100 ohm and noise

This is an image provided by JIm Dearden, in answer to the question Lots of noise in the microphone amplifier circuit when idle. I'd like to know how introducing a 100$\Omega$ resistor reduces noise. If I short that 100$\Omega$ resistor then the noise is increased significantly whereas if I put it there it reduces the noise significantly.

• Instead of "take out the resistor", you can say "open the resistor" or "short the resistor", which is clearer. There are two ways to remove any two-terminal component. It's clear here, but not always. – Kaz Jun 27 '13 at 4:07
• ok thanks kaz..next time i will be careful with my words. – user734861 Jun 27 '13 at 4:15
• I kinda wish you and Jim Dearden would use electronics SE's built-in schematic editor :-) – RedGrittyBrick Jun 27 '13 at 8:55
• @user734861 don't forget you can edit your posts any time you wish. – user17592 Jun 27 '13 at 9:20
• @RedGrittyBrick - I don't like CircuitLabs interface, and it always bitches about my browser choice. I tend to use Falstad's circuit simulator when I need a quick & simple schematic. – Connor Wolf Jun 27 '13 at 16:21

The resistor is needed so that your bypass capacitors have an R with which to form an RC filter. The 22 uF and 0.1 uF capacitors do not effectively remove voltage noise from the 2.2 kOhm resistor because they are in parallel with it.

But when the 100 ohm is added, then there is suddenly a voltage divider, the bottom leg of which is bypassed by the capacitors, for alternating currents.

simulate this circuit – Schematic created using CircuitLab

DC equivalent: 100 ohms doesn't do much next to the 2.2 kOhm:

simulate this circuit

AC equivalent:

simulate this circuit

Here is the AC equivalent circuit if we don't have that 100 ohm resistor there. Now that node is not exactly at AC ground but caught between two impedances: those of the capacitors and the inductance/resistance of the wire. Any noise flowing through there now has more of a chance to enter the 2.2 kOhm resistor and thus end up amplified.

simulate this circuit

• Exactly as I had designed. Good analysis. One thing I would add is why the 0.1uF. From a capacitance point of view it doesn't seem to add much to the total but ordinary electrolytics aren't particularly good with higher frequencies and their impedance can go up, hence the 0.1uF does the job of providing a low impedance pathway for this part of the spectrum. – JIm Dearden Jun 27 '13 at 6:58

The noise originates from the 5V rail and may be due to: -

• Poor/unstable regulation from the 5V regulator
• Digital noise from other circuits not shown

The noise on the 5V line if, directly connected to the microphone's 2k2 will travel thru the 2k2 and superimpose itself on the microphone output and this will be coupled directly into the BJT amplifier via the 1uF cap. Hence the output of the BJT is noisy.

Without the 100R (i.e. shorted out) the noise will be somewhat attenuated by the 22uF and 100nF but, the noise may be quite low impedance and those caps may struggle to reduce it.

Placing the 100ohms in series with the 5V and hence the noise, weakens the potential strength of the noise and allows the 22uF and 0.1uF to do a much better job of reducing the noise. They won't affect the DC because they block it.

The 100ohm resistor will also slightly reduce the 5V but this is of no consequence to the electret microphone.

100ohm and 22uF start to reduce noise at a frequency dictated by 1/(2*Pi*R*C). This frequency is about 72Hz and any frequency above will be more seriously attenuated. For example at twice 72Hz (144Hz) the attenuation will be 6dB (half in terms of voltage). At 288Hz the attenuation will be another 6dB on top of that; a voltage reduction of 4:1.

It's a first order low pass filter and generally it's said that it attenuated at 6dB per octave or 20 dB per decade.

If the noise is low frequency AC hum or similar this filter won't do a lot for improving things but, if the noise is like white noise/hissy then it will significantly reduce the noise.

• i changed the power supply to 3.3 v and i didnt need the 100 ohm resistor. So, i think like you said the power supply of the microphone was responsible. – user734861 Jun 27 '13 at 20:55
• It could not come from anywhere else. Tracking down noise is one of the big challenges in analogue engineering design. I'm glad it worked for you dude. – Andy aka Jun 27 '13 at 21:10