Special Output Gates

It is sometimes desirable to have a logic gate that provides both inverted and non-inverted outputs.

For example, a single-input gate that is both a buffer and an inverter, with a separate output terminal

for each function. Or, a two-input gate that provides both the AND and the NAND functions in a

single circuit. Such gates do exist and they are referred to as complementary output gates.

The general symbology for such a gate is the basic gate figure with a bar and two output lines

protruding from it. An array of complementary gate symbols is shown in the following illustration:

Complementary buffer

Special gates Complementary buffer
Complementary AND gate

Special Gates Complementary AND Gate

Complementary OR gate

Complementary OR gate

Complementary XOR gate

Complementary XOR gate

Complementary gates are especially useful in "crowded" circuits where there may not be enough

physical room to mount the additional integrated circuit chips necessary to provide both inverted

and noninverted outputs using standard gates and additional inverters. They are also useful in

applications where a complementary output is necessary from a gate, but the addition of an inverter

would introduce an unwanted time lag in the inverted output relative to the noninverted output.

The internal circuitry of complemented gates is such that both inverted and noninverted outputs

change state at almost exactly the same time:

Another type of special gate output is called tristate, because it has the ability to provide three

different output modes: current sinking ("low" logic level), current sourcing ("high"), and °oating

("high-Z," or high-impedance). Tristate outputs are usually found as an optional feature on bu®er

gates. Such gates require an extra input terminal to control the "high-Z" mode, and this input is

usually called the enable.

Tristate buffer gate


With the enable input held "high" (1), the buffer acts like an ordinary buffer with a totem pole

output stage: it is capable of both sourcing and sinking current. However, the output terminal floats

(goes into "high-Z" mode) if ever the enable input is grounded ("low"), regardless of the data signal's

logic level. In other words, making the enable input terminal "low" (0) effectively disconnects the

gate from whatever its output is wired to so that it can no longer have any effect.

Tristate buffers are marked in schematic diagrams by a triangle character within the gate symbol

like this:

Tristate buffer symbol


Truth table

Tristate buffers are also made with inverted enable inputs. Such a gate acts normal when the

enable input is "low" (0) and goes into high-Z output mode when the enable input is "high" (1):

Tristate buffer with inverted enable input

Truth table

Tristate  buffer truth table
One special type of gate known as the bilateral switch uses gate-controlled MOSFET transistors

acting as on/off switches to switch electrical signals, analog or digital. The "on" resistance of such

a switch is in the range of several hundred ohms, the "off" resistance being in the range of several

hundred mega-ohms.

Bilateral switches appear in schematics as SPST (Single-Pole, Single-Throw) switches inside of

rectangular boxes, with a control terminal on one of the box's long sides:


CMOS bilateral switch

Cmos bileteral switch
A bilateral switch might be best envisioned as a solid-state (semiconductor) version of an elec-

tromechanical relay: a signal-actuated switch contact that may be used to conduct virtually any

type of electric signal. Of course, being solid-state, the bilateral switch has none of the undesir-

able characteristics of electromechanical relays, such as contact "bouncing," arcing, slow speed, or

susceptibility to mechanical vibration. Conversely, though, they are rather limited in their current-

carrying ability. Additionally, the signal conducted by the "contact" must not exceed the power

supply "rail" voltages powering the bilateral switch circuit.

Four bilateral switches are packaged inside the popular model "4066" integrated circuit:

Quad CMOS bilateral switch  4066

4066 Cmos bileteral switch

Keywords : Gates, Logic, Buffer, Cmos, 4066, Bileteral, Tristate, Complementary-gates
Writer : delon  |
15 Jul 2006 Sat   
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