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C_B transistor impedance matching


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Hi Walid,
A common base transistor circuit uses the emitter as a low impedance input and the collector as a medium impedance output. The emitter has a low impedance because it is a forward-biased diode which has a small voltage change with a high current change. Also, the emitter resistor to ground is a low value because it carries collector and base current. ;D

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  • 2 weeks later...

Not much is ever said about impedance matching. Impedance matching implies that you are trying to get two impedances to be equal. And they do this for a reason. I think it is because in some circumstances you want a certain impedance, whether it exactly matches or not may not matter. Just as long as you don't have an open or short impedance when the circuit demands something else. I know that when the signal lines are long, you have reactance which can result in standing waves, voltage offsets at various points along the transmission line. To handle this situation, they figure out the length of line that will cause the inductance of the conductor and the capacitance of the dielectric to cancel each other at the correct frequency and appear resistive.

If your really interested in the impedance, read about transmission line theory. In there, you will notice that at every one half wavelength, there repeats a cycle. And you will see that one of the stipulations is that the load can be considered an open or a short. It looks very logical this way on paper. But then you will realize that the load is in reality neither an open or a short. And that is why you must use a Smith Chart. This is where you will find the secret ingredient, impedance.

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No No No you all not understand me at all, you all say absolute talk, the net is full of such talk, I ask obviousness question that:
I need a theoretical information about designing an impedance matching circuit using a common base transistor configuration. The transistor is PNP small signal type, collector is i/p and Emitter is o/p.
In other words, I want someone to tell me that take a type xxxxx PNP transistor and connect it like Fig.xx (attachment) then calculate resistor & capacitors values according to equation xx, OR simply advice me to go to link www.XXXXXX.com where I can find my answer.
If anyone can't do so, don't tire yourself, leave it to the suitable person.
I'm very sorry for saying this, but I must say this.
Thanks for all of you who try to help me.  ;D

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Hi Walid,
I did a search for Common Base on Google and got links to 7,900,000 articles that all say that the input is the emitter and the output is the collector.
Some articles say that the impedance at the emitter is simply hIE/hFE, but don't allow for the emitter resistor being in parallel.  As shown on the graph on a transistor's datasheet, the hIE decreases with increasing collector current. ;D


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