Electronics Guide

TTL Circuit Structure

The basic TTL gate uses a multi-emitter transistor at its input stage, a phase-splitter transistor to generate complementary signals, and a totem-pole output stage. The multi-emitter input transistor provides the AND function, with each emitter connected to a separate input. When any input is held low, current flows through that emitter, keeping the transistor in saturation and preventing conduction to the output stage. Only when all inputs are high does the input transistor enter the active region, allowing the signal to propagate through the phase splitter to the output.

The totem-pole output stage uses two transistors in series between the positive supply and ground. One transistor sources current to pull the output high, while the other sinks current to pull the output low. This arrangement provides low output impedance in both states, enabling TTL to drive significant capacitive loads with fast switching times. However, during transitions, both output transistors may conduct briefly, creating a current spike that contributes to power supply noise.

TTL Voltage Levels and Noise Margins

Standard TTL operates with a 5-volt power supply and defines specific voltage thresholds for logic levels. The input low voltage (V_IL) maximum is 0.8 volts, while the input high voltage (V_IH) minimum is 2.0 volts. On the output side, the low voltage (V_OL) maximum is 0.4 volts, and the high voltage (V_OH) minimum is 2.4 volts.

These specifications yield noise margins of 0.4 volts for both low and high states. The noise margin represents the maximum noise voltage that can be tolerated without causing an incorrect logic level interpretation. While adequate for many applications, these margins are smaller than those of CMOS logic, making TTL more susceptible to noise in electrically harsh environments.

TTL Subfamilies

The original TTL family spawned numerous variants optimized for different performance characteristics:

Standard TTL (74xx series) provides the baseline characteristics with propagation delays around 10 nanoseconds and power consumption of approximately 10 milliwatts per gate. This original series established the voltage levels and interface specifications that subsequent families maintained for compatibility.

Low-Power TTL (74Lxx) reduces power consumption to about 1 milliwatt per gate by using higher resistance values, but at the cost of increased propagation delays approaching 35 nanoseconds. This tradeoff suits battery-powered applications where speed is not critical.

High-Speed TTL (74Hxx) achieves propagation delays under 6 nanoseconds through smaller resistor values and more aggressive transistor sizing, but power consumption increases proportionally to around 22 milliwatts per gate.

Schottky TTL (74Sxx) uses Schottky barrier diodes to prevent transistor saturation, dramatically reducing storage time delays and achieving propagation delays under 3 nanoseconds. The Schottky diode clamps the base-collector junction, keeping transistors out of deep saturation and enabling faster switching.

Low-Power Schottky TTL (74LSxx) combines Schottky technology with optimized resistor values to achieve a favorable balance of 2 milliwatts per gate and 10-nanosecond propagation delay. This subfamily became extremely popular and remained in widespread use well into the CMOS era.

Advanced Schottky TTL (74ASxx and 74ALSxx) represents the final evolution of TTL technology, using advanced processing to achieve sub-nanosecond propagation delays while maintaining low power consumption. These devices push bipolar technology to its limits but cannot match CMOS power efficiency at lower frequencies.

TTL Output Configurations

Beyond the standard totem-pole output, TTL offers alternative output configurations for specific applications:

Open-collector outputs omit the upper transistor of the totem-pole, providing only the pull-down transistor. An external pull-up resistor connected to the positive supply completes the circuit. Open-collector outputs enable wired-AND connections where multiple outputs connect to a common node, with the output going low if any connected gate outputs low. This configuration also allows interfacing to different voltage levels by connecting the pull-up to a different supply voltage.

Tri-state (three-state) outputs add an enable control that can place the output in a high-impedance state, effectively disconnecting it from the circuit. This capability enables multiple devices to share a common bus, with only one device actively driving the bus at any time. The high-impedance state presents minimal loading to the bus when inactive.

Basic CMOS Inverter

The CMOS inverter illustrates the complementary principle that defines this logic family. A p-channel MOSFET connects between the positive supply and the output, while an n-channel MOSFET connects between the output and ground. Both transistor gates connect to the input signal.

