Making things bigger, making you better

Sometimes small things can make a big difference. In electronics this is called amplification. Often there will be a small signal we want to make bigger, whether its audio from your electric guitar that needs to be blasted out to the concert hall or the signal from tiny blinks of light that could indicate the presence of an exotic particle like the Higgs boson. Amplification is at the heart of many electronics applications, but how does it work? Historically it was done using rather exotically named devices like a ‘long-tailed pair’.

These days it’s often done digitally. These digital devices can do more that just make a small signal big, though. One useful example is the blood oxymeter. It’s a clever device that clips on your finger and measures the amount of haemoglobin in your blood that is saturated with oxygen. The finger clip device contains two light sources with two different wavelengths (normally 650 nanometres and 805 nanometres). This light passes harmlessly through the skin and is partly absorbed by the haemoglobin. The amount absorbed depends on whether the haemoglobin is saturated or desaturated with oxygen. By calculating the tiny differences in absorption at the two wavelengths the device’s computer can accurately work out the proportion of haemoglobin that’s oxygenated. As the processing is digital the computer can also do other useful things with the signals, like measuring the pulse rate.