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Digital buffer
View on Wikipediafrom Wikipedia
A digital buffer (or a logic buffer) is an electronic circuit element used to copy a digital input signal and isolate it from any output load. For the typical case of using voltages as logic signals, a logic buffer's input impedance is high, so it draws little current from the input circuit, to avoid disturbing its signal.
The digital buffer is important in data transmission between connected systems. Buffers are used in registers (data storage device) and buses (data transferring device). To connect to a shared bus, a tri-state digital buffer should be used, because it has a high impedance ("inactive" or "disconnected") output state (in addition to logic low and high).
Functionality
[edit]A voltage buffer amplifier transfers a voltage from a high output impedance circuit to a second circuit with low input impedance. Directly connecting a low impedance load to a power source draws current according to Ohm's law. The high current affects the source. Buffer inputs are high impedance. A buffered load effectively does not affect the source circuit. The buffer's output current is generated within the buffer. In this way, a buffer provides isolation between a power source and a low impedance. The buffer does not intentionally amplify or attenuate the input signal, and so may be called a unity gain buffer.
A digital buffer is a type of voltage buffer amplifier that is only concerned about digital logic levels, and thus may be non-linear. It may also act as a level shifter, with output voltages differing from the input voltages. One case of this is an inverting buffer which translates an active-high signal to an active-low one (or vice versa).
Types
[edit]Single input voltage buffer
[edit]Inverting buffer
[edit]This buffer's output state is the opposite of the input state. If the input is high, the output is low, and vice versa. Graphically, an inverting buffer is represented by a triangle with a small circle at the output, with the circle signifying inversion. The inverter is a basic building block in digital electronics. Decoders, state machines, and other sophisticated digital devices often include inverters.
Non-inverting buffer
[edit]This kind of buffer performs no inversion or decision-making possibilities. A single input digital buffer is different from an inverter. It does not invert or alter its input signal in any way. It reads an input and outputs a value. Usually, the input side reads either HIGH or LOW input and outputs a HIGH or LOW value, correspondingly. Whether the output terminal sends off HIGH or LOW signal is determined by its input value. The output value will be high if and only if the input value is high. In other words, Q will be high if and only if A is HIGH.
Tri-state digital buffer
[edit]Unlike the single input digital buffer which has only one input, the tri-state digital buffer has two inputs: a data input and a control input. (A control input is analogous to a valve, which controls the data flow.) When the control input is active, the output value is the input value, and the buffer is not different from the single input digital buffer.
Active high tri-state digital buffer
[edit]An active-high tri-state digital buffer is a buffer that is in an active state that transmits its data input to the output only when its control input voltage is high (logic 1).[1] But when the control input is low (logic 0), the output is high impedance (abbreviated as "Hi-Z"), as if the part had been removed from the circuit.
| Data input | Control input | Output |
|---|---|---|
| 0 | 0 | Hi-Z |
| 1 | 0 | Hi-Z |
| 0 | 1 | 0 |
| 1 | 1 | 1 |
Active low tri-state digital buffer
[edit]It is basically the same as active high digital buffer except the fact that the buffer is active when the control input is at a low state.
| Data input | Control input | Output |
|---|---|---|
| 0 | 0 | 0 |
| 1 | 0 | 1 |
| 0 | 1 | Hi-Z |
| 1 | 1 | Hi-Z |
Inverting tri-state digital buffer
[edit]Tri-state digital buffers also have inverting varieties in which the output is the inverse of the input.
| Data Input | Control Input | Output |
|---|---|---|
| 0 | 0 | Hi-Z |
| 1 | 0 | Hi-Z |
| 0 | 1 | 1 |
| 1 | 1 | 0 |
| Data input | Control input | Output |
|---|---|---|
| 0 | 0 | 1 |
| 1 | 0 | 0 |
| 0 | 1 | Hi-Z |
| 1 | 1 | Hi-Z |
Application
[edit]Single input voltage buffers are used in many places for measurements including:
- In strain gauge circuitry to measure deformations in structures like bridges, airplane wings and I-beams in buildings.
- In temperature measurement circuitry for boilers and in high altitude aircraft in a cold environment.
- In control circuits for aircraft, people movers in airports, subways and in many different production operations.
Tri-state voltage buffers are used widely to transmit onto shared buses, since a bus can only transmit one input device's data at a time. The high-impedance output state effectively temporarily disconnects that input device from the bus, since at most only one device should actively drive the bus's shared wires.
References
[edit]- ^ "Digital Buffer Tutorial". Electronics Tutorials. 22 August 2013. Retrieved March 13, 2019.
