In part 9 we looked at how we can use a current stage (or current amplifier) to provide the large currents required from power amplifiers. We also saw that this means we need some kind of control system to ensure that the current stage provides the amount of current needed to get the output voltage we actually want to apply to the load (e.g. a loudspeaker). We'll now look at the technique called feedback which was used in the poweramp example to achieve this control. We'll concentrate on feedback circuits which use operational amplifiers since these are widely used in modern electronics. Note, however, that the technique of feedback can be used to control the behaviour of any kind of electronic amplifier, whether they use 'op-amps' or not.
Figure 10.1 illustrates a simple op-amp voltage amplifier and a typical op-amp integrated circuit (IC). The IC is actually a small silicon 'chip' which, in this example, is placed in a Dual-In-Line Plastic (often called a 'DIL' or 'DIP') package. This is a small slab of plastic with two rows of legs or pins placed in parallel lines along opposite sides. It's conventional to number these pins in anticlockwise order viewed from above, starting from a mark at one end/corner as shown. There are lots of different op-amps around. Here we'll concentrate on the 'SN741' op-amp. This is because it was the earliest one to be made in vast quantities & is still used a lot (it's very cheap!) whenever we don't want state-of-the-art performance (it's also crap compared to more modern designs!). In figure 10.1 the inverting & non-inverting inputs of the op-amp are shown by the '+' and '™' signs at the left hand end of the triangle symbol which represents an op-amp. It's conventional to use the 'flat' end of this symbol for inputs and the 'apex' for the output.
To understand how an op-amp works you have to remember two things about it:-
- i) The output voltage is proportional to the difference between the voltages applied to two inputs, the inverting and non-inverting inputs.
- ii) The op-amp has a very large gain. This means that the output voltage and current it produces are far bigger than the input voltages & currents it requires. A typical op-amp has a voltage gain of between 100,000 and 1,000,000 and a similar current gain value.
Content and pages maintained by: Jim Lesurf (firstname.lastname@example.org)
using HTMLEdit2 on a StrongARM powered RISCOS machine.
University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.