When the input is low, the p-channel transistor turns on (its gate-to-source voltage is negative) while the n-channel transistor turns off (its gate-to-source voltage is below threshold). Current flows through the p-channel device, pulling the output high. Conversely, when the input is high, the n-channel transistor turns on while the p-channel turns off, pulling the output low through the n-channel device.

In either stable state, one transistor is completely off, blocking the path between supply and ground. This creates the defining characteristic of CMOS: nearly zero static power consumption. Power is consumed only during transitions when both transistors may conduct briefly, and when charging or discharging load capacitance.

CMOS Logic Gates

Complex logic functions are constructed by arranging networks of p-channel transistors (the pull-up network) and n-channel transistors (the pull-down network) in complementary configurations. The pull-up network connects between the positive supply and the output, while the pull-down network connects between the output and ground. These networks must be designed as duals: where the pull-down has transistors in series, the pull-up has transistors in parallel, and vice versa.

A CMOS NAND gate, for example, uses two n-channel transistors in series in the pull-down network and two p-channel transistors in parallel in the pull-up network. The output goes low only when both inputs are high (turning on both series n-channel devices), while any low input turns on at least one p-channel device to pull the output high.

A CMOS NOR gate reverses this arrangement, with p-channel transistors in series and n-channel transistors in parallel. The output goes high only when both inputs are low, while any high input pulls the output low.

CMOS Voltage Characteristics

CMOS exhibits excellent voltage characteristics that contribute to its robust operation. The output voltage swings nearly rail-to-rail, with V_OH approaching V_DD and V_OL approaching ground. This full-swing output maximizes noise margins compared to TTL.

The input threshold is nominally at V_DD/2, though the actual switching point depends on the relative sizing of p-channel and n-channel transistors. Input current is essentially zero due to the insulated gate structure, allowing CMOS outputs to drive many CMOS inputs without significant loading concerns.

CMOS operates over a wide supply voltage range, typically from 3 volts to 15 volts for older 4000-series devices, or from 1 volt to 5 volts for modern advanced CMOS processes. The flexibility in supply voltage allows designers to trade speed for power consumption, as higher supply voltages provide faster switching but increase dynamic power dissipation.

CMOS Power Consumption

CMOS power consumption consists of three components: static power, dynamic power, and short-circuit power.

Static power arises from leakage currents through transistors in the off state. In older CMOS technologies with larger feature sizes, this component was negligible. However, as transistor dimensions have shrunk, leakage currents have increased dramatically, making static power a significant concern in modern nanometer-scale processes.

Dynamic power results from charging and discharging load capacitances during switching. The dynamic power equation P = CV²f shows that power is proportional to capacitance C, the square of the supply voltage V, and the switching frequency f. This relationship drives the trend toward lower supply voltages in modern CMOS designs.

Short-circuit power occurs during transitions when both pull-up and pull-down networks conduct simultaneously. As the input voltage passes through the threshold region, both transistors may be partially on, creating a direct path from supply to ground. This component is minimized by ensuring fast input transitions.

CMOS Logic Families

Several CMOS subfamilies address different application requirements:

4000-series CMOS was the original CMOS family, offering extremely low power consumption and wide supply voltage range (3V to 15V). However, speed was limited, with propagation delays around 50 nanoseconds at 5V. The 4000-series remains available for low-speed, low-power applications.

74HC (High-speed CMOS) provides TTL-compatible voltage levels while retaining CMOS low-power characteristics. Operating at 2V to 6V, the 74HC family achieves propagation delays under 10 nanoseconds at 5V, matching or exceeding TTL performance with much lower power consumption.

74HCT (High-speed CMOS, TTL-compatible) modifies the 74HC input thresholds to match TTL levels, enabling direct interfacing with TTL outputs. This compatibility comes at the cost of slightly higher power consumption due to the non-optimal threshold placement.

74AC/74ACT (Advanced CMOS) achieves higher speeds through aggressive transistor sizing and reduced geometries, with propagation delays approaching 3 nanoseconds. The 74ACT variant provides TTL-compatible input thresholds.

