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# MIC coupling capacitor

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In Fig.MIC cap (part of FM Tx from http://hem.passagen.se/communication/bug_rf.html) below you can see the MIC connected to 100nF cap. I Know that 100nF value is not critical and I may use say 10uF, but to what level this value is not critical? What happen if i use a 4700uF or 1pF, please explain.
Thanks
WAKLID.

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The capacitor is being used as a DC blocking capacitor

I has an impedance equal to 1/(2pi*F*C)

If the capacitor is too small  it's impedance will be so high relative to the load (input of the amplifier) the signal will be greatly attenuated, if it's too big it will take a long time to charge up and some DC will pass also because large electolytic capacitors have high leakage current.

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If the capacitor is too small
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it is very good answer audioguru but still one point :
10uF is not so big value and can be used instead of 100 n is this true? And how you choose this cap in your FM Tx
thanks

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Hi Walid,
It is difficult to calculate the gain and input impedance of the mic preamp you posted because it has negative feedback applied to its input. The transistor has an input impedance of about 2k ohms and the negative feedback of about 2k ohms is in parallel with it. It is fed from a fairly high impedance of the mic's 10k ohm load/supply resistor in parallel with the FET in the mic that has an output impedance of about 5k ohms. So the total impedance of the circuit is about 4.3k ohms.
Therefore the 100nF coupling cap will have a low frequency cutoff of about 372 Hz.
Your 10uF cap will have a low frequency cutoff of only 3.7Hz.

The preamp in my FM transmitter doesn't have negative feedback applied to its input so its gain and input impedance are predictable and easy to calculate. The transistor's input impedance is boosted by the emitter resistor of 470 ohms and becomes about 100k ohms. Its bias resistors are in parallel with it and in parallel with each other and the total impedance in my circuit is about 23.5K.
Therefore I selected 330nF for its coupling cap for a cutoff frequency of 20.6Hz. Very good for music. ;D

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Hi Audioguru
You said: "It is difficult to calculate the gain and input impedance of the mic preamp you posted because it has negative feedback applied to its input. The transistor has an input impedance of about 2k ohms and the negative feedback  of about 2k ohms is in parallel with it."

(1) Are you mean by (The transistor has an input impedance of about 2k ohms) that Beta*re and you suppose that Beta=200 and re =10 ohm so (The transistor has an input impedance of about 2k ohms).
(2) Are you mean by (and the negative feedback of about 2k ohms is in parallel with it.) that the 2k //FB resistor, that is 2K // 100k = approx 2k which is the overall i/p impedance. or you mean 2k i/p Z and 2k fb resistor are in parallel = 1k ohm, if this how you convert 100K feedback resistor to only 2k ohm.
(3) As you continuing in your discussion you introduce the MIC impedance, why? You say that the 5K MIC // 10K resistance = 3.3k ok but this is not related to the transistor i/p Z if we talk about a high pass filter composed from 100n cap and this 3.3k.
(4) You finally add the 1k ohm result from (2k//2K) + 3.3k = 4.3k and said this is the i/p Z of transistor!!! I am never seeing this approach.
(5) You benefited me 100% in related the 100n cap to the i/p Z that makes with the cap a HPF and that you design its lower cutoff freq. = 20.6Hz to be better for music and the other FM Tx has a lower cutoff freq. = 372Hz which is the lower limit of human voice. this solves most of my main question about the mentioned coupling cap but introduce a new problem about Z calculations.
(6) I read in an electronic book (ELECTRONIC DEVICES AND CIRCUIT THEORY by ROBERT and LOUIS) that for the first stage transistor, Zi looking into the capacitor = re/[(1/Beta)+(Rc/RF)] AND Voltage gain Av = -Rc/re approximately but in some web page i read this important rule for Av in a collector feedback configuration: Av = Rfb/Rc

thanks

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Perhapps he hasn't answered because he doesn't know. ;D

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No Alun, I believe that AUDIOGURU is the king of this community and he can answer any electronic question. May be he buisy (worked)

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I am intermittently having great difficulty getting pages to complete opening on this site lately. I need to try 20 and 30 times. When it does complete loading a page so I can read it, when I push the quote or reply button then the same page loading problem happens and I must try over and over for the quote or reply page to appear.
Then I type a reply, hit the send button and nothing happens.
Very frustrating! :'(
I deleted my cookies and cleaned all temp internet files without any change.
Other sites work just fine.

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Hey audioguru

I think it is your ISP(Internet Service Provider) that have the problem, have you talked with them, its only a phonecall away? Also, have you checked wich route the packets take from Electronics-Lab:s server to you? Much info to get there!

//Staigen

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I hope you can solve this problems, we all need you to be with us all the time

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Hi there Audioguru

Have you solved your problems with internet?

//Staigen

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A couple of hours ago I surfed just fine everywhere else. Then I came here and pages were very slow and didn't fully load. It was terrible for about 1 hour then suddenly started working fine.

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Hey Audioguru, you got a message

//Staigen

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Hi Walid,
The datasheets for your tiny BC817 transistor don't have its Hie input impedance. A 2N3904 is a similar transistor and its datasheet shows it having about a 2k ohm input impedance in your circuit.

I tested my electret mic to have an output impedance of about 3.3k including its 10k load/supply resistor.

Since the mic circuit, coupling cap and the input of the transistor are all in series, then the cap has the total of the mic output impedance and the transistor's input impedance to determine its low frequency cutoff.

Maybe I am mistaken in determining the input impedance effect of the feedback resistor. It is a 100k resistor and the circuit's gain with it is only about 10. So it would seem like a 10k resistor in parallel with the input impedance of the transistor.

Therefore the total impedance in series with the coupling cap is 4.97k ohms.
The low frequency cutoff with your 100nF coupling cap is 320 Hz. ;D

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Hey Audioguru, theres a Personal message for you, right here on the board!

//Staigen

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• 2 weeks later...

Thank Audioguru i'll take my time to discuss what you said with myself then ask you.
thanks

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