Problems of Amplification

Author: N.H. Crowhurst

To make up an amplifier we have to see that each tube, or stage of amplification, can handle the audio fluctuations expected of it. We saw earlier that there is a limit to the maximum voltages an amplifier can handle. This is particularly important towards the later (output) stages of an amplifier. (At each stage the audio voltages are larger than at the previous stage, so the biggest audio voltages are encountered near the output end.)

In an audio amplifier at each stage the audio voltages are larger than at the previous stage

There is also a limit to how small a voltage can be amplified. This is because of noise. In any circuit, even when there is not supposed to be any signal (audio), the natural electronic agitation going on in all matter causes a basic random fluctuation, called noise. According to physical theory, every molecule of matter is in a stage of agitation, the amount of which depends on its temperature. As we do not operate electronic equipment in a temperature of absolute zero ( - 273°C) we always have this agitation to contend with.

In every amplifier circuit, various kinds of noise set a lower limit to the audio voltages that can be successfully amplified. If the noise produces, say 2 microvolts (2 millionths of a volt), then audio voltages lower than this will get "lost" in the noise. In fact a good margin above the noise should be available for quality reproduction.

If, at a certain stage in an amplifier, the noise level is 2 microvolts and the maximum level before the amplifier distorts the waveform is 2 volts, the dynamic range or the ratio between maximum and minimum levels that can be handled is 1,000,000 to 1, or 120 decibels. (1,000,000 to 1 in voltage or current ratio is equivalent to 1,000,000,000,000 to 1 power ratio.)

Maximum and minimum levels of distortion and noise

Each stage of an amplifier will have its own maximum and minimum levels, fixed by distortion and noise, respectively. Each stage will have different limits. Also, the audio level corresponding to maximum and minimum through the whole amplifier will be different. For example, a three-stage amplifier may have successive voltage amplifications of 50, 10, and 10, making a total amplification of 5000. At each place from input of the first stage to output of the last stage, there will be a point at which distortion would start and a level of noise for that circuit. In short, each stage will have its own dynamic range, which may vary from stage to stage, but will be more than 100 db at each point.

But if we start at the lowest level, the noise level, of the first stage and amplify up (say it is 1 microvolt), the output will be a minimum of 5 millivolts (mv), although the noise level for that stage is only 50 microvolts OAV). Due to the amplification, each stage progressively gets a bigger margin above the noise level, for lowest audio amplified.

Now if we start at the maximum level for distortion at the output - say, 50 volts - and work backward, we find this needs an input of 10 mv, although the first stage will accept 300 mv before distortion. As we work back from the output, we find a bigger margin between the maximum audio signal in the amplifier as a whole and the maximum for that stage.

So, although each stage by itself has a dynamic range of more than 100 db (the lowest is 110), the whole amplifier is limited by the distortion of the output stage and the noise of the input stage. The ratio between lowest and highest audio in the amplifier as a whole is 10,000:1, or 80 db. In a very high gain amplifier, an overall dynamic range of 60 db would be good.