Download PPT: Electrical 5

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.
elec 5.2
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.
elec 5.3
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.
elec 5.4
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
elec 5.5
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
elec 5.6
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.
elec 5.7
Open Collector Output
• An open collector may be used with higher
voltages (refer to manufacturer specifications).
elec 5.8
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).
elec 5.9
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.
elec 5.10
elec
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.
elec 5.12
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
elec 5.13
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
elec 5.14
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.
elec 5.15
End
Paul R. Godin
prgodin @ gmail.com
elec 5.16