capacitor explanation?

Herman - Actually, re prime changes with emitter current which changes the gain which poses big distortion problems - moreso than temperature. You are correct: the swamping resistor stops this negative effect.

However, the little detail I should have made much clearer in my last attempt is this (and is true for most CE amps): A portion of the big emitter resistor is inserted between it and the transistor which I will call for more clarity, the little swamping resistor. Therefore, in a good design, there are actually 2 emitter resistors. The bypass caps are tied in between these 2 and NOT between them and re prime (otherwise, as you said, the instability in Av is back). The reason this is done is so that both swamping resistors still accomodate good DC stability AND the little emitter resistance still gives you good AC gain (since it is smaller) and AC stability.

Therefore, the bypass cap acts mostly to improve the gain which is desirable in BJTs to avoid low-signal transconductance nonlinearities - which is very beneficial of course. I suppose your book did not go into this more practical CE circuit. As with anything, a benefit comes at a price. In this case, good stabiltiy hurts the gain. Two emmitter resistances allow circuit optimization between the two (by means of the bypass cap). Moral of the story: if used correctly, bypass caps are VERY beneficial. Email me if you want to chat more about this but, otherwise, I rest my case ;). Arthur

Aball, your description of the 2 emitter resistors is exactly what I tried so poorly to explain. We are in complete agreement.

As far as variations in re prime, I also agree that current has the most profound effect. The approximate formula re = 25mV/Ie shows this very clearly. However, once you have established a stable Ie by use of whatever bias method you choose, a large factor in the variation of re prime is temperature.

I think we both understand and agree on what is going on. It is difficult to have a give and take on these topics unless you do it realtime.

Clueless, you are very right about the controversy over measurements. I think that the science is very good at predicting the behavior of amplifiers on the macro side of things. Things such as bandwidth and voltage gains are very predictable based on the design of the circuit. Where it falls short is on the micro side. So far, nobody has found a way to quantify how changes such as putting 2 caps in parallel or changing the type of resistor changes the sound of an amp. By all of the "standard" measurements the amplifiers behave exactly the same, yet somehow they are different. I don't think it is that the differences don't exist, it's just that we can't measure them with today's equipment or we just don't know what it is that we should measure.

Herman - Great minds think alike, but can't clearly get the point across either! LoL!

You are right, the micro-level interaction would make for very interesting research. A couple of my classmates are part of a research team doing parasitic research, mostly for high frequency converter applications but I will ask them if they know of anything done for low frequency situations. Characterization of electronics in the audio realm should exist but I have so far been unable to find anything very conclusive. I had forgotten about it till now so if I manage to find anything interesting at school, I will be sure to post it. It will surely give the science-doesn't-know-anything crowd something to get upset about but some of us will find it interesting nonetheless. Arthur