Chapter 6 Low-Noise Design Methodology

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• Chapter 6Low-Noise Design Methodology

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• Low-noise design from the system designers
  viewpoint is concerned with the following
  problem Given a sensor with known signal, noise,
  impedance, and response characteristics, how do
  we optimize the amplifier design to achieve the
  lowest value of equivalent input noise?
• The answer is the amplifier portion of the system
  must be matched to the sensor. This matching is
  the essence of low-noise design.
• Circuit Design
• When designing an amplifier for a specific
  application, there are many specifications to be
  met and decisions to be made. These include
  gain, bandwidth, impedance levels, feedback,
  stability, dc power, cost and signal-to-noise
  ratio requirements. Amplifier designers can
  elect one of 2 paths
• The wrong approach is to worry about the gain and
  bandwidth first and later in the design power
  they check for the noise.
• Alternatively, one can design the system with
  initial emphasis on noise performance.
• Although there are many low-noise devices
  available they do not perform equally for all
  signal sources.

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• To obtain the optimum noise performance , it is
  necessary to select the proper amplifying device
  (FET, BJT, or IC) and operating point for the
  specific sensor or input source.
• Feedback and filtering can then be added to meet
  the additional design requirements
• Design Procedure
• Select the input stage device, discrete or IC,
  BJT or FET
• Select the operating point
• If preliminary analysis shows that the noise
  specification can be met, a circuit configuration
  (CS, CG, CD) can be selected and the amplifier
  designed to meet the remainder of the circuit
  requirements
• Noise is essentially unaffected by circuit
  configuration and overall negative feedback.
  Therefore the transistor and its operating point
  can be selected to meet the circuit noise
  requirements.
• Then the configuration or feedback can be
  determined to meet the gain, bandwidth, and
  impedance requirements.
• This approach allows the circuit designer to
  optimize for the noise and other circuits
  requirements independently.

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• After selecting a circuit configuration,
  analyzing it for non-noise requirements may
  indicate that it will not met all the
  specifications. If the bandwidth is too narrow,
  more stages and additional feedback can be added,
  the bias current of the input transistor can be
  increased or a transistor with a larger fT can be
  selected. The noise can be recalculated to see
  if it is still within specifications. This
  iterating process ensures obtaining satisfactory
  noise performance and prevents locking in on a
  high-noise condition at the very start of the
  design.
• The ultimate limit of equivalent input noise is
  determined by the sensor impedance and the first
  stage, Q1, of the amplifier.
• The source impedance Zs(f) and noise generators
  En(f) and In(f) representing Q1 are each a
  different function of frequency. Initial steps
  in the the design are the selection of the type
  of input device, such as BJT, FET or IC, and the
  associated operating point to obtain the desired
  noise characteristic as described above.
• In the simplest case of a resistive source, match
  the amplifiers optimum source resistance Ro to
  the resistance of the source of sensor.
• If the amplifier is operated over a band of
  frequencies, the noise must be integrated over
  this interval.

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• By changing devices and/or operating points,
  theoretical performance can approach an optimum.
• The noise of the first stage must be low to
  obtain overall low system noise.
• The following stages cannot reduce noise no
  matter how good.
• Subsequent stages can add noise so the design of
  these stages must be considered for a low-noise
  system. The usual problem is that there is not
  enough gain-bandwidth in the first stage to
  provide high gain with the bandwidth needed.
  Then the noise of the following stages may
  contribute.
• After selecting the input stage the circuit is
  designed. Set up the biasing, determine the
  succeeding stages, the coupling networks and the
  power supply.
• Then analyze the noise of the entire system,
  including the bias-network contributions to
  ensure that you can still meet the noise
  specifications.
• Finally add the overall negative feedback to
  provide the desired impedance, gain and frequency
  response.

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Optimum Source Resistance

• The point at which the total equivalent input
  noise approaches closes to the thermal noise
  curve in the figure is significant
• At this point the amplifier adds minimum noise to
  the thermal noise of the source. This optimum
  source resistance is called Ro and is defined as
  , where .
  

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Selection of an Active Device

• An active input device can be an IC with a
  bipolar or FET input stage or a discrete
  transistor. Selection depends primarily on the
  source impedance and frequency range. To assist
  in decisions in choosing the right active input
  devices, a general guide is shown below
• At the lowest values of source resistance, it is
  usually necessary to use transformer coupling at
  the input to match the source resistance to the
  amplifier Ro. Bipolar transistor s and bipolar
  input ICs are most useful at midrange impedances.
  Adjustment of Ro to match the source impedance
  is made by changing the transistor collector
  current with higher currents for lower Ro as
  given by

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• At higher values of source resistance, FETs are
  more desirable because of their very low noise
  current In. In some instances, they are even
  preferred when a low En is desired.
• When operating with a very large range of source
  resistance such as in an instrumentation
  amplifier application, a JFET is generally
  preferred for the input stage.
• A typical JFET has an En slightly larger than a
  BJT, but its In is significantly lower.
• Another advantage of JFET is its higher input
  resistance and low input capacitance thus it is
  particularly useful as a voltage amplifier.
• For the highest source resistances the MOSFET
  with its extremely low In has an advantage. The
  MOSFET may have up to 10 to 100 times the 1/f
  voltage En of a JFET or BJT. As processing
  techiques have improved the MOSFETs are becoming
  more attractive as low noise devices.
• The advantages of include low cost and
  compatibility with digital IC process. It is
  often desirable to combine a MOSFET input signal
  amplification stage with DSP on the the same chip.

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• IC amplifiers are usually the first choice for
  amplifier designs because of their low cost and
  ease of use. In selecting the one IC for your
  design, the source impedance and the transistor
  type must still be considered. All of the
  preceding statements about input stages apply
  when selecting the IC for your design.
• For lower source impedances, select a bipolar
  unit IC and for higher impedances a FET input
  IC.
• If state-of-the-art performance is need the use a
  discrete BJT or JFET stage ahead of the IC.
• Transformer Coupling
• To couple the detector and amplifier in an
  electronic system, it is sometimes better to use
  a coupling or input transformer. When it is not
  possible to achieve the necessary noise figure
  using device selection, transformer coupling may
  be the solution. Very low resistance sources can
  cause this problem.
• The use of an input transformer between the
  detector and amplifier improves the system noise
  performance by matching the sensor resistance
  with the amplifiers optimum source resistance Ro.

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• Consider the secondary circuit as shown below
  that contains noise generators En and In. We
  have identified . When
  these quantities are reflected to the primary as
  En and In we obtain
• where T is the turn ratio.