74LVC/74LCX (Low-Voltage CMOS) operates at supply voltages from 1.65V to 3.6V, targeting modern low-power systems. These families often include level-shifting capabilities to interface with different voltage domains.

74AUC (Advanced Ultra-low-voltage CMOS) operates at supply voltages as low as 0.8V with propagation delays under 2 nanoseconds, representing the current state of the art in discrete CMOS logic.

CMOS Latch-up

A phenomenon unique to CMOS is latch-up, a potentially destructive condition where parasitic thyristor structures within the CMOS device become triggered, creating a low-resistance path from supply to ground. This parasitic SCR (silicon-controlled rectifier) consists of the inherent pnpn structure formed by the n-well, p-substrate, and source/drain regions of the transistors.

Latch-up can be triggered by input or output voltages exceeding the supply rails, by high-current transients during power-up, or by ionizing radiation. Once triggered, the parasitic thyristor sustains conduction until power is removed, potentially drawing enough current to destroy the device.

Modern CMOS processes incorporate latch-up prevention measures including guard rings, well ties to supply rails, and epitaxial substrates. Designers must still observe proper sequencing of power supplies and limit input voltage excursions to prevent triggering latch-up in older or less protected devices.

ECL Operating Principles

ECL operates by steering current between two paths rather than switching transistors fully on and off. A differential pair of transistors shares a common emitter current source, with the input signal applied to one transistor base and a reference voltage to the other. The transistor with the higher base voltage conducts most of the current, pulling its collector lower, while the other transistor remains in the active region but conducts very little current.

Because transistors never enter saturation, there is no minority carrier storage delay, enabling extremely fast switching. The output swing is limited to approximately 0.8 volts, centered around a negative reference level. ECL traditionally uses a negative supply voltage (typically -5.2V) with the most positive level at or near ground, though positive-supply ECL variants exist.

ECL Characteristics

ECL offers exceptional speed, with propagation delays under 500 picoseconds for standard ECL families and under 100 picoseconds for advanced ECL. This speed comes at the cost of high power consumption, as current flows continuously through the current-steering transistors. Power dissipation of 25 to 60 milliwatts per gate is typical.

The small output swing and differential operation provide excellent noise immunity in differential mode applications. However, the voltage levels are incompatible with TTL and CMOS, requiring level translation for interfacing. The output drivers typically use emitter followers, providing low output impedance and the ability to drive 50-ohm transmission lines directly.

ECL systems often employ transmission line interconnects with series or parallel termination to maintain signal integrity at high frequencies. The differential operation allows use of balanced transmission lines, further improving noise rejection in high-performance systems.

ECL Families

MECL 10K was a popular general-purpose ECL family with propagation delays around 2 nanoseconds and differential outputs for noise-immune interconnection.

MECL 10KH offered improved speed with sub-nanosecond delays while maintaining compatibility with 10K.

MECL 100K and ECLinPS pushed speeds to the hundreds of picoseconds range, targeting the most demanding applications in telecommunications and test equipment.

Positive ECL (PECL) variants use positive supply voltages, simplifying system design and interfacing with other logic families.

I2L Structure

The I2L gate consists of a lateral pnp injector transistor and one or more vertical npn switching transistors. The pnp transistor, biased by an external current source, injects current into the base of the npn transistor. When the input pulls the npn base low, the transistor is off and its collectors (serving as outputs) are pulled high by the injected current flowing to the load. When the input goes high, the npn transistor turns on and pulls the output collectors low.

Multiple outputs can be obtained by using multiple collectors on the npn transistor, a feature of vertical bipolar transistor construction. Adjacent gates share the injector structure, minimizing silicon area. The injector current can be controlled externally, allowing tradeoffs between speed and power consumption.

I2L Characteristics

I2L operates with very low supply voltages, typically under 1 volt, with logic swings of only a few hundred millivolts. This limits noise immunity but enables extremely low power operation at low speeds. The speed-power product can be adjusted by changing the injector current, providing design flexibility.

