Another point to remember when reading a circuit diagram is that some of the most important wires and connections may not be shown! For example, the logic gates shown in figure 1.2b have to be connected to a power supply which provides them with the electrical power they need. TTL requires connection to +5 Volts and 0 Volts (usually called 'Ground' or 'Earth'). These power/ground wires aren't shown on figure 1.2b because, to a digital engineer, they are so obvious that they can't always be bothered to waste their time & ink reminding you about them! The effect of not connecting these wires is pretty obvious - nothing works!
Any system which boosts or amplifies signals must get the extra power from somewhere, otherwise it would violate the principle of energy conservation. This means all amplifiers - whether they use transistors, ICs, valves, or whatever - must have power lines connected to them. (Digital logic gates also boost the signals passed through them, so the same arguments apply to them, too.) The amplifier then draws the power it requires from the supply and uses it to produce the enlarged version of the signal. This requirement to satisfy energy conservation has some important consequences. The most obvious one is that there is always a maximum possible signal voltage/current/power which a given system can produce. For example, the amplifier shown in figure 1.2a is powered by connecting it to a +15 Volt rail (another term for a power wire) and a 0 Volt 'Earth' line. This means it's impossible for this circuit to produce an output voltage which is outside the range 0 to +15 Volts. Hence it can't output a signal which is bigger than this range. Although less obvious, there will be also be a limit on how much current the amplifier & its power supply can produce.
All electronic signals require some power/energy to be sent from one place to another. A signal which carried no energy wouldn't be able to do anything to let you know it existed! Since electrical Power, Watts = Volts x Amps, this means that both the voltage and the current have to be non-zero if we want to do anything. Some of the reasons why this is important should become clear in later sections.
Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)
using HTMLEdit2 on a StrongARM powered RISCOS machine.
University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.