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Electret mic question

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obrien135

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Hello,


I biased an electret mic with 10Kohms and I don't get much modulation on the transmitter it is connected to. The load impedance is 50Kohms. Should I increase the bias resister on the drain of the mic's fet to a 50Kohm resister, or would that be ridiculously high? Would it be better to put a 10Kohm resiter across the loaad, to match the impedances?

George
 

10k is fine, there is no need to try to match it to the 50k load (its more of a voltage sensitive thing than a desire to transfer most power). However it sounds like you just need a bit more gain. Unfortunately that may entail a preamp stage (it could be a simple single-transistor job).
What is the 50k load? Is that an amplifier stage? In which case, you could just tweak that stage.
 
Thank you. The load is an LM386 stage with gain of 200. Would it help to lower the output resister a little, perhaps?
 
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Hm, that should certainly be enough to modulate a VCO. What does the circuit look like? Is it an oscillator+varicap or something?
 

PICT0014c.JPG

Above is the oscillator. It has a 10K resister applied to the input , through which the output of the LM386 is applied. It is a standard LM386 circuit with the 10uF cap between pins 1 and 4 and the filter on the output. The LM386 has a 50kohm input impedance. It is driven by the mic.

George

- - - Updated - - -

Do you think I should lower the drain resister of the mic's fet (the ten Kohm connected to mic's output). Could it br that the fet is an cutoff for part of the signal cycle?

George
 

Where are you defining the input to be on the oscillator? You might want to increase the value of the 10k resistor between base and emitter, to make it a bit more 'drivable', maybe increase the 22k resistor too. See here for a similar cct.

EDIT: yes, I tend to use a lower value for the electret, maybe 2.2k for a 12V supply. It should be ok with 10k though, but worth reducing slightly.
 

I meant to explain that. I used the left side of the capacitor on the base of the transister as the input. Thank you.
 

I would try a different point in the oscillator topology. Maybe use another capacitor (100n or something) and stick one end to the base, and the other end to your LM386 output, see if that improves things.
 
I tried your suggestion and it somehow killed the oscillator. It didn't destroy it, it just wouldn't osclilate when hooked up that way. I also tried putting a 2Kohm resister in place of the 10K as the output resister on the mic, but now I don't get any modulation. Could I have damaged the fet inside the mic with too much current? It was connected to 13.8VDC.

George
 
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