Electronics Engineering

Excellent speed regulation is characteristic of this design which lends itself well to speed control by variation of the field current.

Separately excited DC motors can race to dangerously high speeds (theoretically infinity) if current to the field coil is lost. Because of this, applications should include some form of field current protection as an unprotected motor could literally fly apart.

This is a lengthy answer. I tried to cover all the bases b/c I'm not sure how much you already know about semiconductors.

Background:

A semiconductor junction, such as that in a diode, is a connection between an N-type material and an P-type material. The N-type material has extra electrons and the P-type material has extra "holes," or a lack of electrons. Where the two materials meet, an area with neutral charge called a depletion region forms.

In a forward-biased diode, electrons trying to travel from N to P have no problem because of the abundance of extra electrons in the N-type material. The same thing happens to the positive charge carriers (holes) in the P-type material. The depletion region (.7 volts wide in a silicon diode) collapses and electrons exchange with holes through the junction -> A current flows.

In a reverse-biased diode, the depletion region (an insulator) actually grows because the unlike charges presented to each side of the junction pull the charge carriers (both electrons and holes) further away from the junction in the center. This prevents most of the current from flowing through the region.

Answer:

Because semiconductor technology is not perfect, a few charge carriers exist where they shouldn't. some electrons exist in the P-type material and some holes exist in the N-type material. These "minority carriers" are able to move through the depletion region of a reverse biased diode, and allow a small current to flow.

Adding heat or exposing the junction to light will increase the number of minority carriers, and so more reverse-bias current will flow. This is actually useful information- for instance, by measuring the reverse current through a photodiode, you can determine how much light is hitting the junction and use this to create an optical receiver or light sensor.

Keep your diodes cool,

anp

Practical Differences

Style & Structure

There are numerous stylistic and structural differences between broadcast and print journalism. As it is beyond the scope of this paper to discuss them all, I will refer to just three: editorial influence, structure, and pace.

Print journalism edits more than broadcast. Newspapers edit for clarity, fairness, and accuracy (Colgan, 2004). They also edit to ensure individual house style. In broadcast journalism however, exactly what you write is often exactly what airs, with little or no editing. The extensive editing process in print journalism allows more time for eloquence and prose. Conversely, the relative lack of editing in broadcast journalism warrants short, sharp, succinct language of a more conversational tone (Higgins, 2004).

Broadcast and print journalism also differ in structure. Print news stories use an inverted pyramid structure with the most important items (the facts of the story) reported in the first paragraph. Remaining facts are then presented in descending order of importance. Broadcast news stories on the other hand are broader (no pun intended). Important facts are still reported in the first paragraph, but broadcast news stories end decisively and do not trail off as do print news stories (Higgins, 2004).

Lastly, broadcast and print news stories differ in length and pace. The average radio news story is just 30 seconds long. The average television news story is one minute and 30 seconds long. Read at a pace of 180 words per minute these lengths equate to 90 and 270 words respectively for radio and television news stories. The average print new piece can vary greatly in length, but has been described as "12 snappy pars" (Ricketson, 2004). Ultimately, the newspaper journalist has little control over the pace at which the story is read; it is the reader who dictates the pace.

There is a practical answer as well as a technical answer to the question. See below for a precise technical comment on output impedances.

Amplifier manufacturers quote a minimum load resistance or impedance that should be used with an amplifier. Most good quality amplifiers will handle a 4 ohm load without any complaint. Most amplifiers are rated to deliver a certain amount of power into an 8 ohm load but this does not specify that 8 ohms is the minimum load that can be used. In this case, the amplifier may specify a power output into a 16 ohm load or that 16 ohms is the minimum recommended load to be used.

If the amplifier is rated to drive 16 ohm speakers, using 8 ohm speakers will tend to draw more current than with a 16 ohm speaker if all other factors are the same. This can lead to increased distortion or as power levels increase can result in damage to components. Normally, using a lower impedance speaker on an amplifier will work without any problems because manufacturers provide a margin for safety. It is wise to avoid using the amplifier at high output levels and even wiser to consult the manufacturer before doing it. Damage is very unlikely, but possible.

The technical details of amplifier impedances

There is no "16 ohm amplifier" at all on the market. All amplifiers have an output impedance less than 0.5 ohms.

The amplifier will have an output impedance of around 0.05 ohms. In hi-fi we have always impedance bridging. Zout << Zin. That means the output impedance of the amplifier is much less than the input impedance of the loud speaker.

The damping factor Df = Zin / Zout tells you what Zout is.

Zout = Zin/Df.

If the damping factor Df = 200 and the loudspeaker impedance is Zin = 8 ohms, the output impedance of the amplifier is Zout = 16 / 200 = 0.08 ohms.

See related links and look at "Voltage Bridging - Zout < Zin".