion mode or the enhancement mode). The depletion mode MOSFET has a
heavily doped channel and uses reverse bias on the gate to cause a depletion of current
carriers in the channel. The JFET also operates this way. The enhancement mode MOSFET
has a lightly doped channel and uses forward bias to enhance the current carriers in the
channel. A MOSFET can be constructed that will operate in either mode depending upon
what type of bias is applied, thereby allowing a greater range of input signals.
3-89. In addition to the two basic modes of operation, the MOSFET, like the JFET, is
either of the P-channel type or the N-channel type. Each type has the following four
elements:
Gate.
Drain.
Substrate.
Source.
Figure 3-52 (views (A), (B), (C), and (D)) shows the schematic symbols for the four basic
variations of the MOSFET.
3-32
TC 9-62
23 June 2005
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