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In Reply to: Gain Structure posted by Mr_Steady on December 17, 2014 at 16:44:34:
"Gain structure is the concept of relative amounts of signal or voltage gain from one stage of a chain to another. If you have three devices capable of voltage gain (lets say each can create a gain of 10%), and you need an overall gain of 17%, the gain structure would be how you allocate the gain between the stages. If you set up the first stage to generate a gain of 10%, the second stage to create a gain of 6% and the third to create a gain of 1%, that would be one method of structuring the gain. Alternatively, you could set it up so that the first stage has a gain of 5%, the second has a gain of 5%, and the third has a gain of 7%. You have change the gain structure but not the overall amount of gain."This is a rather misleading paragraph. The total gain of multiple stages of amplification is equal to the product of the gain factors for each individual stage. If one really wants to express the gain of each stage in percentage terms, as in the paragraph above, then the total gain is not given by adding the percentages together. It becomes a reasonable approximation if the percentages are small (as in the examples above), but even there, the errors are non-trivial. The overall gain in each of the two cases above is closer to 18% than 17%. But the whole example seems rather bizarre; who would seriously build a three-stage amplifier where the stages had gain factors like 1.05, 1.05 and 1.07? (i.e. about 0.4dB, 0.4dB and 0.6dB.)
Of course, if one uses dB rather than talking of multiplicative gain factors then the process of combining the effects of multiple stages is indeed additive, since the dB scale involves taking logarithms. In fact, the paragraph above would make much more sense if one assumed that the writer had intended those numbers to be dB of gain, rather than percentages. An overall gain of 17dB is something worth talking about, whereas the writer's three-stage amplifier achieving a grand total of about 1.4dB of gain is not!
Having said all this, when Tubewrangler uses the term "gain structure" it appears that the word "structure" is superfluous. Since he quotes just a single figure (or range of figures, like 18-20 dB), he appears to mean simply the overall voltage gain of the amplifier. It seems to me the word "structure" would be better omitted in what he is saying.
Chris
Edits: 12/18/14
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Follow Ups
- RE: Gain Structure - cpotl 02:08:58 12/18/14 (0)
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