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. elec 5.5 Symbol for Open Collector

Open Collector Output Note the open collector output cannot provide an output high due to the lack of a pull-up resistor. elec 5.6 Open Collector Output

External pull-up resistors may be used to provide the logic high output required, creating a wired AND. elec 5.7 Open Collectors with external pull-up.

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

CMOS-based Devices elec 5.11

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. elec 5.13 N P N Gate Drain Substrate Source P N P Gate Drain Substrate Source N-ChannelP-Channel

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. elec V V DD CMOS Inverter

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 elec 5.16  Paul R. Godin gmail.com