of Microprocessors

DIGITAL-TO-ANALOG AND ANALOG-TO-DIGITAL CONVERTERS 241
MOSFET Switch
Metal-oxide gate FETs (MOSFET) are also frequently used for switching.
The MOSFET is the most nearly perfect switching transistor available
from the standpoint of control-to-signal isolation. Basic operation is the same
as with a JFET except that the gate is insulated from the channel and all
voltages are shifted up somewhat. An N-channel MOSFET, for example, is
normally off with a zero gate-to-source voltage. The gate must become more
positive than a threshold voltage before the switch turns on. The gate may also
swing negative with no effect other than driving the device deeper into
cutoff. In fact, the gate voltage swing is limited only by internal breakdown
voltages; it is otherwise isolated (except for a small capacitance) from the
channel.
One difficulty with MOSFETs is that the on resistance is indefinite; the
more positive the gate the lower the resistance. If the MOSFET is carrying an
audio signal into a finite load resistance, the channel resistance will be
modulated by the signal itself, causing nonlinear distortion. This happens
because the gate- (fixed drive voltage) to-source (varying audio signal) voltage
will be changing. Since there is a saturation effect at large gate-to-source
voltages, distortion will be minimized by suitable control overdrive. Distortion
is also reduced by using larger load resistances. A typical switching
N-channel MOSFET might have a nominal on resistance of 100 ohms that
may vary from 75 ohms to 150 ohms with ± 15-V drive and ± lO-V signals.
Newer MOS technologies such as DMOS and VMOS can actually attain on
resistances as low as 1 ohm and carry ampere level currents!
A very nice analog switch may be constructed from two MOSFETs of
opposite polarity connected in parallel. To turn on, both switches must be
driven on by opposite polarity control voltages, which can reverse to drive
both switches off. The major advantage of this circuit is that signal voltage
levels as large as the drive voltages may be handled. Although each individual
switch is highly nonlinear and even cuts off for part of the signal cycle,
the parallel combination is always on. In fact, when the on resistance of the
N-channel unit is increasing with positive-going signals, the P-channel resistance
is decreasing to compensate. The result is considerably less signal
distortion. This structure is called a CMOS (complementary MOS) transmission
gate and is available in integrated form as quad switches and eightchannel
analog multiplexors at very low cost. The disadvantage of most
integrated transmission gates is a ±7. 5-V signal and drive-voltage limitation.
Recently, "B-series" CMOS has become available and can handle up to
±9-V signals, adequate for an 8-V standard system, although some care in
use will have to be exercised. Specialized units with even higher voltage
ratings are available but at a much higher cost.
Figure 7-14 shows a good general-purpose driver for both JFETs and
MOSFETs that can in turn be driven by TTL logic. The driver output swings