Every electrical component we use, whether it be a TV remote, a microwave oven or even the mobile phone, has a complex circuitry consisting of critical connectivity of components forming logical families. But, what are logic families?
Well, there are two meanings to it. Firstly, a ‘logic family’ of digital integrated circuits devices is a well-constructed system of electronic logic gates specifically designed for different configurations, characterized by the various logic levels and the power supplies.
The most common logic levels are logical 1 and logical 0. These logical families contain basic logical systems interconnected to form a larger family of integrated circuitry.
Alternatively, a ‘logic family’ is also the connectivity of electrical components to implement logic in VLSI (very-large-scale integrated) circuitry, such as processors and memory systems.
Application of external voltages powers up these logic systems. The most common logic levels of these external voltages are a logic high/logic 1, generally indicating a presence of voltage, and a logic low/logic 0, generally indicating an absence of voltage.
These logic families have many factors characterizing them. These factors decide which logic family is better over the other.
Characteristics of Logic Families
The speed of a logic circuit is the time between which a single is applied at the input, and an output is received.
Fan-in determines the maximum number of inputs a logic gate can handle. A 2-input AND gate has a fan-in of 2. An inverter has a fan-in of 1.
Fan-out determines the number of circuits that a gate can drive, i.e. the maximum number of digital inputs that the output of a single logic gate must feed to, with both the input and outputs belonging to the same gate family.
A little confusing?
Basically, you can calculate the fan-out of a given NAND gate by checking how many other NAND gates have received input from the output of the prior NAND gate.
In the given system, the output of the first NOT gate serves as the input to 4 NOT gates. Thus, the fan-out is 4.
It is the maximum noise any circuit can handle without affecting the output.
Propagation delay is measured as the time between which input is applied to a system and it affects the output. It is measured at 50% marks.
Classification of Logic Families
Logic families are mainly classified according to the polarity of the devices used. We have two main classifications – bipolar and unipolar.
Bipolar mean two polarities (bi- two, polar-polarities). Here, the circuits have bipolar elements like diodes, transistors, etc. Other passive elements like resistors and capacitors also make up the circuit. We can divide bipolar families further divided into saturated and unsaturated logic families. Under the saturated logic families, the transistors used in ICs are driven into saturation, which is otherwise in unsaturated logic systems.
Unipolar families have components with only one polarity. The circuits have unipolar components like MOSFETs and passive elements.
Transistor-Transistor Logic Families
The transistor-transistor logic families are made completely with bipolar junction transistors (BJTs) and resistors. But why ‘transistor-transistor logic’ and not simply ‘transistor logic’?
This is because BJTs are used as the main components in both the logic system of the circuit as well as the amplification part of the circuit. Commercially, the TTL logic components begin with ‘74XX’, such as 7404, 74S86, etc.
For logic gates built using TTL components, the input voltage is applied to the emitter of the transistors. Circuits comprising of the TTL families usually consist of multi-emitter transistors, i.e. multiple parallel transistors with distinct emitter inputs and common base and collector terminals.
The transistors A and B in Fig.3 have separate inputs at the transistors, but the bases are made common and biased with a resistor, and the collector terminals have also been combined, acting as the base of the next transistor.
In the given 2-input TTL configuration, the application of a logic high voltage at terminals A and B reverse biases the emitter-base junction of the transistor. Thus, a small current flows in the opposite direction. This small current reaches the base terminal of the output, thus turning on the transistor. Upon switching on, the transistor pulls down its output to a logic low. Reversely, if either of the inputs at terminals A and B is zero, no current reaches the base of the second transistor, thus the output becomes VCC.
Apart from using normal transistors, Schottky transistors are also preferred for TTL logic systems. These transistors portray the Schottky effect and thus have higher switching speed and lesser power consumption.
TTL is now considered a little outdated, but is used as switching device is relays and driving lamps, and in printers and video display terminals.
Characteristics of TTL Logic Families
- The output of a TTL device can serve as an input to a maximum of 10 gates, i.e. the fan-out is 10.
- A logic low voltage for a TTL is defined between 0V-0.2V.
- A logic high voltage for a TTL is at 5V.
- The noise margin is at around 4V.
- The propagation delay is about 9ns.
- A typical TTL component draws a power of about 11mW.
Advantages of TTL Logic Families
- TTL has a strong drive capability.
- It is least susceptible to electrical damage.
- Requires only one supply voltage (otherwise for CMOS)
- Lesser immune to noisy when compared to ECL, but more than CMOS.
- Fastest saturation, when compared to other logic families
- Low output impedance for all states
Disadvantages of TTL Logic Families
- TTL dissipates a lot of power, thus not making it suitable for battery-powered devices.
- Not recommended in VLSI chips as it requires more space and isolation
- Expensive compared to MOSFETs
- 74 family
- 74LS (Low-power Schottky) family
- 74F (Fast) family
- 74AS (Advanced Schottky) family
- 74ALS (Advanced Low-power Schottky) family
Emitter – Coupled Logic Families
ECL families use overdriven BJT differential amplifiers with single-ended inputs. The emitters of the transistors are connected all together, hence the name ‘emitter-coupled’ logic families. The transistors in this system never saturate; their logic high and logic low levels are chosen close to one another, thus eliminating the possibility of saturation in the transistors. The transistors are thus operated in the active region or the cut-off region. These transistors use the property of current switching, thus this family is also known as Current Steering Logic (CSL) or Current Mode Logic Families (CML). It makes use of a transistor-based differential amplifier to amplify and combine digital signals.
