Sunday, October 09, 2011

Electronics Lessons: The Amplifier

OK, we looked at a transistor amplifier, and will go on to look at operational amplifier circuits.

One thing that I didn't do was measure how "good" these amplifiers were. I think that everyone has had a bad experience with an amplifier at one time of another, something that just sounded weird, and not quite right. but perhaps you didn't even notice until you heard something better, then went back and listened to the same amplifier again.

So lets look at amplifiers on some more detail, explaining some of the words used.
-this is just a quick guide to some terms that you will find in amplifiers, their use and design.

Distortion (clipping)
The easiest way to explain clipping distortion is to look at what causes it.

Imagine that you have an amplifier that have a positive supply voltage of +5V, and a negative supply voltage of -5V.
The amplifier has an input signal applied to it of +/- 1V peak to peak and a gain of 2.

The input is a lovely sine wave, rising an falling between 1 and -1 volts,
The output wave is a similarly lovely since wave between 2 and -2 volts.

Now if you increase the gain to six.
you would expect the output wave to now rise and fall between +6 and -6 volts in the same perfect sine wave, except, it can't the supply voltage to the amplifier circuit is only 5v so instead the wave goes up to 5v, then there is a plateau where the top of the sine wave should be, before it levels out and falls to -5v and plateaus again where is should be falling below this.

You are effectively over driving the amplifier, trying to force it to amplify beyond it's means. the top and bottom of the signal gets clipped off.

headroom is a weird one, it's not something that you're ever going to see on a data sheet, because it's just a made up figure, it's something that you either have, or don't have, but I include it because I feel that it's important.

Headroom is a measure of how much power you have left in the amplifier. For example: say you need at least 100W of sound to fill a particular room with sound, that would suggest that you need to buy a 100W amplifier right?

Well sort of yes, and sort of no, if you need the full 100W of the amplifier, then you'll be turning the amplifier up full driving it to it's limits (and possibly beyond and possibly clipping the signal) then you've not specified a big enough amplifier. Sure you fill the room with sound, but what about the quality of that sound?

If you need a 100W amplifier to fill a room with sound, you're better suited to buying a 200W amplifier, in that way you only need to turn it up half way to get your 100W of power, the amplifier is operating well within it's limits, you won't be driving components or circuits to their limits and thus will get a cleaner sound.

Frequency Response.
The frequency response of an amplifier shows how well it responds to particular frequencies, If you look back to the floor standing speaker project the frequency response of speakers was discussed in reasonable detail, with amplifiers frequency response tends to be less determined by mechanical considerations, (as with speakers, and how much mass/air/distance can be travelled, and more by electrical characteristics of the components used, for example a capacitor has reactance, (right now you can think of that as resistance that varies with frequency. inductors also have a resistance that varies with frequency. This means that certain ranges signals will pass through the components easier than other signals, (with a different frequency range), and the amplifier may have superior gain at a given frequency range, or indeed an entirely reject other frequency ranges.

This differing frequency range in amplifiers is why, (for example) guitar amplifiers, (higher frequencies) are different to bass guitar amplifiers, (lower frequencies), which are again different from keyboard amplifiers, (which need a much broader frequency response.

The bandwith of am amplifier describes a frequency range at which a given amount of gain can be produced, outside of the bandwidth of an amplifier gain drops off, (both above and below the frequency range). Bandwidth and frequency response, whilst different, may be used interchangeably in marketing literature

Total Harmonic Distortion (THD)
Total harmonic distortion, the absolute bane of anybody shopping for an amplifiers life.
kind of important, kind of a rubbish figure, it says so much and yet not nearly enough.

The basic premise is:
when you alter a signal, (say in an amplifier), each component may distort the signal slightly, and those distortions are usually in the form of adding harmonic compnents to the wave.

What this means is that for a signal, Ahz, there may be distortions at Bhz, Chz and Dhz, (where BCand D frequencies are harmonics of A).

It's possibly worth knowing that a pure sign wave plus decaying harmonics of it's fundamentals eventually sums to a square wave - but I'll explain that one another time) -just trust me, a sine wave is like a pure sweet whistle, a square wave is like a buzzer. -so you can see how adding harmonics to a signal, (and distorting that signal) can make it sound bad?

Anyway... the Total harmonic distortion is described as:
the power of all the added harmonic wave forms, divided by the power of the original source wave form (after amplification).

so if we have a output, amplified wave form A at 90W
and the harmonics at B - 5W, C = 3W, D = 2W

we have 5+3+2 =10 / 90
Therefore THD = 0.111
As you can see, what we're saying here is that you put a signal into your 100W amplifier, of the signal that comes out 10watts of it is just distortion. (whether that distortion sounds good or not is anyone's guess).

Further compounding the problem of THD figures is what type of distortion is added?
Crossover distortion adds harmonics in the levels that are audiable, (and don't sound "good").
whilst clipping distortion, produces more of a square wave, (like an overdrive guitar pedal), yes adding distortion, but adding it in such a way that some may actually find it pleasing to listen to, - and most of the distortion may even be very high order harmonics that are not even audible!

(in short, THD is useful when combined with lots of other information, but at the same time totally useless on it's own!)

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