PNP and NPN replacements

[walid1]

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

View attachment 37824

 

[prateeksikka]

hi walid !
it is a golden rule in electronics that NPN and PNP transistors can be interchangably used in any electronics circuit by slight modification.
But in general we go for NPN transistors including me and u know the reason behind this is too simple-------
"Electrons have higher mobility than holes!!!"
8)

 

[audioguru2]

All Hartley oscillators feed the inverted signal from half the center-tapped coil to the transistor's base. This circuit has the other half of the coil as the collector's load through C3, since the RFC coil is a very high impedance (inductive reactance) at the operating frequency.

 

[prateeksikka]

hi audioguru!
i remember feeding oscillator outputs to antennas in many cases.
Are antennas inductive in nature  or capacitive generally?
i mean if i measure the impedance of an antenna what will it be?
R+jX what will be X?
thanks in advance.

 

[audioguru2]

The impedance of an antenna depends on its length and its percentage of the wavelength of the signal, doesn't it? I think it has a perfect resistance at a quarter and at a half of a wavelength and one is low and the other is high. In between, the impedance is capacitive or inductive.

 

[prateeksikka]

does impedance depend on the frequency of the signal by which it is fed?

 

[audioguru2]

The wavelength of the signal is determined by its frequency so the impedance of an antenna would also change unless you changed the length of the antenna to match.

 

[prateeksikka]

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

 

[audioguru2]

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.

 

[Alun]

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

 

[audioguru2]

You have to be a certified plumber to work on electronics that operates that high. ;D

 

[prateeksikka]

hi audioguru hi alun!
alun just said the spectrum is not never ending.what puts a limit to this spectrum is it the more affecting of noise at such high frequencies or distortion or something else?