We can see here that the transistors are differential amplifiers, with their emitters coupled. Cryptographic applications and high-speed requirements use ECL Logic.
Characteristics of ECL Logic Families
- The output of an ECL system has a very low impedance, thus having a fan-out of about 25.
- The propagation time is about 1ns, making it the fastest logic family.
- A logic low voltage for ECL is about -1.7V to -1.75V.
- A logic high voltage for ECL is about -0.8V.
- It is the fastest amongst all the logic families.
- It has an average propagation delay time of about 1ns-4ns.
Advantages of ECL Logic Families
- Has a fanout better than the TTL Logic family
- Offers the highest speed in operation
- ECL systems produce complementary outputs (OR-NOR, AND-NAND).
- Parameters do not vary much with temperature
Disadvantages of ECL Logic Families
- Worst noise immunity compared to TTL and CMOS
- Highest power consumption, when compared with TTL and CMOS
- VLSI design difficult as ECL circuits require resistors, thus increasing system size
- Capacitive loading reduces the fan-out capacity
Complimentary MOSFETs Logic Families
The CMOS family uses MOSFETs in the integrated circuits. Both NMOS and PMOS complement each other and are used symmetrically in each configuration of the circuit, hence the name ‘complimentary’. Since both PMOS and NMOS are present, an application of input voltage will turn on any one type of the transistor at a time. Thus, at any time, there is no direct link between the power supply and the ground. Two main attributes of CMOS devices are high immunity to noise and low static power consumption. CMOS consists of only an enhancement type of MOSFETs.
For NMOS transistors, if the input is a 1 the MOS turns on, otherwise, it is off. On the other hand, for the PMOS, if the input is 0 the MOSFET switches on, otherwise, the transistor is off. Thus, if either of the input is 1, the output voltage is equal to VDD.
The FETs are placed such that if either of the NMOS or PMOS transistors are connected in series, the other type is connected in parallel. Thus, only one type of MOS turns on at a time.
We use CMOS chips in RF applications, satellite technology, Bluetooth, and mobile networks. We also see their use in radar applications and Wi-Fi technology.
Characteristics of CMOS Logic Families
- CMOS supports a very large fan-out, more than 50 transistors.
- It has excellent noise immunity amongst all families.
- A logic low voltage for CMOS is about
- A logic high voltage for ECL is somewhere between 4.5V to 5V.
- The propagation delay is the worst when compared with TTL and ECL families, at about 200ns.
Advantages of CMOS Logic Families
- Has the highest fanout, when compared with TTL and ECL
- Works well over a wide range of temperature
- Noise immunity is better than TTL and ECL
Disadvantages of CMOS Logic Families
- Average propagation delay is the least in comparison with TTL and ECL
Common TTL Logic ICs
- 4000 family
- 74C family
- 74HC family
- 74HCT family
- 74AC family
- 74ACT family
- 74AHC family
BiCMOS Logic Families
BiCMOS, or Bipolar CMOS, combines the best properties of two distinct technologies in the integrated circuit- Bipolar junction transistors and the CMOS logic family. We see the application of BiCMOS in Pentium, Pentium Pro microprocessors.
The combined strengths of BJTs and CMOS makes BiCMOS a very special and important technology introduced in the electronics industry. High speed and low output impedance offered by the BJTs and high input resistance by CMOS are the main qualities combined to give the BiCMOS.
Since it combines two different designs, the fabrication of the BiCMOS chips happens with high precision and accuracy. The chip designing usually begins with a foundation of the CMOS process and continues with bipolar process steps.
Also, the fabrication of CMOS and BJTs require a controlled addition of impurities, thus increasing the complexity of making these chips. All these together make BiCMOS technology really expensive, when compared with the other technologies. In addition to this, the complicated manufacturing of BiCMOS risks the chances of increased leakage current.
The basic circuit configuration is exactly the same as that of a 2-input CMOS NAND gate, but with extra MOSFETs in the circuit.
Qp and Qo are low impedance output drivers.
The circuit input VA is given at PA and NA1, and input VB is given at PB and NB1. The FETs NB3, NA3 and N2 remove the base charge from the BJTs during switching.
Characteristics of BICMOS Logic Family
- BiCMOS offers lower power dissipation than BJTs.
- It offers improved speed when compared with CMOS technology.
- BiCMOS circuits offer high load current sinking and current sourcing
Advantages of BiCMOS Logic Families
- The best circuit to be used where high current sinking and sourcing is involved.
- Have reduced cycle time, when compared to CMOS circuits
- Robust to process changes and temperature changes
- Lower power dissipation than BJTs
Disadvantages of BiCMOS Logic Families
- Greater process complexity
- Manufacturing cost is very high
- Increased time in fabrication
Difference between TTL, ECL, and CMOS
|Components||Transistors & passive elements||Transistors & passive elements||MOSFETs|
|Noise Immunity||Strong||Good||Very strong|
|Fanout||10||25||More than 50|
|tPD in ns||1.5-30||1-4||1-210|
|Power/gate in mWatt||10||40-55||0.0025|
|Clock rate in MHz||35||>60||10|
|Figure of Merit||100||40-100||0.7|