Common logical connectives | ||
|---|---|---|
 | ||
Digital buffer
View on Grokipediafrom Grokipedia
A digital buffer is an electronic circuit component in digital electronics that replicates the input signal at its output without inversion or logical alteration, serving primarily to amplify the signal's drive capability and isolate the input from output loads.[1] Its Boolean expression is simply Q = A, where the output Q matches the input A, functioning as the non-inverting counterpart to an inverter gate.[2] Buffers are essential for maintaining signal integrity in logic circuits by boosting current or voltage levels to prevent degradation when driving multiple loads.[3]
In operation, a digital buffer typically consists of two cascaded inverter stages, which together restore the original logic level while enhancing the output's ability to source or sink current.[2] This design allows it to achieve high fan-out, meaning it can drive up to 20 or more subsequent gates or devices without signal loss, a key advantage over standard logic gates with lower drive capacities.[1] Non-inverting buffers, symbolized by a triangle without an output bubble, are distinguished from inverting types that flip the logic state, though the former is more commonly implied by the term "buffer" in digital contexts.[4]
Common variants include the tri-state buffer, which incorporates an enable (EN) pin to switch the output to a high-impedance (Hi-Z) state when disabled, allowing multiple buffers to share a common bus without contention.[1] Active-high and active-low enable configurations exist, with some tri-state buffers also offering inversion.[3] These features enable precise control in bus-oriented systems, such as preventing short circuits by ensuring only one device drives the line at a time.[1]
Digital buffers find widespread applications in integrated circuits for tasks like interfacing weak signal sources with high-current loads, such as LEDs, relays, or transistor switches, thereby protecting upstream components from overload.[3] They are integral to data buses, registers, and memory systems for efficient signal transmission and isolation, and appear in families like TTL (e.g., 74LS244) and CMOS (e.g., CD4050).[1] By providing impedance matching and noise reduction, buffers ensure reliable operation in complex digital designs, from microcontrollers to communication interfaces.[4]
[1] An optional enable pin (EN) can be included, where the output follows the input only when EN = 1; otherwise, the output may enter a high-impedance state in tri-state variants, though core non-inverting buffers operate continuously.[7]
The primary advantages of non-inverting buffers include maintaining signal polarity for direct logic propagation and enabling effective impedance matching to drive capacitive loads without phase inversion.[2] Unlike inverting buffers, which reverse the logic level, non-inverting designs ensure compatibility in chains where polarity preservation is essential.[1]
A widely used integrated circuit example is the 74HC125, a quad non-inverting buffer from Texas Instruments, featuring four independent buffers in a 14-pin package with supply voltage range of 2 V to 6 V.[7] The pinout includes inputs (1A to 4A on pins 2, 5, 9, 12), outputs (1Y to 4Y on pins 3, 6, 8, 11), output enables (1OE to 4OE on pins 1, 4, 10, 13, active low), VCC on pin 14, and GND on pin 7.[7] Maximum ratings specify a supply voltage of -0.5 V to 7 V and continuous output current of ±35 mA per output, supporting operation up to 10 LSTTL loads with low power dissipation.[7]
Inverting buffers find application in polarity correction within mixed-logic systems, where signals from active-high sources must be converted to active-low or vice versa to ensure compatibility between circuit modules. They also serve as fundamental building blocks in constructing more complex logic gates, such as NAND or NOR gates, by combining inversion with other operations.[11]
A widely used integrated circuit example is the 74HC04, a hex inverter providing six independent inverting buffers in high-speed CMOS technology, capable of driving up to 10 LSTTL loads with low power consumption. It operates from 2 V to 6 V supplies and features propagation delays of 5 ns to 10 ns typical at 5 V and 25°C. Common package types include the 14-pin PDIP for through-hole mounting and the 14-pin SOIC for surface-mount applications, with speed grades varying by temperature range (e.g., commercial 0°C to 70°C or industrial -40°C to 85°C). For noise-sensitive environments, the 74HC14 offers a similar hex configuration but with Schmitt trigger inputs.[10][12]
Here, Z denotes the high-impedance state. For inverting variants, the Y column would show the complement of A when EN=1 (e.g., Y=1 for A=0).[1][13]
A primary benefit of tri-state buffers is their ability to enable bidirectional data flow on shared buses, as the Hi-Z state allows other devices to drive the line without conflict, which is essential for efficient multiplexing in digital systems.[1]
