1. Digital Components Introduction Gate Characteristics Logic Families
Chapter 25
Digital Components
Introduction
Gate Characteristics
Logic Families
Logic Family Characteristics
A Comparison of Logic Families
Complementary Metal Oxide Semiconductor
Transistor-Transistor Logic

2. 25.1
Introduction
Earlier we looked at a range of digital applications based on logic gates – at that time we treated the gates as ‘black boxes’
We will now consider the construction of such gates, and their characteristics
In this lecture we will concentrate on small- and medium-scale integration circuits containing just a handful of gates
typical gates are shown on the next slide

3. Typical logic device pin-outs

4. Gate Characteristics The inverter or NOT gate
25.2
Gate Characteristics
The inverter or NOT gate
consider the characteristics of a simple inverting amplifier as shown below
we normally use only the linear region

5. We can use an inverting amplifier as a logical inverter but using only the non-linear region

6. we choose input values to ensure that we are always outside of the linear region – as in (a)
unlike linear amplifiers, we use circuits with a rapid transition between the non-linear regions – as in (b)

7. Logic levels
the voltage ranges representing ‘0’ and ‘1’ represent the logic levels of the circuit
often logic 0 is represented by a voltage close to 0 V but the allowable voltage range varies considerably
the voltage used to represent logic 1 also varies greatly. In some circuits it might be 2-4 V, while in others it might be V
in order for one gate to work with another the logic levels must be compatible

8. Noise immunity noise is present in all real systems
this adds random fluctuations to voltages representing logic levels
to cope with noise, the voltage ranges defining the logic levels are more tightly constrained at the output of a gate than at the input
thus small amounts of noise will not affect the circuit
the maximum noise voltage that can be tolerated by a circuit is termed its noise immunity, VNI

9. Transistors as switches
both FETs and bipolar transistors make good switches
neither form produce ideal switches and their characteristics are slightly different
both forms of device take a finite time to switch and this produces a slight delay in the operation of the gate
this is termed the propagation delay of the circuit

10. The FET as a logical switch

11. Rise and fall times
because the waveforms are not perfectly square we need a way of measuring switching times
we measure the rise time, tr and fall time, tf as shown below

12. The bipolar transistor as a logical switch

13. when the input voltage to a bipolar transistor is high the transistor turns ON and the output voltage is driven down to its saturation voltage which is about 0.1 V
however, saturation of the transistor results in the storage of excess charge in the base region
this increases the time taken to turn OFF the device – an effect known as storage time
this makes the device faster to turn ON than OFF
some switching circuits increase speed by preventing the transistors from entering saturation

14. Timing considerations
all gates have a certain propagation delay time, tPD
this is the average of the two switching times

15. 25.3
Logic Families
We have seen that different devices use different voltages ranges for their logic levels
They also differ in other characteristics
In order to assure correct operation when gates are interconnected they are normally produced in families
The most widely used families are:
complementary metal oxide semiconductor (CMOS)
transistor-transistor logic (TTL)
emitter-coupled logic (ECL)

16. Logic Family Characteristics
25.4
Logic Family Characteristics
Complementary metal oxide semiconductor (CMOS)
most widely used family for large-scale devices
combines high speed with low power consumption
usually operates from a single supply of 5 – 15 V
excellent noise immunity of about 30% of supply voltage
can be connected to a large number of gates (about 50)
many forms – some with tPD down to 1 ns
power consumption depends on speed (perhaps 1 mW)

17. Transistor-transistor logic (TTL)
based on bipolar transistors
one of the most widely used families for small- and medium-scale devices – rarely used for VLSI
typically operated from 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

18. Emitter-coupled logic (ECL)
based on bipolar transistors, but removes problems of storage time by preventing the transistors from saturating
very fast operation - propagation delays of 1ns or less
high power consumption, perhaps 60 mW/gate
low noise immunity of about V
used in some high speed specialist applications, but now largely replaced by high speed CMOS

19. A Comparison of Logic Families
25.5
A Comparison of Logic Families
Parameter
CMOS
TTL
ECL
Basic gate
NAND/NOR
NAND
OR/NOR
Fan-out
>50 10 25
Power per gate (mW)
1 MHz
1 - 22
4 - 55
Noise immunity
Excellent
Very good
Good
tPD (ns)
1.5 – 33
1 - 4

20. Complementary Metal Oxide Semiconductor
25.6
Complementary Metal Oxide Semiconductor
A CMOS inverter

21. CMOS gates

22. CMOS logic levels and noise immunity

23. Transistor-Transistor Logic
25.7
Transistor-Transistor Logic
Discrete TTL inverter and NAND gate circuits

24. A basic integrated circuit TTL NAND gate

25. A standard TTL NAND gate

26. A TTL NAND gate with open collector output

27. Key Points Physical gates are not ideal components
Logic gates are manufactured in a range of logic families
The ability of a gate to ignore noise is its ‘noise immunity’
Both MOSFETs and bipolar transistors are used in gates
All logic gates exhibit a propagation delay when responding to changes in their inputs
The most widely used logic families are CMOS and TTL
CMOS is available in a range of forms offering high speed or very low power consumption
TTL logic is also produced in many versions, each optimised for a particular characteristic