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I'm having an argument with a classmate and we can't agree.

He tells me that it's not possible for the amplifier of a radio station to amplify the input signal "instantaneously" (at the speed of light), because they work with very big powers (tens of kilowatts) and you can't accelerate the electrons that quickly.

I insist that since it's an analog device, it will work at speed of light, and that you don't need to accelerate the individual electrons, but only the electric/magnetic fields.

So which one of us is right in this particular case? Is there a delay in amplifying a signal for a high powered radio station? The radio station part is important. He agrees for example that a typical home audio amplifier is instantaneously.

It would be great if you could provide a reference link for an answer (if possible). Wikipedia would be fine. But don't waste your time searching for one.

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It really depends on what you're talking about.

The signal does not travel at the speed of light in the cables connecting to the antenna. Cable propagation speeds are often around 2/3 the speed of light, for instance.

It doesn't travel at the speed of light through an amplifier, either. Any filtering incurs a small delay, for instance, which is why filters are implemented using delay lines in the digital realm. (It's not instantaneous through a home audio amplifier, either, so you're both wrong.) :D

After it gets out of the antenna it should travel at the speed of light in air, which is almost c, and I don't know of any reason why this would vary with the amount of energy. The sun puts out a lot more electromagnetic energy than a radio tower, and it still travels at c through space.

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  • \$\begingroup\$ High power RF amps are still valve based, so there would also be a delay from receiving an input signal at the gate to seeing a change in the power output at the electrode based on the speed the electrons are travelling through the vacuum between them. \$\endgroup\$ – Pete Kirkham Oct 17 '10 at 9:51
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    \$\begingroup\$ Hmm, really? The speed at which electrons move is not usually relevant to the speed at which signals move. \$\endgroup\$ – endolith Oct 17 '10 at 20:58
  • \$\begingroup\$ @endolith, on many amplifiers, not vacuum tubes to my knowledge, but ones that take advantage of minority and majority carries there is a transition time for the carriers to travel. In a vacuum tube like a tetrode that is not the case. \$\endgroup\$ – Kortuk Sep 25 '11 at 21:04
  • \$\begingroup\$ @Kortuk: But does that have an effect on the signal? The signal is carried by electric field waves, not by the charge carriers themselves. \$\endgroup\$ – endolith Sep 25 '11 at 22:28
  • \$\begingroup\$ @endolith, forgive me if I misunderstood you. I was referring to the fact that they do have a measurable affect on the output. After every on period when you reverse bias a diode it stays on for just one moment while the carriers dissipate. \$\endgroup\$ – Kortuk Sep 25 '11 at 22:57
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you can't accelerate the electrons that quickly.

I think the problem lies in a misconception. The electrons don't move at the speed of light, in fact if you could 'tag' an individual electron coming into a wire and then sense when it leaves you could measure it with a stop watch in a reasonable length of cable.

The effect of the electron, or in other words the wave that is generated when an electron is pushed into a conductor, can be sensed almost at the speed of light on the other end of the conductor, but the individual electron you pushed onto the cable will not appear there for some time based on current, voltage, etc.

So the amplifier does not accelerate electrons to anywhere near the speed of light. It induces waves in the electrons in the cables, or amplifier, which are sensed by the semiconductors which induce waves in other cables and other semiconductors.

There is some inherent delay with every amplifier, but it's so small as to be unnoticeable by human ears.

Note that if amplifier introduced significant delay, then BBC broadcasts on NPR stations in the US would be delayed much more than the few hundred mS it already is.

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    \$\begingroup\$ The electrons will never appear at the other end, since it's AC and they're only moving back and forth. \$\endgroup\$ – endolith Mar 22 '10 at 19:13
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    \$\begingroup\$ And a stopwatch isn't really necessary. "For example, for a copper wire of radius 1 mm carrying a steady current of 10 amps, the drift velocity is only about 0.024 cm/sec". So it would take about an hour for an electron to travel one meter. \$\endgroup\$ – endolith Mar 22 '10 at 19:25
  • \$\begingroup\$ @endolith - I thought it was very slow, I just couldn't recall from classes so long ago. In this case it's AC, so for even short wires it won't appear, but in tiny silicon amplifiers in might. \$\endgroup\$ – Adam Davis Mar 22 '10 at 20:43
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    \$\begingroup\$ Electrons moving faster than the speed of light in a medium cause a cool blue glow \$\endgroup\$ – Nick T Dec 1 '10 at 20:30
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The signal may travel fast but the electrons do not. Google electron drift velocity. Speeds are measured in cm per sec or perhaps cm per hour. Here is one pretty good hit http://www.eskimo.com/~billb/miscon/speed.html

