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Telephone related question #3


walid
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I have more than 10 telephones schematic diagrams one of them is a very simple {from: circuit ideas} see Fig.1 and I note that all of them use a small signal PNP transistor (T1 in Fig.1 and Fig.2).
1) Why always PNP and not NPN?
2) Why it always connected as a common-base configuration?
3) How T1 deals with the two audio sig. (Caller and user)?
Please I want a direct answer and it is good idea if give me a link about this subject to read then back to ask about what I could not understand.
Thank you and I respect you.

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1) Why always PNP and not NPN?
To make a positive supply voltage. An NPN would make a negative supply.


So its current into the zener diode can be modulated with the preamped microphone signal through VR2.


It supplies power for the zener and speaker amp from the caller and is modulated for the user. ;D
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Hi Mr. audioguru, When I say "Please I want a direct answers" I don't mean a brief answers, in short you must know that you and this site is the only source of information for me.
Now lets go....
Question 1: Why always PNP and not NPN?
Your answer: To make a positive supply voltage. An NPN would make a negative supply.

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