where is the channel aspect ratio, is the carrier mobility, is the gate oxide capacitance per unit area, is the gate-source voltage, and is the threshold voltage; this equation ensures balanced drive strength by adjusting widths to minimize delay.[23] As a variation, bipolar transistor-transistor logic (TTL) buffers employ a totem-pole output stage for faster switching, featuring an upper NPN transistor (e.g., Q5) for active pull-up to VCC and a lower NPN transistor (e.g., Q6) for pull-down to ground, driven by prior amplification stages with resistors and diodes to prevent simultaneous conduction.[2] This configuration achieves lower output resistance in the high state compared to resistive pull-up, supporting higher fan-out at speeds up to tens of MHz in standard TTL families.[24]
Fundamentals
Definition and Purpose
A digital buffer is an electronic circuit element that replicates a digital input signal at its output while providing high input impedance and low output impedance, thereby preventing the load from affecting the source signal. This impedance matching ensures that the buffer acts as an intermediary that preserves the integrity of the digital signal without altering its logical value.[1] The primary purpose of a digital buffer is to offer electrical isolation between the signal source and the load, enhancing the drive capability to handle capacitive loads effectively and guaranteeing that output signal levels align with the voltage thresholds of specific logic families, such as TTL (typically 0-5 V) or CMOS (often 0-3.3 V or 0-5 V). By buffering current rather than amplifying voltage, it mitigates issues like signal degradation in multi-gate systems where fan-out demands exceed the source's capacity.[5][1][3] In operation, a digital buffer produces an output that directly follows the input logic state when enabled, maintaining unity gain to avoid voltage scaling while providing sufficient current to drive subsequent circuit stages without distortion. This buffering action is essential for maintaining reliable signal propagation in complex digital networks.[2]Electrical Characteristics
Digital buffers are characterized by several key electrical parameters that determine their performance in digital circuits, including voltage levels, current capabilities, timing specifications, power consumption, and susceptibility to environmental variations. These parameters vary depending on the logic family, such as TTL (Transistor-Transistor Logic) or CMOS (Complementary Metal-Oxide-Semiconductor), with TTL offering higher speed and drive capability at the cost of greater power use, while CMOS provides lower power dissipation and wider supply voltage tolerance.[6][7]Voltage Levels
The output high voltage (VOH) represents the minimum voltage level recognized as logic high at the output, typically 2.4 V min for LS-TTL buffers under standard conditions (VCC = 4.75 V, IOH = -2.6 mA), ensuring compatibility with downstream inputs. The output low voltage (VOL) is the maximum voltage for logic low, at 0.4 V max for LS-TTL when sinking 12 mA (or 0.5 V max at 24 mA). Input thresholds include the minimum high-level input voltage (VIH) at 2 V and maximum low-level input voltage (VIL) at 0.8 V for commercial LS-TTL, defining the range where the buffer reliably interprets signals. In contrast, CMOS buffers like the 74HC series operate over a supply range of 2 V to 6 V, with VOH ≥ 0.9 VDD (e.g., 4.4 V min at VDD = 4.5 V), VOL ≤ 0.1 VDD (e.g., 0.1 V max), VIH ≥ 0.7 VDD (e.g., 3.15 V min), and VIL ≤ 0.3 VDD (e.g., 1.35 V max) at nominal 5 V, allowing rail-to-rail operation for better noise immunity.[6][7]Current Specifications
Buffers must source or sink current to drive loads, with high-level output current (IOH) indicating sourcing capability (negative value for convention) and low-level output current (IOL) for sinking. For LS-TTL buffers, IOH is -2.6 mA max and IOL is 24 mA max, supporting fan-out to multiple standard loads. Input leakage currents are low, with high-level input current (IIH) ≤ 20 μA and low-level input current (IIL) ≥ -0.4 mA for TTL. CMOS buffers exhibit much lower currents, with IOH and IOL at ±6 mA min and input leakage (IIL) ±1 μA max, reflecting their high-impedance inputs and suitability for battery-powered applications.[6][7]Noise Margins
Noise margins quantify a buffer's tolerance to signal degradation, calculated as high-level noise margin NMH = VOH - VIH and low-level noise margin NML = VIL - VOL. In TTL, these are typically 0.4 V each (e.g., NMH = 2.4 V - 2 V = 0.4 V; NML = 0.8 V - 0.4 V = 0.4 V), providing modest protection against noise in noisy environments. CMOS offers superior margins, around 1.25 V or more at 5 V (e.g., NMH ≈ 4.4 V - 3.15 V = 1.25 V; NML = 1.35 V - 0.1 V = 1.25 V), due to sharper thresholds relative to supply rails, making it preferable for high-reliability systems.[6][7]| Parameter | TTL (e.g., 74LS125A at 5 V) | CMOS (e.g., 74HC125 at 5 V) |
|---|---|---|
| NMH (typ) | 0.4 V | 1.25 V |
| NML (typ) | 0.4 V | 1.25 V |
Timing Parameters
Propagation delay (tpd) measures the time from input change to output response, critical for high-speed operation; typical values are 15 ns max for TTL low-power Schottky (LS) buffers and 24 ns typ for HC-CMOS at 5 V with 50 pF load. Rise time (tr) and fall time (tf) define output transition speeds, around 18 ns max each for both families under similar conditions, influencing signal integrity in cascaded circuits. Input and output capacitances are generally low, with CMOS inputs at 3 pF typ, minimizing loading effects.[6][7]Power Dissipation
