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Why the electrolyte is tied in this polarity in the circuit ?

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ccurtis

Well-Known Member
Why the polarity of the 1U clectrolyte in this circuit is this?Why can't be tied with the other polarity?
The right hand side of the capacitor is at 1.8 volts. The Left hand side of the capacitor is probably less than 1.8 volts due to the load the microphone presents. Therefore, the right hand side is probably more positive than the left hand side. The capacitor can be connected with the opposite polarity, but it will not function as it was designed to for long, as the dielectric film will deteriorate and possibly the foil plates will short circuit.
 

ccurtis

Well-Known Member
How to calculate the voltage value 1.8V? Why is 1.8V?
Assume DC current gain (Hfe)=140 for the 2N3904.

Base current (Ib)= (Vcc-.7)/(47000+(140)(330)=25µA

Collector current (Ic)= Ib(Hfe)=3.4 mA

Emitter voltage (Ve)=Ic(330)=1.122 V

Base voltage (Vb)= Ve+.7= 1.8 V
 

kchriste

New Member
Forum Supporter
It could, theoretically, need to be reversed depending on the beta of the transistor and the type of mic. You could build the circuit without installing the capacitor, power it up, and measure the DC voltage of where the cap would be. Then install the cap with the correct polarity.
 
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audioguru

Well-Known Member
Most Helpful Member
The 1uF capacitor is so big that it produces a cutoff frequency of 6Hz which is stupid.
Use a 330nF (0.33uF) film capacitor for 18Hz or 220nF (0.22uF) for 27Hz which does not have polarity.
 

Junker_eu

New Member
The 1uF capacitor is so big that it produces a cutoff frequency of 6Hz which is stupid.
Use a 330nF (0.33uF) film capacitor for 18Hz or 220nF (0.22uF) for 27Hz which does not have polarity.
I am a electronic new man.Can you explain how to get below corresponding frequency of the capacitor?

1uF ---6Hz
0.33uF--18Hz
0.22uF--27Hz
 

audioguru

Well-Known Member
Most Helpful Member
The cutoff frequency is when the resonse has dropped -3dB which is to 0.707 times the AC voltage. It is simply calculated by 1 divided by pi x R x C. The electret mic is about 3k ohms, the input impedance of the transistor is shown on its datasheet at the low current in this project and the 47k bias resistor is in parallel witrh the input impedance of the transistor.
 
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