Page 23 - Complete Wireless Design

Wireless Essentials

22 Chapter One

or above pinch-off (V ). Pinch-off is simply an area where the drain current
P
will stay constant even if the drain-to-source voltage is increased; now only the
gate-to-source voltage can affect the drain current.
A few of the more common JFET parameters are:

I , the maximum JFET drain current possible (with a V at 0 V)
DSS GS
g or g , the transconductance gain (or I / V ), measured in siemens or
m fs D GS
mhos
V , the maximum safe drain-to-source voltage
DS(MAX)
V , the pinch-off voltage, the minimum V required for the JFET’s linear
P DS
operation
P , the JFET’s maximum power dissipation rating
D

Metal-oxide-silicon field-effect transistor (MOSFET). Metal-oxide-silicon field-effect
transistors use a gate structure that is well insulated from the source, drain,
and channel. This produces an active device with an almost infinite DC input
resistance. However, this high input resistance is significantly decreased by its
bias components, as well as by high-frequency operation. In fact, as the fre-
quency of operation is increased, the MOSFET’s input impedance approaches
that of a BJT.
MOSFETs are available that can operate in one of two modes: the depletion
mode, as a normally on device, and the enhancement mode, as a normally off
device.
Drain current in a depletion-mode N-channel MOSFET (Fig. 1.26) is con-
trolled through the application of a negative and positive gate voltage (Fig.
1.27). By raising the negative voltage at the MOSFET’s gate we would soon
reach a point where, as a result of the channel being depleted of all majority
carriers, no significant drain current can flow. But as the gate-to-source volt-
age V becomes less negative, more current will start to move. Even as we
GS
pass 0 V for V , the drain current will still continue to rise, since at zero V
GS GS
the MOSFET—unlike the JFET—has not reached the maximum current.
Nonetheless, the drain current is still quite substantial, since many majority
carriers are present within the depletion MOSFET’s N channel. The V
GS
increases until it reaches some maximum positive value; now the maximum
number of electrons has been drawn into the N channel, and the maximum
current is flowing through the channel and into the drain.
Depletion MOSFETs are used extensively in wireless circuits because of
their low-noise-producing characteristics. A similar structure, but employing
two gates within a single device, is the dual-gate MOSFET (Fig. 1.28). These
are utilized in mixers and automatic gain control (AGC) amplifiers, with each
of the MOSFET’s gate inputs having an equal control over the drain current.
The other type of MOSFET, the enhancement-mode type, or E-MOSFET
(Fig. 1.29) is, as mentioned above, a normally off transistor. So, almost no
source-to-drain current flows when there is no bias across the E-MOSFET’s
gate, as shown in the characteristic curves of Fig. 1.30. However, almost any

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