Static power dissipation in TTL arises from continuous current paths, with supply current (ICC) up to 20 mA for a quad buffer (≈5 mW/gate at 5 V), while CMOS static power is negligible, at 80 μA max (≈0.4 mW total). Dynamic power in CMOS follows Pdyn = CL VDD2 f, where Cpd ≈ 45 pF typ contributes to switching losses, scaling with frequency and load; TTL dynamic power is dominated by static components but includes transition currents. Fan-out capability, or the number of similar loads drivable without exceeding specs, is typically 10 LSTTL loads for both families.[6][7]Temperature and Supply Voltage Effects
Performance degrades with temperature and supply variations, requiring derating factors; commercial TTL operates reliably from 0°C to 70°C with VCC 4.75–5.25 V, where delays increase ≈0.3%/°C beyond 25°C and noise margins shrink at supply extremes (military grade extends to -55°C to 125°C and VCC 4.5–5.5 V). CMOS maintains characteristics over -40°C to 125°C and 2–6 V, with propagation delays varying inversely with VDD (e.g., halving from 6 V to 2 V) and minimal temperature coefficient due to complementary structure, though leakage rises at higher temperatures.[6][7]Types
Non-Inverting Buffer
A non-inverting buffer is a digital circuit that replicates the input logic level at its output without altering the signal polarity, effectively providing a Boolean function where the output Y equals the input A (Y = A).[1] This structure is commonly implemented at the gate level using two NOT gates connected in series, as the double inversion restores the original logic state.[1] The circuit symbol is a simple triangle without an inverting bubble, distinguishing it from inverting types.[2] In operation, a non-inverting buffer amplifies the input signal's drive capability while preserving its logic value, making it suitable for scenarios requiring signal isolation or level shifting without inversion.[2] For a basic always-on configuration, the truth table is as follows:| Input (A) | Output (Y) |
|---|---|
| 0 | 0 |
| 1 | 1 |
Inverting Buffer
An inverting buffer is a digital logic circuit that performs signal inversion while providing sufficient drive strength to interface with multiple loads or longer transmission lines, effectively combining the function of a NOT gate with amplification capabilities. It is structurally equivalent to a single NOT gate augmented with buffering stages to enhance fan-out and reduce loading effects on the input source.[8] Certain variants incorporate a Schmitt trigger input stage, which introduces hysteresis to improve noise immunity by preventing multiple transitions due to input noise near the threshold levels.[9] The basic operation of an inverting buffer is to produce an output that is the logical complement of the input: a logic high (1) at the input results in a logic low (0) at the output, and vice versa, thereby reversing the signal polarity. The logical operation can be expressed as for the basic case, where is the input and the overbar denotes negation.[10] The truth table for a basic inverting buffer is as follows:| Input | Output |
|---|---|
| 0 | 1 |
| 1 | 0 |
Tri-State Buffer
A tri-state buffer is a specialized digital buffer that includes an enable input to control its output state, allowing it to operate in three distinct modes: logic high, logic low, or high-impedance (Hi-Z). This structure enables the buffer to isolate its output from the connected circuit when disabled, preventing electrical contention in shared signal lines. Unlike a standard buffer, which is always active, the tri-state variant uses the enable signal to switch between buffering the input and presenting a disconnected output.[1] The operation of a tri-state buffer depends on the enable input: when active, it functions as either a non-inverting buffer (output equals input) or an inverting buffer (output is the logical complement of the input); when inactive, the output enters the Hi-Z state, exhibiting an impedance typically greater than 1 MΩ in CMOS implementations, which allows multiple drivers to share the same bus without interference. This high-impedance mode effectively removes the buffer from the circuit, as the output neither sources nor sinks significant current (leakage typically <10 µA).[1][7] Tri-state buffers come in several variants to suit different control and logic requirements. Active-high variants are enabled when the enable input (EN) is logic 1, while active-low variants are enabled when EN is logic 0. Additionally, inverting types produce an output that is the inverse of the input when active, available in both active-high and active-low configurations. Common integrated circuit examples include the 74LS241 (active-high non-inverting) and 74LS240 (active-high inverting) for TTL logic, and the SN74HC125 for CMOS.[13] The truth table for a basic non-inverting active-high tri-state buffer illustrates its behavior, where Y is the output, A is the data input, and EN is the enable input:| EN | A | Y |
|---|---|---|
| 0 | 0 | Z |
