In this article, you will find the Study Notes on Logic Families which will cover the topics such as Integrated Circuits, Characteristics of digital circuits, Logical Families, Digital IC terminology, DL, RTL, DTL, and TTL.
1. Integrated Circuits
- Integrated circuits (ICs) are chips, pieces of semiconductor material, that contain all of the transistors, resistors, and capacitors necessary to create a digital circuit or system.
- The first ICs were fabricated using Ge BJTs in 1958.
- Jack Kirby of Texas Instruments, Nobel Prize in 2000
- Robert Noyes of Fairchild Semiconductor fabricated the first Si ICs in 1959.
- SSI Small scale integration [12 gates/chip]
- MSI Medium-scale integration [100 gates/chip]
- LSI Large scale integration [1K gates/chip]
- VLSI Very large scale integration [10K gates/chip]
- ULSI Ultra large scale integration [100K gates/chip]
- A prediction made by Moore (a co-founder of Intel) in 1965: “… a number of transistors to double every 2 years.”
2. Characteristics of digital circuits
- Fan in:
- Fan-in is the number of inputs connected to the gate without any degradation in the voltage level.
- Fan out:
- Fan out specifies the number of standard loads that the output of the gate can drive without impairment of its normal operation
- Power dissipation:
- Power dissipation is a measure of power consumed by the gate when fully driven by all its inputs.
- Propagation delay:
- Propagation delay is the average transition delay time for the signal to propagate from input to output when the signals change in value. It is expressed in ns.
- Noise margin:
- It is the maximum noise voltage added to an input signal of a digital circuit that does not cause an undesirable change in the circuit output. It is expressed in volts.
2. Logic Families
Logic families are sets of chips that may implement different logical functions but use the same type of transistors and voltage levels for logical levels and for the power supplies. These families vary by speed, power consumption, cost, voltage & current levels. The most widely used families are:
- DL (Diode- logic)
- DTL （Diode-transistor logic）
- RTL （Resistor-transistor logic）
- TTL （Transistor -transistor logic）
- ECL （Emitter-coupled logic）
- MOS （Metal-oxide semiconductor）
- CMOS (Complementary Metal-oxide semiconductor）
3. Digital IC Terminology
- VIH(min): high-level input voltage, the minimum voltage level required for a logic 1 at an input.
- VIL(max): low-level input voltage
- VOH(min): high-level output voltage
- VOL(max): low-level output voltage
- For proper operation, the input voltage levels to logic must be kept outside the indeterminate range. Lower than VIL(max) and higher than VIH(min).
- The maximum noise voltage that can be tolerated by a circuit is termed its noise immunity (noise Margin)
- IOH – Current flowing into an output in the logical “1” state under specified load conditions
- IOL – Current flowing into an output in the logical “0” state under specified load conditions
- IIH – Current flowing into an input when a specified HI level is applied to that
- IIL – Current flowing into an input when a specified LO level is applied to that
4. Diode Logic (DL)
- simplest; does not scale
- NOT not possible (need an active element)
5. Resistor-Transistor Logic (RTL)
- replace diode switch with a transistor switch
- can be cascaded
- large power draw
6. Diode-Transistor Logic (DTL)
- essentially diode logic with transistor amplification
- reduced power consumption
- faster than RTL
7. Transistor-transistor logic (TTL)
- based on bipolar transistors one of the most widely used families of small- and medium-scale devices – rarely used for VLSI
- typically operated from a 5V supply
- typical noise immunity about 1 – 1.6 V
- many forms, some optimised for speed, power, etc.
- High-speed versions comparable to CMOS (~ 1.5 ns)
- low-power versions down to about 1 mW/gate
TTL NAND Gate:
- Input terminals: The emitter of Q1
- Output terminals: collector of Q2
- When any input = logic ‘0’
- Q1 emitter junction is forward biased.
- Also, its collector junction is FB,
- so Q1 goes in saturation.
- Base of Q2 is at Low voltage
- This causes base-emitter junction of Q2 to be RB, so Q2 goes in cut-off
- Hence output is 5V or logic ‘1’
- When all inputs = logic ‘1’
- Q1 emitter junction is RB.
- so Q1 goes in cut-off.
- Its collector voltage increases
- This forward biases Q2,
- so Q2 goes into saturation
- Hence output is 0V
A TTL NAND gate with open collector output:
- Its similar to the previous circuit.
- Q2 is used as an emitter follower. The output of Q2 is fed to the input of Q3. Collector of Q2 and Q3 are in phase.
- This circuit needs an external ‘Pull- up’ resistor between output and power supply.
- The disadvantage of open- collector gate is their slow switching speed.
- The pull-up resistance is few kilo ohms. Gives a relatively long time constant, when multiplied by the stray output capacitance.
- Is worst when output goes from low to high.
TTL NAND gate with totem pole (active pull-up):
- In this circuit Q1 and the 4KΩ resistor act like a 2 input AND gate. The remaining circuit acts like an inverter. Transistors Q3 & Q4 form a totem-pole i.e.one NPN transistor in series with another.
- With a totem-pole output stage either Q3 or Q4 is on. When Q3 is ‘on’ output is high. When Q4 is ‘on’ output is low. If A or B is low, the Q1 conducts and the base voltage of Q2 is almost zero.
- Q2 cuts off, hence Q4 goes into cut off. Q3 base is high, Q3 acts as an emitter follower, the output Y’ is high.
- If A and B are high, Q1 does not conduct (cut-off), Q2 base goes high (saturation). Q4 goes into saturation hence output is low.
- The drop across Diode D3 keeps the base emitter diode of Q3 reverse biased. Hence Q3 is off or else it conducts slightly when output is low.
- Now only Q4 conducts when output is low. Totem pole transistors produce a low output impedance. When Q3 is conducting the output impedance is approx 70 Ω. When Q4 is saturated the output impedance is only 12 Ω. Hence the output impedance of a totem pole circuit is low.
- Any stray output capacitance is rapidly charged or discharged through the low output impedance. Hence the output can change quickly from one state to the other.
Types of TTL:
- Standard TTL
- typical gate propagation delay of 10ns and a power dissipation of 10 mW per gate, for a power–delay product (PDP) or switching energy of about 100 pJ
- Low-power TTL (L)
- slow switching speed (33ns)
- reduction in power consumption (1 mW) (now essentially replaced by CMOS logic)
- High-speed TTL (H)
- faster switching than standard TTL (6ns)
- but significantly higher power dissipation (22 mW)
- Schottky TTL (S)
- used Schottky diode clamps at gate inputs to prevent charge storage and improve switching time. A Schottky diode has a very low forward-voltage drop of 0.15–0.45V approx (silicon diode has a voltage drop of 0.6–1.7V). This lower voltage drop can provide higher switching speed.
- Faster speed of (3ns) but had higher power dissipation (19 mW)
- Low-power Schottky TTL (LS)
- used the higher resistance values of low-power TTL and the Schottky diodes to provide a good combination of speed (9.5ns) and reduced power consumption (2 mW), and PDP of about 20 PJ.
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