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Transceiver TRX switching, how is it done?

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neazoi

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Hi, attached is a small qrp CW transceiver schematic.
I do not understand how TX can be activated this way.
When key is connected do GND no power flows to the buffer amplifier according to this schematic, so how is the TX activated?
 

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BigBoss

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When you tie the switch to gnd, Q2 turns off, Q3 turns on and buffer amplifier is supplied through this transistor.
 
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neazoi

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When you tie the switch to gnd, Q2 turns off, Q3 turns on and buffer amplifier is supplied through this transistor.

Why is Q3 on? it's base is tied to GND through a resistor, isn't it?
 

Dan Mills

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Q3 is a PNP transistor.....

Regards, Dan.
 
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Dan Mills

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Sinking (conventional) current from the base switches on a PNP so in essence yes.

Regards, Dan.
 
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neazoi

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Sinking (conventional) current from the base switches on a PNP so in essence yes.

Regards, Dan.

Since I would like to build it European/US style I would like to know suitable replacements for the three types of BJTs, are 2n2222, 2n2907 and BD139 suitable types?
 

betwixt

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BC237 and BC227 will work just as well, they only carry a small current. BD139 should be OK as the PA transistor. Be careful that you find a compatible FET though. If the FET isn't available, you can use almost any analog switch (DG..., CD4016, CD4066 etc.)

Brian.
 
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neazoi

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BC237 and BC227 will work just as well, they only carry a small current. BD139 should be OK as the PA transistor. Be careful that you find a compatible FET though. If the FET isn't available, you can use almost any analog switch (DG..., CD4016, CD4066 etc.)
Brian.

I thought the FET, since it is used as a switch, can be mpf102/j310/2n7000 or similar.
Have you got any idea of a European alternative for this FET? It will save me trouble.
Thanks a lot!
 

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