Integration density was I2L's primary advantage, achieving gate densities competitive with NMOS while using standard bipolar processing available in the 1970s. This made I2L attractive for early microprocessors and complex bipolar integrated circuits. However, as CMOS processing matured and offered even higher density with better speed-power tradeoffs, I2L faded from commercial use.

NMOS Pass Transistor

An n-channel MOSFET can pass a low voltage efficiently, as its gate-to-source voltage remains high when the source is near ground. However, passing a high voltage is problematic because the gate-to-source voltage decreases as the source voltage rises. The transistor begins to turn off as the output approaches V_DD - V_th (where V_th is the threshold voltage), leaving the output at a degraded high level.

This threshold voltage drop is acceptable in some applications but causes problems in cascaded stages where the degraded voltage may not reliably trigger subsequent gates. Level-restoring techniques or complementary transistors can address this limitation.

Transmission Gates

The transmission gate combines an n-channel and p-channel transistor in parallel, with complementary control signals applied to their gates. When enabled, both transistors conduct, with the n-channel efficiently passing low voltages and the p-channel efficiently passing high voltages. This complementary operation achieves full-swing output without threshold voltage degradation.

Transmission gates find extensive use in multiplexers, latches, and analog switches. Their bidirectional nature allows signals to pass in either direction, making them suitable for bus architectures. The on-resistance varies with signal voltage but remains reasonably constant due to the complementary transistor action.

Complementary Pass Transistor Logic (CPL)

CPL uses differential pass transistor structures with complementary inputs and outputs. This approach eliminates the need for level restoration in most cases and enables efficient implementation of XOR and XNOR functions. The differential nature also improves noise immunity compared to single-ended pass transistor logic.

CPL requires both true and complement versions of all signals, increasing wiring complexity but reducing transistor count for many logic functions. The combination of CPL with conventional CMOS output drivers provides a powerful hybrid approach used in high-performance arithmetic circuits.

Basic Dynamic Logic Operation

A dynamic logic gate consists of a precharge transistor, an evaluate network, and an output node with parasitic capacitance. During the precharge phase (typically when the clock is low), the precharge transistor charges the output node to V_DD. During the evaluate phase (clock high), the precharge transistor turns off, and the evaluate network may discharge the output through a path to ground if the input conditions are satisfied.

If the evaluate network conducts, the output transitions from high to low. If the evaluate network does not conduct, the output remains high due to the stored charge. The logic function is determined by the evaluate network topology, which uses only n-channel transistors, reducing capacitance and improving speed compared to complementary CMOS.

Domino Logic

Domino logic addresses a key limitation of basic dynamic logic: the inability to cascade inverting stages because the output can only transition from high to low, not from low to high. Domino logic adds a static inverter after each dynamic gate, ensuring that the output transitions only from low to high during evaluation. This monotonic behavior allows cascading multiple stages within a single clock phase.

The name "domino" refers to the sequential evaluation that ripples through cascaded stages like falling dominoes. Each stage's evaluation triggers the next stage's evaluation, enabling fast signal propagation through combinational logic. However, domino logic can only implement non-inverting functions directly; inverting functions require additional logic.

NORA Logic

NORA (NO RAce) logic alternates between n-type dynamic stages (precharge high, conditional discharge) and p-type dynamic stages (precharge low, conditional charge). This approach allows both inverting and non-inverting functions while maintaining the speed advantages of dynamic logic. The alternating phases provide implicit pipelining within combinational blocks.

Dynamic Logic Considerations

Dynamic logic requires careful attention to timing and charge sharing effects. The precharge and evaluate phases must be properly controlled to prevent charge loss or incorrect evaluation. Charge sharing between the output node and internal nodes in the evaluate network can cause incorrect operation if not properly managed through techniques like precharging internal nodes or adding keeper devices.

Leakage current becomes increasingly problematic in nanometer-scale processes, as the stored charge can drain away before evaluation completes. Various techniques including voltage boosting, multiple threshold transistors, and leakage current compensation address this challenge.

