Current flows between the drain and source through a semiconducting channel in a silicon substrate.. Figure 13.9 shows the structure and symbols for n-channel Figure 13.9a and Figure 13.
Trang 1Output Drivers 303
The voltage dropped across the load will not be exactly 10 V because even when
the transistor is fully turned on (saturated) some voltage is still dropped across
it(V CESAT) Additionally, should the load be capacitive, as is the case when driving
an electrostatic actuator for example, then a large amount of current will need to
be available to ensure that the capacitor charges rapidly enough
Additional current amplification can be achieved by employing two transistors
together, as in a Darlington configuration (Figure 13.8) Darlington transistors
are available in a variety of formats and packages
13.3.2 T HE MOSFET
The name metal-oxide-semiconductor field effect transistor (MOSFET) is derived
from its structure It has three terminals: drain, source, and gate Current flows
between the drain and source through a semiconducting channel in a silicon
substrate This channel is controlled by the metal gate electrode, which is
sepa-rated from the channel by a thin oxide layer MOSFETs can be found in one of
two modes: enhancement or depletion, and may be either n-channel or p-channel
Enhancement-mode MOSFETs are more commonly employed than the
depletion-mode, and for simplicity, these will be the focus of this section
Figure 13.9 shows the structure and symbols for n-channel (Figure 13.9a and
Figure 13.9b) and p-channel (Figure 13.9c and Figure 13.9d) enhancement-mode
MOSFETs
Note the substrate connections in Figure 13.9 For correct operation, this has
to be connected to the most negative voltage for n-type and to the most positive
voltage for p-type transistors This is required to ensure that the pn junctions
between substrate and the active parts of the device (channel and source or drain
implants) are reverse biased Although the structure of the device looks
symmet-rical, discretely packaged transistors are usually supplied with the substrate
inter-nally connected to source, which will normally be the more negative (n-channel)
or more positive (p-channel) than the drain during normal operation of the device
(In some cases, asymmetrical doping may be employed as well)
CMOS processes employ both n-channel and p-channel MOSFETs, and so
will be referred to as either n-well or p-well processes In the n-well process, a
p-type wafer is specified and deep n-type diffusions or implants are introduced
into this at points where p-channel MOSFETs will be fabricated, and vice versa
for the p-well process
FIGURE 13.8 Darlington pair (npn).
E B
C
DK3182_C013.fm Page 303 Monday, January 16, 2006 12:46 PM
Trang 2flow from drain to source The voltage between gate and source (V GS) required
to open up the channel is the threshold voltage, or V T As V GS is increased, more
current will be able to flow
The current flow will be limited by the drain–source voltage if:
(13.5)
In this case, the current flowing, I D, is given by:
(13.6)
The value of k depends on the channel dimensions and the process, and will
not be pursued any further in this chapter Figure 13.11 shows the characteristics
of an n-channel MOSFET with k = 0.075 S and V T = 2.1 V This sort of characteristic
is typical of a discrete MOSFET used for low-power switching, such as the
2N7000
From Equation 13.6, it is apparent that, unlike the BJT, the MOSFET is a
transconductance amplifier: a change in gate voltage produces a corresponding
change in drain current So its transfer characteristics are given in terms of a
current divided by a voltage (I D/V GS) — which is a conductance
MOSFETs make very good switches, because virtually no gate current is
required to switch large drain currents Additionally, the channel behaves like a
resistor, and this can be made very low when the transistor is turned on fully
(data sheets will list this as R DS(ON) — the drain–source on-resistance) For
low-power applications, some MOSFETs are designed with low-threshold voltages
so that they can be driven by logic level signals (Obviously, MOSFETs on ICs
FIGURE 13.11 Graph of I D vs V GS for an n-channel enhancement-type MOSFET with
V T = 2.1V and k = 0.075 S; this is similar to the readily available 2N7000.
0 0.5 1 1.5 2
VGS (V)
ID
V DS >V GS−V T
I D=k V( GS −V T)2
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are designed specifically for low-voltage operation, but will not be discussed as this chapter deals with output drivers for power applications)
As a switch, the MOSFET has the following characteristics:
• Negligible steady-state gate current required
• Gate current must be sufficient to charge the gate capacitance (between the gate and the channel) quickly when turning the transistor on (or discharge it when turning it off)
• Gate capacitance and available current to the gate limit the switching time
• V GS must be sufficient to overcome the threshold voltage and ensure a
low R DS (not always possible with a logic level drive)
An additional caveat when employing discrete MOSFETs is that they are frequently intended to be used as switches, and to this end many incorporate flywheel diodes that will carry reverse currents that appear when inductive loads are being switched Normally, these can be ignored, but sometimes their incor-poration can cause problems Figure 13.12 shows the symbol for a 2N7000 n-channel enhancement-mode MOSFET, which incorporates such a diode
13.4 RELAYS
The relay is an electromechanical switch An electromagnet is used to close a switch element When no current is flowing through the electromagnet, a spring holds the switch in one position When current flows the electromagnet holds the switch in a second position Figure 13.13a shows the circuit symbol for a single-pole double-throw (SPDT) relay, and Figure 13.13b shows how this relay can be driven by a MOSFET
The relay coil is inductive, so a diode is required to protect the transistor during switching The resistance of relay coils is normally in the range of 500 to
1000 Ω, so they cannot normally be switched directly by digital outputs Heavy-duty relays will require higher currents
FIGURE 13.12 Symbol for 2N7000 n-channel enhancement-type MOSFET showing a
built-in flywheel diode This type of symbol is commonly found on data sheets.
S D
G
Trang 4currents and voltages as required.
Solid-state relays are also available These are implemented in silicon tech-nology and have no moving parts, and they are generally designed using CMOS transistors Optically isolated versions are available (see the following text) These generally have fast switching times and no contact bounce, but high-power devices are at present quite costly
Owing to the advantages of electromechanical switches, MEMS relays have been developed, and some are commercially available These are usually based
on cantilever structures, actuated by a variety of methods MEMS relays have generally been designed for communications applications: they are more compact than any other relay design, and offer good isolation and bidirectional current flow Power requirements for actuation depend on the strategy used, but they are generally portable-device-friendly
13.5 BJT OUTPUT BOOST FOR OP-AMPS
An arbitrary scheme for op-amp voltage control was indicated in Figure 13.3 Figure 13.14 shows how this and a BJT can be combined to boost the current-handling capacity of an op-amp The current available to drive the load will be magnified by the current gain (β) of the transistor Another way of looking at it
is that the load resistance will appear as a resistor of value βRL to the output of the op-amp This circuit cannot sink current, but it is a useful circuit for driving resistive heaters or coils on MEMS devices
It may be desirable to control the current flowing through the load in Figure 13.14 One scheme for this is shown in Figure 13.15, where a sense
FIGURE 13.14 A BJT used to boost the output of an op-amp output.
+
−
Vin
RL V+
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Optical isolation of analog signals is more problematic because of nonlin-earities in the optical components and variability from device to device This
is overcome by employing matched receivers on either side of the isolation (Figure 13.17) In this case, the signal on one side can be monitored, and the duplicate circuitry on the other side duplicates the signal correctly
FIGURE 13.17 This optoisolator incorporates two matched transistors so that analogue
signals can be reproduced accurately The transistor on the LED side is incorporated into
a feedback loop in the circuit that drives the LED.