Abstract
Op Amps with a large difference between their general and their shot noise tend to be circuits that are compensated for bias current. Typically for circuits, if they are compensated but if then the circuit isn’t compensated
Thermal Noise
Synopsis
Increases with temperature Uniform over all frequencies Uniform over all resistors Noise Power independent to R
Explanation
Shot Noise
Synopsis
Increases with current, but only the square root of current, which means it’s effect decreases with current Uniform over all frequencies Only present for some devices, and only when current is flowing
Explanation
Flicker Noise
Synopsis
Increases with current Only present for some devices (not choppers for example) Increases at lower frequencies
Explanation
Defects in components like resistors or transistors can have defects, which when charged, it can affect the current flow, like a gate; when uncharged, it doesn’t affect the current flow, which means its creates a modulation in the signal like a square wave. Which means a random period square wave for each defect, where there’s typically quite a few of these.
This type of noise typically has around 1/f frequency dependance. Occurs in most devices as both current and voltage noise.
Typically our noise doesnt keep going, we have a “noise corner” where the signal comes within 3dB of the main frequency, like
// Find TL071 18nVHz^-1/
Strategy
Single Input Op Amp can only have two noise sources, Voltage Noise in series, and Current Noise in parallel
For any single input system our voltage noise is:
The quality of our Noise can be described using our Noise Figure, which is: In general due to the way we take the Pythagorean sum of the noises, typically any noise that is 3x more than any other will dominate, so we need to optimise for that
If we have a low input resistance we look for a low voltage noise
For circuit creation, for every independant resistor, place something to represent the themal noise, typically two in a system.
// Ex Design 2 (2)