# Electronic – 100 ohm and noise

amplifiermicrophonenoise

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. 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