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LDMOS bias circuit, what's the purpose of the NPN-transistor

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tyassin

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LDMOS bias circuit

Hello,

This is a bias circuit for the gate of an LDMOS transistor. I can tell you that S5 in the picture is LM7805 and S4 is BCP56 NPN-transistor.

Can sombody tell me what is the purpose of the NPN-transistor(marked with the red circle). i can not really see the purpose of it when the emitter is not connected??

Thank you
 

Re: LDMOS bias circuit, what's the purpose of the NPN-transi

similar circuit, although easier to read, in the attached. I am assuming it is a temperature compensation circuit. The proper gate voltage to the LDMOS fet wants to vary slightly over temperature. The 7805 if fixed voltage output (5 volts) over temperature, but if you float it slightly off of ground with a transitor circuit, you can get the overall gate supply to vary over temperature (presumably in the right direction).

IF there were bigger capacitors around the bipolar, I would have suspected some sort of "soft start" circuit, where the current in the LDMOS ramps up on power supply activation.

Added after 43 seconds:

data sheet with schematic
 

Re: LDMOS bias circuit, what's the purpose of the NPN-transi

Hi

Yes the circuit is a temperature stabilizer circuit. The circuit I attached had an error in the datasheet. The emitter should be grounded.

Thnak you
 

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