Low Noise Amplifier Design

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Low Noise Amplifier Design

• Sources of Noise
• Low Noise Design

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Sources of Noise

• Ambient Noise.
• Eg. Background noise in a recording
• Electrical Interference.
• Eg. Mains hum, mobile phones
• Amplifier Noise.
• Noise added by the amp

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Sources of Amplifier Noise

• Thermal Noise
• Resistive components
• Shot Noise
• Semiconductor junctions
• Flicker Noise
• All components

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Thermal Noise
Random motion of electrons in a conductor
(Brownian motion) causes thermal noise.
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White Noise
N(f)2
frequency

• Flat spectrum extending to infinity implies a
  signal with infinite r.m.s. voltage obviously
  impossible.
• In practice, interest is restricted to a finite
  band of frequencies.

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Band-Limited White Noise
N(f)2
frequency
N(t)
time
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Band-Limited Thermal Noise

• The r.m.s. voltage level of a band-limited white
  noise signal is proportional to the square root
  of the bandwidth.
• For thermal noise

or,
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Shot Noise
Dominates in semiconductor devices when carriers
are diffusing. (In conductors, thermal noise is
usually much bigger)
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Current a number of carriers per second
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Shot Noise Current

• The average number of carriers flowing past a
  point per second is proportional to the current.
• In any observation interval, the actual number of
  carriers will vary around this average this
  variation is shot noise.

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Flicker Noise

• Imperfections in the fabrication of a component
  cause flicker noise.
• Unlike thermal noise and shot noise, flicker
  noise is not white it is pink.
• Also, it can be reduced depending on the quality
  of the component.

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Flicker Noise Current
f frequency, K component dependent
constant. Note that the noise power is inversely
proportional to frequency i.e. pink noise.
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Noise in Amplifiers
Equivalent circuit
A noisy amplifier
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Noise Figure
Equivalent circuit of a noisy amplifier with a
noisy input.
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Noise voltage level at the output will be the
r.m.s. sum of the resistor noise and the
amplifier noise. (Add power, not voltage)
Equivalent circuit of a noisy amplifier with a
noisy input.
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(No Transcript)
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Noise Temperature
RS
vS
Noisy amp
If RS is cooled to zero
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The noise temperature of the amplifier is the
value of Tn where
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Introduction to Noise Summary

• The main sources of amplifier noise are thermal
  noise, shot noise and flicker noise.
• Thermal noise and shot noise are both white and
  are irreducible.
• Flicker noise is pink and can be controlled.
• The noise performance of an amplifier can be
  quantified by either its Noise Figure or Noise
  Temperature.