capacitor explanation?

Herman - thanks for the compliments. Your uncertainty in your last note about the emitter resistance lies in the fact that the resistance's definition is ambiguous in my statements. There are actually 2 emitter resistances, one internal to the transistor, re prime for those interested (which is in the circuit regardless) and the emitter BIAS resistor which is the one being bypassed in my description. Bias stablitiy is indeed improved with the bypass cap since the emitter bias resistor if there for that very reason in the first place.

I am glad others were able to fill in for the cap roles in tube circuits as they are foreign to me. Makes for good info! Arthur

Conceptually, the coupling capacitors aid is separating each stage's design by blocking DC (since the caps are in series) and only allowing the AC portion of the signal. This aids in accurate transistor biasing and stage-impedance matching.

The output caps block DC before it gets to the speakers but also lowers the impedance at the output so that the load resistance is isolated and not mucked up by bias/transistor resistances so that proper bias design can be acheived. You want these to be fairly large so that the lowest audible frequencies will be unaffected.

The bypass caps are often used in common-emitter BJT designs so that the gain-reducing effects of the emitter resistance are elimiated during AC signal periods. It also aids in temperature stability of the DC bias by now including the emitter resistance (i.e., the cap is in parallel with the emitter resistor).

If you have anymore questions, don't hesistate to email me. Arthur

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

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