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PNP and NPN replacements


walid
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I an not so familiar with PNP type transistor.
when I see a PNP tran. in any circuit like one that shown below that using a PNP, I immediately think to replace - at least in mind- with NPN.
If I want to do so I must invert the power supply. is this what must to do or there are many other things?
thanks
Another question about the circuit below:
I see a hartly osc. before many times and noticed that the tank circuit conected-always- between Vcc and collector and not base and Vcc. What the difference?
thanks

post-2833-14279142465864_thumb.jpg

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Yes and length of antenna is inversely proportional to frequency of the input signal.Right?
If i feed an antenna with very high frequency say in GHz an ntenna of few mm may be enough but if the frequency is audio say 20Hz height is in kms.
Then why dont we use the never ending upper limit of the spectrum for communication purposes?
like in GHz.i guess FM range is only a few MHz.plz reply

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length of antenna is inversely proportional to frequency of the input signal. Right?

Correct.

why dont we use the never ending upper limit of the spectrum for communication purposes?
like in GHz.

We are using higher frequencies now. New wireless phones for homes operate at about 12.xGHz. My old one operates at 900MHz which is high enough for me.
NASA uses some extremely high frequencies.
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Then why dont we use the never ending upper limit of the spectrum for communication purposes?

If you're talking about RF the spectrum isn't never ending, you get to about 300GHz and the air becomes opaque to the radiation, and we get to an odd and largly unexplored regon called terahertz radiadion, it's in between far infrared and microwaves and is quite interesting.

http://en.wikipedia.org/wiki/Electromagnetic_spectrum
http://en.wikipedia.org/wiki/Terahertz_radiation
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