Download Integrated Circuits Design for Applications in Communications

CAD For Noise Analysis
• SPICE contains built-in models for passive elements, semiconductor
devices …etc. 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
En and In 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
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SPICE calculates the thermal noise current in a resistor according to
I t2  4kT / R
Example:
Create En and In of 3nV / Hz and 10 pA / Hz respectively
We create 2 thermal noise current standards of 1 pA / Hz by using 2
resistors of value
23
Rn 
4(1.38 10 )(300)
 16.56k
 24
10
• 2 separate sources are necessary since it is normally assumed that the En
and In 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
3nV / Hz
10 pA / Hz
 3k and
 10
1 pA / Hz
1 pA / Hz
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•
•
•
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
KF ( I dc ) AF
2
Id 
 2qI dc
f
• Example: Model En noise of 20 nV/Hz1/2 with a noise corner
frequency fnce = 200 Hz and In noise of 0.5 pA/Hz1/2 with a noise
corner frequency fnci = 500 Hz in each diode
1024
• The dc diode current is calculated according to
I dc 
2 1.602 10
19
 3.122A
• Next the flicker noise term is set equal to the 1-pA/Hz1/2 reference
noise current at the fnc frequencies. For the En source with AF =1, KF
17 two right-hand terms. Thus
isKF
calculated
 2qf by
 6equating
.408 10the
A
nce
• Similarly KF for In is
1.602 10 16 A