capacitor explanation?

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.

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.

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Which brings me back to my original statement about bypass caps NOT adding to the stability, the one before I rescinded it.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.

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.