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9.9 Option - The Age of Silicon: 2. Analogue verses digital circuits

Syllabus reference (October 2002 version)
2. Electronics use analogue and digital systems, the basic circuit elements of which are potential dividers and transistors	Students learn to: describe the difference between an electronic circuit and an electric circuit and the advantages and disadvantages of each distinguish between digital and analogue systems in terms of their ability to respond to or process continuous or discrete information identify systems that are digital and systems that are analogue in a range of devices identify potential dividers and transducers as common elements in both analogue and digital systems explain how the ratio of resistances in a potential divider allows a range of voltages to be obtained describe the role of transducers as an interface between the environment and an electronic system	Students: identify and analyse data and perform an investigation to demonstrate the difference between digital and analogue voltage outputs over time gather, process and present information to identify electronic systems that use analogue systems, including television and radio sets and those that use digital systems, including CD players solve problems and analyse information involving resistances, voltages and currents in potential dividers

Extract from Physics Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.

[Edit: 21 Aug 08]

distinguish between digital and analogue systems in terms of their ability to respond to or process continuous or discrete information

• Analogue systems produce or respond to continuously changing voltages and currents. Digital circuits work with binary states such as off or on or with two distinct voltages that switch individual circuit elements off or on as needed.

describe the role of transducers as an interface between the environment and an electronic system

• Transducers are devices that accept an input of energy in one form and output it in another form, with a fixed relationship between the input and the output. In electronics, we are interested in transducers that produce an electrical output signal or that change the voltage or current flow in the circuit where they are located. Some transducers accept an electrical signal as an input from electronic systems and produce effects (outputs) in the environment. See sections 3 & 4 below for more details.

describe the difference between an electronic circuit and an electric circuit and the advantages and disadvantages of each

• An electric circuit contains only devices such as switches, resistors, capacitors, inductors and transducers (see below for a definition) such as lights, motors, solenoids or combinations of these. An electronic circuit can contain all or any of the above plus one or more semiconductor-based devices such as a diode, light emitting diode, transistor, IC, light dependent resistor (LDR), thermistor or photodiode.

Electric circuits	Electronic circuits
Advantages: can handle higher voltages and current than electronic circuits Disadvantages: less efficient than electronic circuits	Advantages: are more efficient than electric circuits, the use of semiconductor based devices in them enable them to perform many different functions, can detect and work with tiny voltages and currents when compared to electric circuits, take up less space than equivalent electric circuits. Disadvantages: there are limits to the voltages and currents they can handle.

identify potential dividers and transducers as common elements in both analogue and digital systems

• Both digital and analogue systems may use potential (voltage) dividers in their circuits. A transducer (see section 3 below for different kinds of transducers that you must know about for the HSC) may take the place of one of the resistors in a voltage divider circuit. The resistance of the transducer is changed as it is affected by the environment which in turn changes the current and voltage in the circuit of which it is a part. Those changes in voltage and current can then be detected by electronic circuits (see some examples below).

explain how the ratio of resistances in a potential divider allows a range of voltages to be obtained

Potential divider circuit that produces a linear relationship between the input and the output	Potential divider circuit that produces a non-linear relationship between the input and the output
Vout = ___Rt___ x  V in   R1 + R t	Vout = ___Rt___ x  V in   R1 + R t Where R t is replaced by a transistor or other type of transducer

• If V _in is 5 volts and R ₁ is 1000 ohms and we vary R _t from 500 to 1000 ohms in increments of 100 ohms, V _out varies from 1.7 (to 1.9 to 2.1 to 2.2 to 2.4) to 2.5 volts. R _t could well be an LDR, thermistor or phototransistor (see sections 3 and 4 on transducers below)
  
  
• If we keep R ₁ (say 500 ohms) and R _t constant (say 1000 ohms) and vary V _in, then V_out is a fixed proportion of V _in ; in this case V _out = 0.67 V _in
  
  Note that V _out must always be less than V _in
  
  An internet resource with information about circuits that use resistive transducers in voltage divider circuits is the doctronics site in the UK:
  http://www.doctronics.co.uk/voltage.htm

identify and analyse data and perform an investigation to demonstrate the difference between digital and analogue voltage outputs over time

• Analogue voltage outputs vary continuously between preset limits over time. Digital voltage outputs also vary over time between preset limits, but they will be pulses whose amplitude varies in multiples of a basic voltage.

Analogue voltage	Digital voltage

identify systems that are digital and systems that are analogue in a range of devices

• A compact disc (CD) or digital versatile (or video) disc (DVD) player uses digital processes to extract information from the disc and an analogue system to produce sound. If an image is involved, then the systems that make them may be digital or analogue or a combination of the two.
  
  
• Mainstream radio stations in Australia, whether AM or FM, are analogue systems. Some are broadcasting digitally coded signals as part of a planned move to digital broadcasting in the future.
  
  
• TV stations transmit both analogue and digitally coded signals to your TV antennae. Again, there is a plan to move exclusively to digitally coded broadcasting in the future.
  
  
• Mobile phone systems are a combination of digital and analogue systems.
  
  
• The central processing unit (CPU) in a computer is a digital device, that convert inputs to digital codes that are transferred, processed, stored and retrieved as needed.
  
  
• An iPod uses digital systems to store and manage the music pieces and an analogue system to produce the sound in your ear piece.
  
  
• A cassette tape recorder/player or video tape recorder/player is an analogue storage system.
  
  
• A cathode ray tube (used to produce the images on many television sets and computer monitors) is an analogue system.
  
  
• The video screens and monitors on portable computers, liquid crystal display (LCD) screens and plasma screen TVs are driven by digital devices called video graphic arrays (VGAs).
  
  
• Camera film (analogue) is being replaced in digital cameras. The heart of the digital camera is a digital device called a charge coupled device (CCD). It contains an array of light detecting elements (pixels) that digitally code information about intensity and colour of the light falling on it. That information is stored on a digital device for transfer to a computer or digital decoder on a printer.
• Camera lenses are analogue systems.
  
  
• Computer hard drives (HD) and Random Access Memory (RAM) use digital systems to store information.
  
  
• Thermionic valves and transistors can be used in digital systems as switches and in analogue systems as amplifiers (see section 6 below).

gather, process and present information to identify electronic systems that use analogue systems, including television and radio sets and those that use digital systems, including CD players

• This point is related to the one above. Use the sources listed below to gather information about, at least, television and radio sets and CD players.
  
  How stuff works has a series of pages that contain information relevant to the topic and a collection of other sites you can go to for more information
  
  KIDS.NET.AU is the site of an interactive encyclopedia.
  
  Infoplease site also has explanations of how CD players and other devices work.

solve problems and analyse information involving resistances, voltages and currents in potential dividers

• You should try to work out relevant problems from previous HSC exams. The formula related to potential (voltage) dividers is provided above and you can use the Ohms Law formula R = V/I (or rearranged as needed: I = V/R & V = IR) to calculate the current flow in a series circuit (note that in a series circuit, the current is the same in each element of the circuit).