LVDS Electrical Characteristics

LVDS transmitters produce a differential output with a voltage swing of approximately 350 millivolts centered around a common-mode voltage of 1.2 volts. The low voltage swing enables fast transitions while minimizing EMI. The constant-current output driver maintains a defined current flow (typically 3.5 mA) that reverses direction to change the logic state, eliminating the large current spikes associated with single-ended CMOS switching.

LVDS receivers use differential comparators with high common-mode rejection, enabling reliable signal detection in the presence of significant common-mode noise. The receiver terminates the transmission line with a resistor (typically 100 ohms) matched to the differential line impedance, absorbing the transmitted energy and minimizing reflections.

LVDS Advantages

The differential nature of LVDS provides excellent immunity to common-mode noise, as any noise affecting both lines equally is rejected by the receiver. This enables reliable operation in electrically noisy environments and over longer distances than single-ended signaling.

The low voltage swing and constant-current operation dramatically reduce EMI compared to single-ended CMOS. The opposing currents in the differential pair create magnetic fields that largely cancel, minimizing radiated emissions. The small, controlled current swings also reduce power supply noise.

LVDS achieves data rates from hundreds of megabits per second to several gigabits per second depending on the implementation. The technology scales well with distance, maintaining signal integrity over cables and backplanes where single-ended signals would fail.

LVDS Applications

Display interfaces extensively use LVDS, including the FPD-Link standard for connecting graphics processors to LCD panels. The parallel transmission of pixel data over multiple LVDS pairs provides the bandwidth needed for high-resolution displays.

Serial communication interfaces such as certain USB, SATA, and custom high-speed links employ LVDS principles. The serializer-deserializer (SerDes) approach converts parallel data to serial LVDS signals and back, reducing pin count while maintaining high throughput.

Industrial and automotive applications benefit from LVDS robustness in harsh electrical environments. Camera links, sensor interfaces, and control buses commonly use LVDS to maintain reliable communication despite electromagnetic interference.

CML Circuit Structure

A CML gate consists of a differential transistor pair with a tail current source and resistive loads. The differential input steers the tail current to one side or the other, creating complementary voltage outputs across the load resistors. The output swing depends on the product of the tail current and load resistance, typically set to a few hundred millivolts to maximize speed.

In CMOS CML, the differential pair uses n-channel transistors operating in saturation, with PMOS loads or resistors. The low output swing keeps transistors in their optimal operating region, avoiding the speed-limiting effects of rail-to-rail CMOS switching. The constant current draw reduces power supply noise and enables predictable performance.

CML Performance

CML achieves switching speeds approaching ECL levels while using standard CMOS processes. Transition times under 50 picoseconds are achievable in advanced processes, enabling data rates exceeding 10 gigabits per second. This performance makes CML essential for SerDes interfaces in high-speed communications and computing.

Power consumption in CML is constant regardless of switching activity, as the tail current flows continuously. This characteristic simplifies power distribution but limits efficiency at low activity factors. The power-speed tradeoff can be adjusted by scaling the tail current and load resistance inversely while maintaining constant output swing.

CML Applications

High-speed SerDes transmitters and receivers use CML extensively for the analog front-end circuitry. The clock recovery, data recovery, and serialization/deserialization functions all benefit from CML's combination of speed and differential operation.

Chip-to-chip interfaces at multi-gigabit rates commonly employ CML outputs driving controlled-impedance differential traces terminated at the receiver. Standards including PCIe, SATA, and high-speed Ethernet specify electrical characteristics consistent with CML implementations.

On-chip clock distribution in high-performance processors often uses CML or derived techniques to achieve the required frequency and phase accuracy. The differential operation and low swing minimize jitter and noise coupling to sensitive analog circuits.

Speed Comparison

ECL and CML lead in raw speed, with propagation delays under 100 picoseconds achievable in advanced processes. These families remain essential for the highest-frequency applications despite their power consumption.

Advanced CMOS families (74AUC, 74LVC) achieve propagation delays of 1-2 nanoseconds, adequate for most applications below 1 GHz. The continued scaling of CMOS processes pushes these limits higher with each technology node.

