IAN LANG ELECTRONICS
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.
The drawing on the left shows a simplified version of a single ended triode (SET) working in class A. Like all Class A amplifiers it has a continual bias at the grid and thus a continual current at the plate. (The transistor equivalents would be base and collector for BJT, gate and drain for FET)
The voltage and therefore the current at the grid is being changed continually by the varying input, in our model from our modulated amplifier, and therefore so is the current flowing to the plate and by extension a varying voltage on the coil of the transformer attached in series. In turn this induces a varying voltage on the other coil of the transformer which transfers the same to the load; this latter could be the aerial or another amplifier in the chain. Either way we would have to match impedances. Chaining many together like this could give us a very high wattage output at the aerial, with little distortion of the intelligent part (i.e. the audio) of the signal. However this is not without problems of its own, as we shall discuss on the next page.
As a last comment, if we put a linear amplifier after the modulated amplifier, it is considered that the modulation, regardless of the power used in the modulation stage, is low-level.