Module 2: Logic Circuits

Question 1

What is TTL logic?

Answer 1

TTL = Transistor - Transistor Logic.

It consists of BJT transistors talking to each other.

Question 2

What are the pros and cons of TTL?

Answer 2

Pros: fast, robust, versatile.

Cons: big (BJT’s take lots of room) and consume lots of power.

Question 3

Explain how this circuit works and what it does.

Answer 3

It’s a TTL NAND gate.

Whilst inputs ‘A’ and ‘B’ are high, there is a reverse-biaed NP junction between the emitter and base. This draws a small reverse-biaed current, whih goes through the collected in to the second transistor. This allows VT2 to conductor (collector - emitter), driving the output ‘Q’ to low.

Question 4

What is the standard power supply voltage for TTL?

Answer 4

+5V and 0V

Question 5

What is the ‘fan-out’ of a logic gate?

Answer 5

The number of other logic gates it can drive

Fan-out = I_{Output High (driving gate) /} I_{Input High (driven gate)}

Question 6

What are some practical issues with TTL and/or to keep in mind when using it?

Answer 6

• when driving a load with some capacitance, large spikes of current will occur at the output when changing states
• unused inputs should not be left floating (can pick up unwanted noise)
• beware of switch bounce!
• always use appropriate heat sinks

Question 7

How can one minimise the impact of capacitance loads causing large output current spikes when using TTL?

Answer 7

Place decoupling capacitors (0.01uF - 0.1uF) across an IC throughout the board (every 5 IC’s or so).

Question 8

What does ‘CMOS’ stand for, and what are its advantages/disadvantages when compared with TTL?

Answer 8

Complementary Metal Oxide Semiconductor

Pro’s: extremely low power consumption, excellent noise immunity, wide range of acceptable power supply voltages

Cons: slower

Question 9

Convert the following NAND gate into a CMOS equivalent

Answer 9

Question 10

For a CMOS IC with a supply voltage of 5V, the output levels (e.g. output high, output low) are typically within ____ of the 5V input and 0V ground.

Answer 10

50mV!

Question 11

What are some typical values of fan-out for a CMOS and TTL IC?

Answer 11

TTL = around 20

CMOS = almost infinite (very low input current!)

Question 12

From a switching/propogation delay perspective, which is faster: CMOS or TTL? Why?

Answer 12

TTL by a long shot! CMOS has lots of capacitance because of the MOSFETs.

Question 13

What are the three reasons for power consumption in CMOS circuits?

Answer 13

• Quiescent dissiptation due to leakage currents (i.e. leakage currnts whilst the transistor is off)
• Dynamic dissipation as capacitors in the MOSFETs are charged up and discharged (this is proportional to switching times)
• Through-current dissipation when both n and p channels conduct simultaneously for a time

Question 14

Why is input protection in CMOS circuits such a big issue? Draw a basic circuit to protect a CMOS input from static build-up.

Answer 14

CMOS gates have a very high resistance, so there’s no leakage path for charge to be removed from the gate. Therefore, massive static charges (100V+) can build up, eventually ruining the insulation of the gate.

Question 15

What does ‘ECL’ stand for, and what is its claim to fame?

Answer 15

Emitter Coupled Logic

Fastest available logic (faster ever than TTL!), due to BJT’s never being in saturation, and small swing voltages between high and low.

Question 16

What is the following circuit called, and what makes it an oscillator?

Answer 16

A ‘ring’ oscillator

Oscillation occurs because there’s a propagation delay (t_P) between when the input changes from low to high, and the output from high to low

Question 17

What’s the maximum frequency for a CMOS 3x inverter ring oscillator? Assume each inverter has T_P = 8ns

Answer 17

T_P = 2 * 8ns * 3 inverters = 48ns

f = 1/48ns = 20.8MHz

Question 18

Draw the ouput waveform ‘Q’ for the following circuit (assume V₃ = initially high)

Answer 18

Question 19

What’s the difference between an astable and a monostable?

Answer 19

An a**stable has no stable states - it constantly oscillates between high and low, with different waveforms depending on the ciruit.

A monostable changes states when triggered.

Question 20

What time of circuit is this (e.g. astable/monostable?) Draw its output waveform.

Answer 20

Question 21

Define the terms ‘retriggerable’ and ‘resettable’ in the context of monostables.

Answer 21

• Retriggerable* means the pulse can be further extended by another trigger before the first pulse has elapsed
• Resettable* means the pulse can be made to return to its original stable state

Question 22

In the context of monostables, the width of the output pulse is determined by…

Answer 22

The internal resistance and capacitance

Question 23

Answer 23

Question 24

What is a comparator, and what does it do?

Answer 24

A comparator decides whether or not an input voltage is greater than some fixed value

For example, if the ‘+’ input is greater than the ‘-‘ input, it will output a ‘high’

Question 25

What is the purpose of this circuit? What does the zener diode do in this circuit?

Answer 25

This is a comparator circuit. It decides whether or not the input voltage is greater than 0V.

The Zener diode ‘clamps’ the output to be within TTL logic levels. Anything greater than 4.7V will conduct through the zener, and any reverse current (e.g. negative sink current from comparator output) will give a 0.6V drop across the Zener.

Question 26

What’s this circuit called, and what does it do? Draw a truth table.

Answer 26

It’s a window detector. It detects whether or not the input voltage is within a certain window.

Question 27

What’s the advantage of a Schmitt trigger over a regular comparator? Illustrate.

Answer 27

Question 28

Determine the two threshold voltages for the circuit

Answer 28

Question 29

How and why does this circuit work? Why is the second diode to Vdd needed?

Answer 29

If the input voltage (e.g. static) is greater than Vdd (supply), than Vdd becomes the lower potential. The static voltage thus flows, through the diode, to Vdd.

If there’s static voltage from the IC end (e.g. from another pin or some other source), then it shorts through the other diode to Vdd.