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2 odd resistors in pre amp circuit

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chubbspet

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Hi guys

I was wondering if someone would be so kind to explain to me the purpose of R1 and R3 in this circuit I built in MultiSim from a website. I get the functionality of the transistor really well, but I just can't seem to see the purpose of those 2 resistors. All the sites I have been looking at focus so much on the transistor that they neglet these resistors. i thought that we just need the voltage supplied from the microphone?

Thanks in advance
 

R3 is needed if the microphone is an electret type because electret mikes have a built-in FET that requires a power supply. R3 connects the power supply to the FET and also acts as its load resistor, similar to the function of R2 for Q1. If the mic is a dynamic (coil and magnet) type, then R3 not needed.

R1 provides base bias current for transistor Q1.
 
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XLV1 is an Electret Condenser Microphone. R3 is the charge resistor for the built in capacitor on the Gate of the FET (in side the Mic) which will discharge according to the voice intensity and will cause the FET to conduct. R2 is the load resistor for the Q1 collector and the R1 is the base bias resistor for Q1. Hope it helps.
 

Above explanations are of-course right.
But , also, R3,C and R1 create the band pass filter response.
Just try AC analysis and check the response.
 

Agreed, but here we are concerned to a bias voltage on top. If we do not use R3 the circuit will not work, as no signal will be produced by Microphone. Without giving a bias voltage to the FET inside the microphone no signal will be produced, hence talking here for the purpose of connecting the resistors.
 

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