capacitor explanation?


hello could someone explain the differences between the following types of capacitors: coupling capacitors, output capacitors and possibly explain the purpose of bypass capacitors in a amplifier circuit. Thanks
mejames
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.

That’s a big question. When I first read your question I didn’t bother because there is so much to say. I'll add my 2cents to hte good stuff above. (Now it’s filling time during a short bout of insomnia.) Somebody could ( and has) write a small book about it. I’ve included a number of sites that I think are good because it’s easier than reinventing the wheel.

1) One of the better online glossaries of tech terms is at Aiken’s 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 I’m 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 I’m getting to tired.

Zzzzzz.,
I remain