| 0 | 1 | Z |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
Implementations
Gate-Level Design
Digital buffers at the gate level are constructed using basic logic gates such as NOT (inverters), AND, and OR gates to achieve signal buffering without altering the logical function beyond inversion where intended.[3] This approach provides an abstraction layer above transistor-level implementations, focusing on combinational logic to drive signals with increased fan-out capability.[14] The non-inverting buffer is implemented by cascading two NOT gates in series, where the first inverter inverts the input signal and the second restores the original polarity, resulting in a non-inverted output with enhanced drive strength.[3] This configuration ensures the output matches the input logic level while providing buffering to prevent signal degradation.[14] An inverting buffer, by contrast, is realized using a single buffered NOT gate, which inverts the input signal and amplifies it to drive multiple loads.[4] At the gate level, this simplifies to a standard inverter symbol, emphasizing the polarity reversal inherent to the NOT operation.[15] For tri-state buffers, the design incorporates an enable signal using an AND gate for non-inverting variants, where the input is ANDed with the enable (EN) to control an output driver, producing a high-impedance state (Z) when EN is low.[16] Inverting tri-state buffers similarly use an OR gate with the complemented enable or a NAND configuration to achieve the third state, allowing multiple buffers to share a bus without contention.[17] Schematic representations follow IEEE/ANSI Std 91-1984 conventions, where buffers are depicted with a triangular outline indicating amplification, a small circle (bubble) at the output for inversion in inverting types, and an additional control input with a bubble for active-low enable in tri-state symbols. In field-programmable gate arrays (FPGAs), digital buffers are implemented using look-up tables (LUTs), where a simple non-inverting or inverting buffer requires only one LUT configured to pass or invert the input, minimizing resource usage in programmable logic blocks.[18] This LUT-based approach leverages the reconfigurability of FPGAs for efficient buffering in larger designs.[19]Transistor-Level Design
In complementary metal-oxide-semiconductor (CMOS) technology, a non-inverting buffer is typically implemented by cascading two inverters, each consisting of complementary PMOS and NMOS transistor pairs connected in series to form a push-pull output stage.[20] The first inverter inverts the input signal, while the second restores the original polarity, ensuring the output matches the input logic level without attenuation. This configuration uses four transistors total: two PMOS (one for pull-up in each stage) and two NMOS (one for pull-down in each stage), with gates driven by the input or intermediate node.[21] The basic building block for an inverting buffer in CMOS is the inverter itself, featuring a single PMOS transistor for pull-up connected between VDD and the output, paired with a single NMOS transistor for pull-down connected between the output and ground, with both gates tied to the input.[20] When the input is low, the PMOS turns on to charge the output high, while the NMOS remains off; conversely, a high input activates the NMOS to discharge the output low, with the PMOS off. This push-pull arrangement provides low output impedance in both logic states, enabling the buffer to drive capacitive loads effectively.[20] For tri-state functionality in CMOS buffers, the design incorporates an enable signal (EN) to control parallel stacks of transistors, allowing the output to enter a high-impedance state when disabled. In a restoring tri-state non-inverting buffer, the core inverter pair is augmented with additional PMOS and NMOS transistors gated by EN and its complement (EN̅); for example, an extra PMOS in the pull-up path and an extra NMOS in the pull-down path are turned off simultaneously when EN = 0, isolating the output.[21] Advanced implementations, such as those using triple cascode structures with dynamic biasing, extend voltage tolerance while maintaining tri-state operation, employing low-voltage transistors (e.g., 2.5 V process) to handle up to 7.5 V signals without stress.[22] Transistor sizing in these buffers is guided by the approximate output resistance in the linear region, given bywhere is the channel aspect ratio, is the carrier mobility, is the gate oxide capacitance per unit area, is the gate-source voltage, and is the threshold voltage; this equation ensures balanced drive strength by adjusting widths to minimize delay.[23] As a variation, bipolar transistor-transistor logic (TTL) buffers employ a totem-pole output stage for faster switching, featuring an upper NPN transistor (e.g., Q5) for active pull-up to VCC and a lower NPN transistor (e.g., Q6) for pull-down to ground, driven by prior amplification stages with resistors and diodes to prevent simultaneous conduction.[2] This configuration achieves lower output resistance in the high state compared to resistive pull-up, supporting higher fan-out at speeds up to tens of MHz in standard TTL families.[24]