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 |
Aball's explanation of coupling capacitors is good. Coupling is actually a misnomer since they are actually in the circuit to block DC as he points out. Output caps are also used to block DC but the part about output caps lowering impedance is wrong. Every cap has some impedance so it will actually increase the ouput impedance. However, as he points out, the trick is to make it large enough so the impedance is low enough compared to the load impedance that it doesn't have much of an effect. I'm also not so sure about the DC stabilizing effects of bypass caps since they are open to DC and are not part of the DC bias. The emmitter resistor is part of the DC circuit whether you have a bypass cap or not. |
Actually, "coupling" is a proper description. The capacitor couples two circuits together by the common capacitive reactance of both circuits. The capacitor provides current at the rate to keep the voltage across it constant, so a change in voltage results in a change of current. The voltage change of the input signal causes a current change in the coupled circuit - except for DC, which is blocked. Output capacitors also can act as DC blocks but they can also be used for filtering. Output caps in some tube circuits bypass harmonic frequencies to ground, before they reach the xfmr. Bypass caps are used on the cathode of a tube to help with self-biasing the circuit. The voltage across a cathode resistor will vary with the signal voltage across the grid. As the voltage across the resistor varies, so does that of the grid and the bias. If the cathode resistor is bypassed with a capacitor, the voltage across the resistor remains constant which keeps the bias voltage constant. |
Hi mejames. Thats a big question. When I first read your question I didnt bother because there is so much to say. I'll add my 2cents to hte good stuff above. (Now its filling time during a short bout of insomnia.) Somebody could ( and has) write a small book about it. Ive included a number of sites that I think are good because its easier than reinventing the wheel. 1) One of the better online glossaries of tech terms is at Aikens site. It includes bypass and coupling caps and is often helpful if you are looking for tech answers (at least in tube circuits). http://www.aikenamps.com/AmpTerms.html 2) A short article about bypass caps. http://www.seattlerobotics.org/encoder/jun97/basics.html It defines them as follows: Bypass capacitor: A capacitor employed to conduct an alternating current around a component or group of components. Often the AC is removed from an AC/DC mixture, the DC being free to pass through the bypassed component. This is just what you want in a power supply. You want dc current. The ac is rectified to lumpy dc and then caps are used for more filtering and/or bypassing the remaining ac to ground away from where you are going to need the dc. When you think about using caps in this way it helps to remember that caps in parallel add up. This just the opposite of resistors and inductors. Read Adding components in series and parallel here: http://www.aikenamps.com/TI_Aiken_int.htm 3) You probably already understand that it is the nature of caps to allow higher Frequecy to flow and to block lower F. Therefore, if you put a cap directly in front of something (another stage or a driver), that is in series, it will block the flow of lows from getting there. This is how coupling caps are used. On the other hand, if you make a separate parallel path (like a fork in the road) in the circuit and put a cap in it, (that is use it to bypass) the High F will take that route and the lows (including direct current which you can think of as infinitely low frequency) will go the other way. This is how caps in power supplies and tube bias circuits are used. Here is a good site on the basics of capacitors. http://my.execpc.com/~endlr/index.html You might wonder. Why not use just one cap. Why send the current through 2 or three caps in parallel? One reason: In theory all caps become inductive as frequency rises.(that's right, caps start to act like inductors). You want the caps to be a perfect short at all frequencies. Inductance, of course, increases with frequency. If they are paralleled they can more resemble a perfect ideal cap (less inductance) and deal with high frequencies better. There are other reasons too. This has become almost a religious topic among diyers. Many folk mix and match many combinations in various circuits and report better/worse sound and Im not certain there is anything definite you can point to. There are certainly differences in caps that would lead to different performance and people mix and match different types and sizes. If you read the article directly above it gives you an idea of different designs in caps. Here is the old famous article that started lots of folks looking to tweak sound with different caps. Young & Marsh, Picking Capacitors: http://www.capacitors.com/pickcap/pickcap.htm 4) Above by GS >>Actually, "coupling" is a proper description. The capacitor couples two circuits together by the common capacitive reactance of both circuits. I agree here for the most part. I think coupling is a helpful and accurate description of what is going on. You are transferring power from one stage to another. It is very helpful to keep that in mind and coupling gives that a name. I think I would use impedance instead of common capacitive reactance though because impedance includes the resistances of both circuits and the R is a factor too. (the time constant of the charging and discharging circuits, for example, is R times C) Maybe I just misunderstand GS as people use different terms. 5) Bypass caps are used in various places and have different functions. People spend a lot of time talking about bypass caps in power supplies. There are other kinds of bypass caps. As noted above by GS and Aball. If you have tubes most voltage gain circuits, for example, have a bypass cap in the bias circuit of the tube ( from cathode to ground). It decreases the gain and keeps the bias voltage at a more constant level. It more or less provides a path (bypass) for the ac signal to ground. It gives the signal a different path than through the cathode biasing resistor. (If the signal current goes through the bias resistor it will cause a voltage drop in the resistor. This voltage change will cause the bias of the amp to change because the bias is set by this resistor). If you want a visual and a little more on this look at the second picture here. The cap labeled Ck is a bypass cap. It's purpose is explained in the texthttp://www.tpub.com/neets/book6/20h.htm I tried to find a nice picture of caps used in a Power supply but I think Im getting to tired. Zzzzzz., I remain |
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 |
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 its 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 |
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 |
