# Electrical Noise - PowerPoint PPT Presentation

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## Electrical Noise

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### Flicker noise. Flicker noise is due to contamination and crystal defects. ... flicker noise. Metal film resistors have no flicker ... Flicker noise modeling ... – PowerPoint PPT presentation

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Title: Electrical Noise

1
Electrical Noise
• Wang C. Ng

2
Nature of electrical noise
• Noise is caused by the small current and voltage
fluctuations that are generated internally.
• Noise is basically due to the discrete nature of
electrical charges.
• Externally generated noise is not considered here.

3
Why study noise?
• It sets the lower limit for the detectable
signals.
• It sets the upper limit for system gains.
• Develop mathematical models to take the effects
of noise into account when analyzing electrical
circuits/systems.
• Find ways to reduce noise.

4
Thermal noise
• Due to random motion of electrons.
• It is ubiquitous (resistors, speakers,
microphones, antennas, )
• It is directly proportional to absolute
temperature.
• White noise - Frequency independent up to 1013 Hz.

5
Thermal noise modeling
• The noise amplitude is represented by the rms
value

6
Thermal noise modeling
• The rms noise voltage for a 1-KW resistor is
• The amplitude distribution is Gaussian with m 0
and s vn .
• A series voltage source (vn) can be added to a
resistor to account for the thermal noise.

7
Thermal noise modeling
• Examples
• A 1-KW resistor in a system with a bandwidth of
100 MHz generates about 40 mV of noise voltage.
• A 1-MW resistor in this system generates about 40
mV of noise voltage.
• 10 1-MW resistor in this system generates about
0.4 V of noise voltage.

8
Shot noise
• Shot noise is due to the random arrivals of
electron packets at the potential barrier of
forward biased P/N junctions.
• It is always associated the a dc current flow in
diodes and BJTs.
• It is frequency independent (white noise) well
into the GHz region.

9
Shot noise modeling
• The noise amplitude is represented by the rms
value

10
Shot noise modeling
• The rms noise current for a diode current of 1 mA
• The amplitude distribution is Gaussian with m
ID and s in .
• A parallel current source (in) can be added to a
diode to account for the shot noise.

11
Shot noise modeling
• Examples
• For a diode current of 1 mA in a bandwidth of 1
MHz shot noise generates about 20 nA of noise
current.
• For a diode current of 10 mA in a bandwidth of
100 MHz shot noise generates about 2 mA of noise
current.
• 100 diodes would generate .2 mA of noise current.

12
Flicker noise
• Flicker noise is due to contamination and crystal
defects.
• It is found in all active devices.
• It is inversely proportional to frequency (also
called 1/f noise) .
• DC current in carbon resistors cause flicker
noise.
• Metal film resistors have no flicker noise.

13
Flicker noise modeling
• The noise amplitude is represented by the rms
value

14
Flicker noise modeling
• The constant K1 is device dependent and must be
determined experimentally.
• The amplitude distribution is non-Gaussian.
• It is often the dominating noise factor in the
low-frequency region.
• It can be described in more details with fractal
theory.

15
Other noise types
• Burst noise (popcorn noise)

16
System Noise Analysis
• Wang Ng

17
Introduction
• Noise sources can be added to a device models to
represent the effect of noise.
• We need a means to characterize the noise
performance of a system (black box).
• Noise figure
• Noise temperature

18
Noise figure
• Used for resistive source impedance.
• Most communication systems have a 50-W source
impedance (Thevenin equivalent).
• Signal-to-noise (S/N) ratio
• Noise figure F (S/N)in / (S/N)out
• F is a direct measure of the S/N ratio

19
Noise figure calculations
• For an ideal (noiseless) amplifier
• Sout G Sin
• Nout G Nin
• For a real system
• F (Sin/Nin)(Nout/Sout) Nout/GNin
• or F (Total noise)/(Noise due to input)
• F in in general frequency dependent.

20
System noise
• Internally generated noise can be computed from
• Nsys (F - 1)GNin
• since Nout Nsys GNin

21
• Gain Gtotal G1 G2 GN
• Noise figure
• Ftotal F1 (F2 - 1)/G1 (F3 - 1)/G1G2
(FN - 1)/G1G2 GN
• What does this tell us?
• We should pay most attention to the reduce the
noise of the first system (Why???)

22
Noise temperature
• It is the temperature at which the noise
generated from the source resistance equals to
the system noise.
• The noise temperature of a system is a better
measure when F is close to 1 (low-noise system)
• Noise temperature Tn T(F-1)

23
• A modern radiometer can measure noise temperature
variation down to 100th or even less in ?K.
• This instrument can be used for remote
sensing/imaging.
• Possible extra credit presentation.

24
Summary
• System noise measure Noise figure and noise
temperature
• Internal noise calculation