Standard CMOS and TTL families provide propagation delays of 5-20 nanoseconds, suitable for lower-frequency applications where power consumption or cost considerations outweigh speed requirements.

Power Consumption

CMOS dominates for low power consumption, especially in applications with low activity factors. Static power is essentially zero in older CMOS technologies, though leakage becomes significant in nanometer-scale processes.

TTL consumes significant power even when not switching, making it unsuitable for battery-powered applications. However, TTL power consumption is lower than ECL and CML for most practical applications.

ECL and CML consume power continuously due to their current-steering operation. This power is independent of switching frequency, making these families power-efficient only at very high operating frequencies where other families cannot perform.

Noise Immunity

CMOS offers the best noise immunity due to its rail-to-rail output swing and high input impedance. The noise margin approaches half the supply voltage, providing robust operation in noisy environments.

TTL provides moderate noise immunity with 0.4-volt noise margins. The well-defined voltage thresholds enable reliable operation but require careful attention to ground noise and power supply decoupling.

ECL and LVDS achieve excellent noise immunity through differential operation despite their small signal swings. Common-mode rejection allows reliable signaling in environments where single-ended approaches would fail.

Integration Density

CMOS enables the highest integration density, with billions of transistors on a single chip in modern processes. The simple transistor structure and minimal static power enable scaling to ever-smaller geometries.

Bipolar technologies (TTL, ECL) require larger transistor structures and have higher power density, limiting practical integration levels. Modern BiCMOS processes combine the advantages of both technologies where needed.

Supply Voltage

Modern CMOS operates at supply voltages from below 1 volt to 3.3 volts, with lower voltages reducing power consumption and enabling advanced process nodes. Legacy 5-volt CMOS and TTL compatibility requires level translation in mixed systems.

ECL traditionally uses negative supplies around -5.2 volts, though positive-supply variants exist. LVDS uses standard CMOS supply voltages with reduced signal swings.

Application Guidelines

For general-purpose logic below 100 MHz, CMOS provides the best overall solution with low power, high noise immunity, and excellent availability. The 74HC and 74LVC families cover most requirements.

For high-speed applications from 100 MHz to several GHz, advanced CMOS combined with CML-based SerDes addresses most needs. Only at the highest frequencies does ECL remain relevant, and even there advanced CMOS often provides adequate performance.

For battery-powered and portable applications, low-voltage CMOS (74LVC, 74AUC) maximizes battery life while providing adequate performance for most embedded systems.

For harsh environments with significant electrical noise, differential signaling using LVDS or CML provides robust communication. Single-ended CMOS with adequate noise margins serves less demanding applications.

TTL to CMOS Interfacing

The primary challenge in TTL-to-CMOS interfacing is the TTL high output voltage (V_OH minimum 2.4V), which falls below the CMOS input threshold at 5V supply. A pull-up resistor to the CMOS supply ensures the input reaches a valid high level. Alternatively, 74HCT CMOS accepts TTL levels directly with modified input thresholds.

CMOS-to-TTL interfacing is straightforward when both operate at 5V, as CMOS outputs easily drive TTL inputs. At lower CMOS supply voltages, level translation using dedicated level-shifter ICs or discrete circuits becomes necessary.

Different Voltage Domain Interfacing

Modern systems commonly include multiple voltage domains (5V, 3.3V, 2.5V, 1.8V, 1.2V). Level-shifter ICs provide bidirectional translation between domains, using techniques including auto-direction sensing, dual-supply operation, or open-drain outputs with pull-ups.

Simple resistive dividers can level-shift high-to-low but are slow and unidirectional. Active level shifters preserve signal integrity and enable bidirectional communication on data buses.

LVDS and CML Interfacing

Converting between single-ended CMOS and differential LVDS or CML requires specialized driver and receiver circuits. LVDS transceiver ICs handle this conversion, accepting single-ended CMOS inputs and producing differential outputs for transmission, then converting received differential signals back to single-ended CMOS.

Termination is critical for differential interfaces. The 100-ohm differential termination at the receiver absorbs transmitted energy and minimizes reflections. Improper termination degrades signal integrity and can cause bit errors.