# Electron flow due to impedance?

In the basic electronics lecture today not only I but whole class became confused by an explanation of our teacher that in high impedance value electrons movement become easier while in low impedance value electrons movement is congested. How could be that? I have always read the opposite in many texts.

• This probably needs some context. Is he talking about a particular semiconductor device? In the collector region of a transistor, when it is "off" the electron population can be low (and they move fast, attracted by a high collector voltage), but when it is saturated, Vc=0.2V or less, which exerts very low "pull" on a large population of electrons. Is this what he is talking about? – Brian Drummond Mar 26 '13 at 12:56
• we were studying transistor modelling. During that, discussion of input and output impedance came. – Ali Khan Mar 26 '13 at 15:08
• Then please make it as most clear as possible, because it's very hard to understand the context in which that statement might hold. Anyway, why didn't you ask the professor directly? – clabacchio Mar 26 '13 at 15:45
• We ask him many times but his ansewer was the same. He gave us the analogy of water and pipe. suppose a pipe is full of water and if you narrow it down then pressure will increase but its flow will break due to narrowness. Similarly when you apply a small signal if impedance is less then it will die down before reaching preamplifier stage.That's it. My question is here not impedance and resistance are taken in same context?. then how less resistance/impedance will cause signal trouble to move to next stage while high resistance/impedance not? – Ali Khan Mar 26 '13 at 17:10
• It is mainly about impedance. I have the concept that when value of impedance increases then resistance to electrons must also increase and when it's value decreases then electrons must move easily through that impedance part. Is it this case or otherwise? clear this confusion. – Ali Khan Mar 26 '13 at 17:41

The analogy is badly flawed in several ways, one of which is that water does not really "congest" (let alone that it does so more in a wider pipe, or that a narrower one represents lower impedance).

In a low impedance device, electron flow is less constricted. This makes the device difficult to drive, and in particular to develop a voltage across it.

If you push against air, it has less impedance than water. This is why you can move through water by moving your arms, but not through air (even if you simulate the support of floating by standing on a skateboard). Likewise, a ship propeller could hardly work on an airplane.

If you want to develop pressure in a pipe, you will have the best luck if the other end of the pipe is closed, or has only a pinhole leak (this is like a very high impedance, analogous to an insulated gate on a field-effect transistor). The next best thing would be a pipe which is open, but long and narrow. The complete opposite is wide-diameter pipe which requires you to produce a huge flow of water in order to develop significant backpressure.

In other words, when we are propagating electronic signals represented by a voltage, the "congestion" is desirable. It is easier escape of electrons (lower impedance) which makes the loading harder. Voltage is transmitted with the greatest ease when impedance is as high as possible (if we overlook issues like stray capacitances and certain kinds of noise).

If we want to transmit power, then impedance matching becomes important: not too low an impedance and not too high and impedance: just right. The fluid analogy is that it's next to impossible to paddle a canoe in thin air, but also very hard to paddle one through a pond of molasses. The in-between impedance of water is more efficient than either.

Fluid analogies don't really apply all that well to voltage signal propagation in small-signal electronics because fluid pipe networks are used for moving fluid, rather than keeping fluid flow small while transmitting information via pressure. The analogies require students to imagine unusual ways of using pipes, which is a bit of a failure, since analogies should be comparisons to something familiar.

Maybe if you consider an electron's movment in a vacuum - there are no constraints on movement until they bump into each other whereas in a conductor they much more easily bump into atoms because they are bigger. It's a bit simplistic my explanation of course.

I'm not saying this is what your teacher meant; I'm trying to figure it out.

• He was saying if value of impedance is greater then impedance itself is less and if it has less value then it is greater. This is the confusion. Can you give me physical examples of it. – Ali Khan Mar 27 '13 at 1:47
• @AliKhan OK dude - it all sounds a backwards way of looking at things. I can't give a physical example - it's just me trying to make sense of what teacher said – Andy aka Mar 27 '13 at 7:56
• There certainly are constraints on electron movement in a vacuum : a negative electric field in their path can be used to inhibit their motion. This used to be quite important, once upon a time... – Brian Drummond Mar 29 '13 at 18:45
• @BrianDrummond ahhh the good ole days - I hear ya loud n clear – Andy aka Mar 29 '13 at 19:09