1445326735900-0

Building on blocs
Analogue integrated circuits
The first circuit is shown in Figure 9.5, and from this you will
see two 9 V batteries are used as a power supply. This is because
the 741 needs what’s called a three-rail power supply
i.e., one with a positive rail, a negative rail, and a ground rail
0 V. Now you may ask how two batteries can be used to provide
this. Well the answer’s simple, join them in series. This gives
a +18 V supply, a 0 V supply, and at the mid-point between the
batteries, a +9 V supply. As voltages are only relative anyway,
we can now consider the +9V supply to be the middle ground
rail of our three-rail power supply and refer to it as 0 V. The
+18 V supply then becomes the positive rail (+9 V) and the 0 V
supply becomes the negative rail (–9 V).
Hint:
The 741 is commonly referred to as an operational amplifier — shortened
to op-amp. The term comes from the earlier days in electronics
when discrete components were built into circuits and used as complete
general-purpose units. An op-amp, as its name suggests, is a
general-purpose amplifier which is more-or-less ready-to-use. Years
ago it might have been a large metal box full of valves and wiring.
Later it would have been a circuit board on which the circuit was built
with transistors. But nowadays nobody uses op-amps made with
discrete components, because modern technology has produced IC
op-amps such as the 741. Given the choice, would you use discrete
components to build an op-amp of the complexity of that in Figure
9.1, or would you use a 741?
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