Terms for Datasheet

[quantum1]

http://www.datasheetcatalog.com/datasheet/BC/BC548.shtml ( the datasheet for General Purpose Transistor) This site is cram pack with stuff I don't know. I am going through them and asking anything I don't know. All of these questions are on the same page.
1. Under "Features" there is "High Voltage" then goes Vceo. Is that the maximum voltage from the collector to emitter? Also, what is the "o" in Vceo, I am thinking it's voltage or something.
2. Next to "Maximum Rating" is "(Ta=25*C)". The T is obliviously temp., but what is a?
3. For the chart of max. ratings, there is a colomn donated to the Smybol. The fisrs three cells in the Smybol colomn has Vcbo Vceo and Vebo. What is the "o" in it?
4. I don't understand why the rating is neg. in the row Emitter Current.
5. In Junction Temp. what do they mean in Junction or what is the Junction?
6. The chart that holds the drawing of the transistor, under the millimeter colomm, some of the cells have MAX, what would that be?
7. Under the drawings, there is TO-92, I am assuming that it is for the package type.

 

[audioguru2]

Hi Quantum,
It is good that you are studying this datasheet and questioning its symbols.
1) The "o" in Vceo refers to its base being open, or not connected to anything (or perhaps with a resistor with a very high value connected from its base to its emitter). The transistor breaksdown (suddenly conducts) above its Vce rating (yes, voltage from collector to emitter) because its current gain amplifys the leakage current in its C-B junction.
2) "a" in "Ta=25*C" is the ambient (surrounding air) temp. At higher temps, leakage current increases.
3) In "Vcbo, Vceo and Vebo", the "o" refers to the unmentioned transistor's terminal being open.
4) This is an NPN transistor, so its emitter current is negative with respect to its C or B terminals.
5) The junction of a transistor that produces heat (power=V x I) during normal operation is its C-B diode junction, right on its chip. Your circuit must limit the voltage, current or both that may cause this junction to heat to a temp beyond its rating.
6) The "max" dimensions are the factory's guaranteed largest.
7) Yes, TO-92 is a standard package for a small, low-power transistor.

 

[quantum1]

Thanks for the answers. I really understand the Vcbo and other similiar stuff.
2nd page...
1. What is cut-off current?
2. How about current gain?
3. What's Hfe, I know that it's current gain smybol of course, but what about the individual letters.
4. What is saturation voltage?
5. I don't undrestand the collector ouput capacitance or why how what, I know nothing about it.
Thanks Audioguru.

 

[audioguru2]

Hi again, Quantum,
1) Cut-off current, Icbo, is the amount of leakage current in the C-B reverse-biased junction, with the emitter disconnected. This current is probably less at a lower voltage, and is much higher at higher temps.
Note that if you connect the emitter to ground as in a normal circuit, the Icb current will flow directly into the B-E junction and the current gain of the transistor will amplify it and probably turn-on when it shouldn't. Therefore you should use a resistor between the base and ground to shunt this current, or keep the collector voltage and temperature low.
2) Current gain is one of the most important spec's of a transistor!
HFE (DC current gain) is the multiplication of base current that occurs at the collector, so an HFE of 100 allows 1mA into the base to control 100mA at the collector. Hfe (AC current gain) is similar at low frequencies and reduces at high frequencies.
3) I don't remember what the "h-parameter" letters stand for. There are many more and I use only the current gain ones.
4) Saturation voltage is the voltage at the collector when the transistor is conducting as hard as it can. If the transistor is used as a switch, controlling a high current into a load, the saturation voltage causes the transistor to heat and require a heatsink, and the voltage (and power) is wasted at the transistor instead of going to the load.
5) The collector capacitance amount depends on the size (power-handling) of the transistor's chip. It also depends on voltage because the capacitance of all diodes changes when the voltage across them changes.
Big, high power chips have a high capacitance and can't operate at high frequencies because their capacitance is too high.

 

[quantum1]

Hi again, Quantum,
1) Cut-off current, Icbo, is the amount of leakage current in the C-B reverse-biased junction, with the emitter disconnected.

THanks again for well explainded answers. About that quote, I don't understand why in the world why and how you would put C- B in reverse bias, however, isn't C to B work like a diode and if you had that "diode" in reverse bias it would leak unwanted current, possibly ;D.
Now for the last page...
1. for the first graph, top left, don't understand why that have such a graph. Like what is the purpose for showing the C current against C-E voltage. It just seems abstract and I can't realate.
2. So even if the C-E voltage is at 0, there is still current in (or going through, about to go through or having the possibility of going through?) the collector. But I thought that currents mostly lag behnd by say 90 degrees.
I'll wait with the other confusing graphs. ???

 

[audioguru2]

Hi Quantum,
Of course a transistor's C-B junction is reverse-biased! The datasheets guarantee a certain maximum (very low) leakage current for it. That's why a transistor turns-off when you ground its base.

The collector (N) of an NPN transistor is normally at much higher positive voltage than its base (P). See, it's reversed-biased.

New stuff:
1) The graph of C current against Vce also most importantly shows the base current. It shows that a low Vce (like when you need the transistor to saturate) requires much more base current for a certain collector current than at a higher Vce. Look at the saturation voltage spec. It is rated with a base current that is 1/10th the collector current, in a transistor with a current gain of over 100 at a higher Vce.
2) The current in a transistor is like in any other conductor: in phase with the voltage.
In a capacitor (and maybe in a transistor that is operating at a frequency too high for it) the voltage lags the current by 90 degrees. A cap's current is immediate and its voltage changes slowly. In an inductor the current lags the voltage by 90 degrees.

[move]Thanks for feeding your questions a few at a time. Thanks for feeding your questions a few at a time.[/move]