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What is the reason of that 1 kilo ohm connected to the microphone? Why always too high resistors are used? I couldn't find any explanation. How do we calculate these values?

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On what grounds do you consider 1k too high? Too high for what? Like most components, a microphone is designed to be used in some practicular way, hence the only good reason for using a particular value is that the designers of the microphone intended you to do so. So why use another value? – Wouter van Ooijen Apr 6 '12 at 8:35
let me be more clear. if i have a microphone, and wanna make an amplifier, how am i gonna decide of that pull up resistor!s value? nothing is written on microphone. depending on what i will decide the value of the resistor? – cmd1024 Apr 6 '12 at 8:40
For lack of a datasheet for your microphone you could use datasheets for similar microphones. primo.com.sg/ourproducts-jap-microphone.html shows mostly 2 k, with a few exceptions (1k, 5k6, 15k). – Wouter van Ooijen Apr 10 '12 at 6:36

3 Answers

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The microphone is an electret type, which needs a bias for the capacitor, which it basically is. Some electret microphones also have an amplifier built-in, which also needs power. The larger the resistor from \$V_{CC}\$ the larger the voltage drop caused by the supply current, though for an unamplified electret the current is low and the resistor may be bigger.
A too small resistor will cause the signal to be attenuated. \$V_{CC}\$ is ground for AC signals and a small resistor will cause too much current to go that way.
In practice you'll often see higher resistor values, like 10k\$\Omega\$.

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The microphone as drawn in the circuit diagram is an electret microphone. It basically is a capacitor of which the plates can slightly move further and closer apart, by sound. A capacitor without an electric charge does nothing, so a slight electric charge is applied to the capacitor through the 1k resistor. Now when the plates of the capacitor start moving closer/further apart (caused by slightly changing air pressure from sounds), the capacity of the device changes with the distance between the plates and while the charge on its plates takes a relatively long time to change, the result is a varying voltage across the microphone.

These devices have a very high impedance for audio frequencies and therefore you often see an small buffer amp built close to it, so it can drive a long cable.

This type of microphone is very cheap and audio quality is high.

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thanks for ur answer but why it is too high like 1 kilo ohm – cmd1024 Apr 6 '12 at 8:25
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1k is not very high, it is rather low. The idea is that the capacity of the mic does not discharge while its plates are moving. If you manage to keep the charge on its plates constant, then the output voltage follows the recorded sound in detail. The 1k resistor allows the capacitor to discharge rather fast, which will influence frequency response and output signal swing. The lower the series resistor, the lower the response for low frequencies. – jippie Apr 6 '12 at 8:32
so we can also use 1 ohm? same principle isnt it? no reason why 1 kilo ohm rather than 0.001 ohm? – cmd1024 Apr 6 '12 at 8:36
theoretically ... yes, but the signal you get from it will be astronomically small. In practice I'd go for 10k or 100k maybe even more. – jippie Apr 6 '12 at 8:43
this is the point i dont get. if the resistor is 0.0001 ohm why signal is astronomically small? shouldnt it be opposite according to ohms law I=V/R. so i was thinking less resistance will cause higher current. where am i wrong? – cmd1024 Apr 6 '12 at 8:46
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up vote 1 down vote

Microphones can generate voltages by themselves sometimes, but often they need to be powered by an external source: in this case, the resistor is used to provide a supply to the microphone without forcing it's output voltage.

The signal generated from the microphone will be a variable current which will cause a varying drop over the resistor. Note that there is a series capacitor to AC-couple the signal, which means removing the common mode voltage (DC component).

And by the way, 1 kOhm is not a too big value: consider that 1 Ampère in electronics is often too big, and milliAmpères are far more common.

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do you mean that the current should be very small before entering the amplifier. is that the reason for that 1 kilo ohm instead of 100 ohm. is 1 k resistor operates as a voltage divider? – cmd1024 Apr 6 '12 at 8:27
@cmd1024 1kOhm is not too high, please read my answer and study some theory: in electronics, 1kOhm is a quite common value, and even small for some applications – clabacchio♦ Apr 6 '12 at 8:29
thanks i think i got the picture. sometimes it is too hard to grasp from formulas the main idea. many books even dont mention these reasons. do you know any good reference for beginners like me? – cmd1024 Apr 6 '12 at 8:33
There is a lot of posts over here for books, look at the top voted questions; Art of Electronics is often recommended, even tough I've never read it – clabacchio♦ Apr 6 '12 at 8:46
I agree with clabacchio. Most textbooks that explain resistors and the like use component values that are easy to write down (maybe 1 - 50 ohms), but are of little practical use in electronics. In electronics resistors easily range from 1 - 1.000.000 ohms. – jippie Apr 6 '12 at 9:01
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