Radio Principles

No6. Linear Amplifier

The job of the linear amplifier is to take the modulated carrier and raise it to more power without distorting the signal. On previous pages we have discussed class C amplifiers for generating radio carrier waves and class A amplifiers for magnifying the tiny voltages that come from an audio source such as a microphone, or (nowadays) an MP3 player. You may recall that though efficient, a class C amplifier will distort audio signals, and a class A amplifier is highly inefficient. For the linear amplifier passing the modulated carrier we cannot use class C, for as you get further from the nominal frequency in bandwidth more distortion occurs. We could use a class AB push-pull amplifier, in which two valves would conduct through slightly more than 180 degrees of the sine wave, but these are not quite linear and as well as introducing complexity we run the risk of crossover distortion and non-linear reproduction; overcoming the two will result in an expensive solution. On the other hand a class A will give us the linear reproduction but at the cost of constant power going through it, and so generating heat and the need to dissipate it, which may prove to be just as expensive! On the basis that we require the best linearity, let us assume our linear amp will be a class A power amplifier. We will be using valves baecause a) they cope better with high voltages than do transistors and b) they give a very linear reproduction indeed.