capacitor explanation?

Now that I've thought about a bit, I'll back off on the statement about bypass caps not adding to stability. I now see where you are coming from.

I will stand by my statement that the term "coupling capacitor" is a misnomer. The purpose of the cap is to isolate the two stages from the stand point of DC. If all you wanted to do was couple two points together to pass an AC signal you would just use a piece of wire. There are such direct coupled circuits that don't use capacitors. If the output of one stage is at a different DC potential than the input of the next stage, then you use a cap to block the DC. Otherwise you would dipense with the cap and just wire them together. So they really should be called DC blocking capacitors. No big deal, just thought it might be of interest to some.

Hi Herman.

>>said above: “I will stand by my statement that the term "coupling capacitor" is a misnomer…. No big deal, just thought it might be of interest to some.”

Hey you are probably right and it’s of interest to me. I don't mean to argue. I like your posts and have learned from them (and the other posters too) and I'm no expert having worked with a few, mostly set circuits, over the past year or two.

I certainly agree that generally "the purpose of the cap is to isolate the two stages from the stand point of DC."

But usually I find in amp circuits you have a purpose to do one thing and inadvertently do three others (maybe it's just me). This is why I think design choices are almost always compromises. Despite their main purpose coupling caps can, among other things, effect grid circuits under certain circumstances and a stage's frequency response and contribute to noise….etc. and it seem to be the way power is transferred from stage to stage too.

ABall, the way you describe the emitter BIAS resistor and such it sound svery much like a tube amp.

ps, thanks Greg.

Cheers all
I remain,

If anyone is still following this thread:

My previous responses were based on my memory so I decided to pull out an old friend, Malvino's book on Electronic Principles that I taught about 15 years ago. Aball's mention of re prime brings back fond memories.

On CE amps with the emitter bias resistor totally bypassed, the AC input signal is developed completely across re prime (the AC resistance of the base-emitter junction). Since re prime is affected by temperature, the gain of the amp is temperature dependent. Not good. To solve this, a swamping resistor is added in series with the emitter bias resistor that is not bypassed by the cap.

Which brings me back to my original statement about bypass caps NOT adding to the stability, the one before I rescinded it. The correct response is that bypass caps on a BJT amp actually make them less stable with temperature changes.

Clueless, you are absolutely correct. It seems everything that is done to a circuit effects it in ways other than the one intended. Adding a bypass cap as above to increase gain and bandwidth also increases distortion and decreases input impedance. Coupling caps do indeed affect frequency response and can affect the bias, especially on high impedance input circuits such as tubes by holding a charge when it was not intended to.

I enjoy these discussions and don't take any differences of opinion as arguing (except maybe on posts like the one on the Dixie Chicks). Unlike politics, electronics is based on the science of physics so for the most part there is very little that can be argued. It just that it is complicated enough that a lot people, including myself, get confused and develop strange ideas about how things work. Hopefully by sharing we can all get a little smarter.

>>>"electronics is based on the science of physics so for the most part there is very little that can be argued."

That is a very controversial statement in these parts.
Ears are everything.(hehe) Numbers nothing.

Of all the subjective areas the effects of paralleling caps is certainly one of them! As I said in my prior post it is a bit like religion these days I think.

Taught electronics decades ago eh? That's good to know. I don't feel so stupid when I read some of your posts. I'm kinda new at it but I find electricity to be an interesting mystery of sorts. It's like reading a technical "who done it."

Cheers

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