Circuits & schematics

Electronics guide

Electronics guide

So far, we’ve used the 741 op-amp in a variety of circuits, all of which form the basis for us to build many, many more circuits. And that’s fine if you only want to be given circuits which you can build yourself. But the more inquisitive reader may have been wondering how the device can do all of this: after all, it’s only through knowledge that greater things can be achieved — to design your own circuits using op-amps, you need to know what makes them tick.

Figure 9.12 The breadboard layout of the circuit in Figure 9.11 on the previous page

An op-amp can be considered to be a general-purpose amplifier with an extremely high gain. By itself (that is, with no components connected to it) it will have a gain of many thousands. Typical figures for the 741, for example, give a gain of 200,000 at zero frequency, although this varies considerably from device to device. Now, nobody’s suggesting that this sort of gain is particularly useful in itself — can you think of an application in which a gain like this is required? — and besides, as each device has a different gain it would be well nigh impossible to build two circuits with identical properties, let alone the mass-produced thousands of radios, TVs, record players and so on which use amplifiers. So we need some way of taming this high gain, at the same time as defining its value precisely, so that useful and accurate amplifiers may be designed.

The process used in this taming of op-amps is known as feedback i.e., part of the output signal from the op-amp is fed back to the input. Look closely again at the circuits we have built so far this chapter and you’ll see that in all cases there is some connection or other from the output back to the input. These connections form the necessary feedback paths which reduce the amplifier’s gain to a determined, precise level.