Chapter 4 CAD For Noise Analysis

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Contents

Amplifier Noise Sources

Resistor Modeling Approach

Modeling 1/f Noise

• SPICE contains built-in models for passive elements, semiconductor devices ¡Ketc. by including control lines in an input file, SPICE can be made to perform various analyses of a circuit including noise analysis.
• Noise analysis is performed in SPICE by using .NOISE statement in the input control file.
• SPICE calculates shot noise and thermal noise for the circuit

Amplifier Noise Sources

• For modeling purposes it is advantageous to lump all the noise produced by an amplifier or other functional block into 2 noise sources E_n and I_n as shown:
• There are several ways to implement this type of an amplifier model in SPICE. The easiest way is to use 2 resistors as noise sources and reflect their noises to the appropriate nodes using dependent voltage and current sources.
• Diodes can also be used as sources of shot noise and flicker noise and reflect their noises to the appropriated nodes in the circuit.

Resistor Modeling Approach

• SPICE calculates the thermal noise current in a resistor according to



• Example:
• Create E_n and I_n of and 10 respectively
• We create 2 thermal noise current standards of 1 by using 2 resistors of value



• 2 separate sources are necessary since it is normally assumed that the E_n and I_n amplifier noise sources are uncorrelated.
• Polarity does not matter for any of the sources because noise is a squared quantity. The multipliers needed for the H and F sources are respectively



• The SPICE codes are: RNE 100 0 16.5k;
• VESEN 100 0 DC 0; HEN 1 3 VESEN 3k;
• RNI 200 0 16.56 k; VISEN 200 0 DC 0;
• FIN 2 3 VISEN 10
• If we desire to include the amplifier¡¦s input impedance into our noise model, an extra noise will be unintentionally introduced through the resistive component of the input impedance.

Modeling 1/f Noise

• SPICE calculates the noise current in a diode according to

  

• Example: Model E_n noise of 20 nV/Hz^1/2 with a noise corner frequency f_nce = 200 Hz and I_n noise of 0.5 pA/Hz^1/2 with a noise corner frequency f_nci = 500 Hz in each diode
• The dc diode current is calculated according to

  

• Next the flicker noise term is set equal to the 1-pA/Hz^1/2 reference noise current at the f_nc frequencies. For the E_n source with AF =1, KF is calculated by equating the two right-hand terms. Thus KF=2qf_nce=6.408x10^-17A
• Similarly KF for I_n is 1.602x10^-16