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  • \$\begingroup\$ +1, that article made me think more clearly about this subject. \$\endgroup\$ – J. Polfer Mar 22 '10 at 22:09
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Actually you do need to move the electrons around in the active component of the amplifier, for example the junction of a bipolar transistor, as the amplifying effect depends on that. The junction is small, but you still get a delay in the pico- to nanoseconds range.

Along wires the signal runs at the speed of light in that medium, which is somewhat lower than the speed of light in vacuum.

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I think the answer depends on the media and not the amplitude of the output signal; the amplifiers, in this case, and not the signal.

Electromagnetic waves (like the signals described) travel at the speed of:

(speed_of_light) / sqrt(permittivity_of_material * permeability_of_material)

The speed of the signal depends on the permittivity (electric attribute) and the permeability (magnetic attribute) of the media, not attributes of the signal itself (it's amplitude, frequency, phase shift). So it depends on the amplifiers themselves, and the differences in permittivity and/or permeability between the two, but not the amplitude of the output signal.

Your reasoning is better than your friend's in this case.


Source: http://wiki.answers.com/Q/Do_all_electromagnetic_waves_travel_at_the_same_rate

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To debunk your friends argument, start with a single low-power amplifier. He agrees that it works fast. Now take 1000 of these amplifiers and connect their outputs together. Obviously each one will work just as fast as the single one, so thogether they still work just as fast.

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  • \$\begingroup\$ The premise is mistaken; "low-power" amplifiers are by no means "instantaneous" either. \$\endgroup\$ – Chris Stratton Dec 19 '11 at 3:28
  • \$\begingroup\$ Re-read the first sentence of the question. \$\endgroup\$ – Wouter van Ooijen Dec 19 '11 at 9:24
  • \$\begingroup\$ Both parties to the debate are deeply mistaken - "He agrees for example that a typical home audio amplifier is instantaneously." - indicates that neither one understands what they are talking about. \$\endgroup\$ – Chris Stratton Dec 19 '11 at 15:27
  • \$\begingroup\$ True, but the "because they work with very big powers" argument can still be refuted by a simple reasoning - no deep electronics knowledge needed. \$\endgroup\$ – Wouter van Ooijen Dec 22 '11 at 11:55
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There seems to be this fixation on how fast electrons or signals move in wire. That does represent a fundamental lower limit on propagation delay thru a amplifier, but that is swamped by other delays in most amplifiers. The individual active components of the amplifier will have a greater delay, and then there will be delay associated with the bandwidth of the amplifier. Usually there are deliberate bandwidth limiters in the path that represent the largest input to output delay.

The reason for the deliberate bandwidth limiters is to make it predictable. Individual transistors or other active devices can vary significantly. Device are chosen to still operate well up to the intended upper frequency or bandwidth. The bandwidth or frequency limiters then guarantee that the active devices are only presented with frequencies they can handle. If you give them frequencies outside that range, all kinds of undesirable non-linear effects can occur.

A radio transmitter in particular has very carefully tuned and usually sharp cutoff bandwidth limiting on its modulated signal. There are practical reasons for this, but also legal reasons. The spectrum of a transmitted signal depends in part on the bandwidth of the modulation signal, and there are legal requirements as to how wide that bandwith may be. If the modulated signal weren't bandwidth limited in the transmitter, then the radiated signal would spill from the assigned band to a band assigned to another station, which of course is not allowed.

So the signal path from the input of a radio transmitter to the broadcasted signal always has some delay for various reasons.

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No radio station amplifiers and transmitters work at the speed of sounds, hence you hear the radio at the other end, whereas television amplifiers and transmitters work at the speed of light, as you can see the pictures at the other end .....

(sorry couldn't help but add my dry humor as the question has already been answered correctly)

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  • \$\begingroup\$ I don't want to encourage these answers with an upvote, but I liked this, so mental +1 for you. Probably better suited as a comment though :) \$\endgroup\$ – Bryan Boettcher Aug 24 '12 at 14:59

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