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[Need help] breadboard of pre-amp circuit with BF245A is not working

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nvha

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We are trying to made an pre-amp circuit for microphone using BF245A. In simulation it worked well. But with breadboard we got problem, the output is absolute silent.

Here is our circuit


Our data: as measured, Vg=0V; Vs=~0,4V; Vd=~8,5V. With "Soundcard" oscilloscope could obverse the electret mic give ~10mV before C1.

Any suggest for this?! Is the BF245 died? We changed other BF245 but result is the same. Or it was bad biased? Here is Vg - Vs = -0.4V.
The electret need to bias?
Thanks.
 
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hi,
The electret microphone requires a biasing/operating supply voltage.
Connect a resistor from the electret to the Vdd, say a 4K7 or 10K.
 

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    nvha

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Eric, thanks.
We'll reply soon.
 

Eric, it works very well. Thanks!
 

Why do you use a FET? An electret mic already has a FET inside, that is what must be powered. Use a normal audio opamp as a preamp.
Edit: I guess you did not look at the datasheet for the VHF and UHF FET that is used at the input of high frequency radios. Yours is barely turned on.
Didn't you calculate the extremely low frequency that the 2uF input capacitor feeds to the 2M resistor? Earthquake frequencies down to 0.04Hz which is one cycle every 25 seconds. A capacitor value of only 0.004uF (4nF) will pass audio frequencies down to 20Hz.
 
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