Transcript ENT-DIGI-290

Electrical Characteristics of
Logic Gates
Gate Characteristics
Last Mod: January 2008
 Paul R. Godin
The Circuitry of Logic Gates
• In this presentation, we will discuss the basic
internal circuitry of logic gates in order to better
understand how they interface with other devices.
• Many data books include the circuit diagram of the
gate configurations. For detailed information on
the circuitry of the digital gates, please refer to the
manufacturer’s specification sheets.
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Basic Transistor Output
• The transistors in logic gates are typically utilized
as switches…they are either on or off.
• The output transistors in logic gates are operated
at the cutoff and saturation levels. Based on the
specific configuration, this will provide either a
logic low or a logic high.
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A Basic Output
• This configuration is similar
to an inverter:
▫ ON: a high level input to the
base will send the transistor
into saturation and provide a
low level output (the collector
will connect to the ground on
the emitter).
▫ OFF: a low level input to the
base will send the transistor
into cutoff and provide a high
level output.
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Open Collector Output
• An open collector output is a device that provides a
ground for a low output and an open circuit for a
high output. This allows for multiple outputs to be
wired together without damage.
• Open collector gates are useful in many
applications, but must not be confused with a
standard output.
Symbol for Open Collector
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Open Collector Output
• Note the open collector output cannot provide an
output high due to the lack of a pull-up resistor.
Open Collector Output
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Open Collector Output
• External pull-up resistors may be used to provide the
logic high output required, creating a wired AND.
Open Collectors with
external pull-up.
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Open Collector Output
• An open collector may be used with higher
voltages (refer to manufacturer specifications).
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Totem Pole Output
• A totem pole output has 2 output
transistors; one controls the high
level output and the other
controls the low level output (only
one transistor is saturated at a
time).
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Totem Pole Output
• The advantage of a totem pole
configuration is a faster output
switching speed and a good
current carrying capability.
• The disadvantage is switching
noise, where for brief moments
both transistors may be
saturated. This presents a
greater load to the power supply.
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5.11
CMOS-based Devices
CMOS
• The CMOS family consists of
MOSFETs (Metal Oxide
Semiconductors Field Effect
Transistors).
• CMOS utilizes gate voltage to control
base current. As a result, CMOS
devises require very little input current
to operate, and have high input
impedances.
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CMOS
Current will flow from the Drain to the Source if a
positive voltage (in reference to the Substrate) is
applied to the input of the N-Channel MOSFET.
The P-Channel device will conduct if a negative
voltage is applied to the Gate input.
Drain
Drain
N
Gate
N-Channel
P
Substrate
Gate
Substrate
P
N
N
P
Source
P-Channel
Source
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CMOS Inverter
• A CMOS inverter operates under a simple
principle:
▫ If the gate voltage is equal to the source voltage,
the MOSFET provides a high impedance.
0V
VDD
CMOS Inverter
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Review
• Logic gates are electronic circuits, constructed
from transistors, resistors and MOSFETS. As
electronic circuits, they require supply voltage and
ground, and have current and voltage
requirements and limitations.
• There are different types of IC designs available.
These are selected based on the circuit
requirements. Not all gates operate
interchangeably in all environments.
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End
Paul R. Godin
prgodin @ gmail.com
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