Main Sources of Electronic Noise
Thermal Noise
- It is always associated to dissipation phenomena produced by
currents and voltages. It is represented by a voltage or current
sources randomly variable in time
- It is analytically described by a stationary process
- Amplitude distribution: GAUSSIAN with zero mean value
- Power Spectral Density: constant (white noise)
G(f)
 = Power spectral density =K.T

f
Main Sources of Electronic Noise
Shot noise

It arises typically in PN junctions forwardly biased; it is due to the discrete nature of current through the junction, which results randomly variant around the imposed bias value

Amplitude distribution: GAUSSIAN with zero mean value 
Constant spectrum (white noise)
G(f)

 = Power spectral density (=2q.I)
f
Main Sources of Electronic Noise
Flicker noise (1/f)
1. It arises in semiconductor devices, due to impurities and
defects in the crystal structure).
2. Its spectrum is not constant (energy is concentrated at low
frequency).
Power Spectrum:
Ia
G f   K b
f
W / Hz
K: depends on the
fabrication process
I: DC current through the
device
a: typically 0.5  2
b:  1
Nota: The amplitude distribution is not always GAUSSIAN
Noise Characterization in
microwave devices
PNout
PNin
2-port
ZL
NDB = Added Power (from the 2‐port)
NDB
The noise figure NF: define the attitude of the 2‐port of adding noise at the output:
NF 
PNout
Ga  PNin
PNout is the actual noise power at output while Ga.PNin is the noise power at output if the 2‐port would not add noise power (Ga is the available power)
Actually NF is a function of frquency, so the above powers must be assumed
per unit band (i.e. they represent actually power densities)
Moreover NF depends also on the source impedance (S)
NF dependance on S
NF  ( NF )min  4rn
s  min
2

1  min  1  s
2
2

 (NF)min = Minimum value of NF
 min value of S which determines NF=NFmin
 rn = Normalized noise resistance
All these parameters are frequency dependent. Typically, they are made
available by the manufacturers of commercial devices (directly into .s2p
data files).
Constant Noise Figure Cirlces
If we plot the equation expressing NF as function of S on the Smith Chart (representing S ), we obtain a circle with the following center and radius:
min
CF 
,
1  Ni
rF 

1
2
Ni2  Ni 1  m
1  Ni
Ni is given by:
Ni 

NF  ( NF ) min
 1  min
4 rn
2


Noise Figure for cascaded stages
Ga1, NF1
 NF TOT
Ga2, NF2
 NF1 
Ga3, NF3
NF2  1 NF3  1

 
G a1
Ga1Ga 2
The noise figure is mainly determined by the first stage
NOTE: In general the value of S that determines the minimum value of NF
is different by the one that maximizes GT; the choice of S for the first stage
is then the result of a compromise between the noise figure and gain
Design of a low noise amplifier
The choice of s is a compromise between GT and NF.
Some circles with NF=cost and Ga=cost are first plotted on the Smith chart (s). The value of s is selected within the common area of two circles. Considering that NF increases with the radius while Ga decreases with it, one has (with reference to the zones 1 and 2):
Cerchi a
NF=cost
Ga2
NF2
NF1
NFmin
Ga1
1
2
Cerchi a
Ga=cost
Cerchio di
Instabilità
del gen.
In zone 1: NF NF1 and GaGa2 NF is previliged
In zone 2: NF NF2 and GaGa1 Ga is previliged
Once assigned s,opt , L,opt is computed by imposing the matching at output (then GT=Ga)
Example of design
Substate: Duroid
r = 2.54
H= 0.508 mm
t = 35 
Amplifier Requirements
Frequency Band: 6.2 – 6.8 GHz Minimum Transducer Gain: 10.5 dB
Maximum Noise Figure: 1.5 dB
Active Device
MGF1923 Mitsubishi (GaAs Mesfet)
MSG (6.5 GHz): 15 dB (with NF=3.4 dB)
Minimum NF (6.5 GHz): 1.12 dB (with Gt=8.66 dB)
Topology:
TLO C
SU BC KT
N ET="transistor"
TLIN
TLIN
1
2
TLO C
Rete M A TC H ingresso
Rete M A TC H uscita
Biasing network of the active device
Tensione Vgs
Tensione Vds
Corto circuito per la RF
NOTE: The S parameters delivered by the manufacturer refers to the red sections. After the biasing network has been assigned, the S parameters changes to the ones referred to the black sections Circuito aperto per la RF
Sezione di ingresso
Sezione di uscita
Condensatori di blocco per la DC
Evaluation of the S parameters of the biased active
device
MLEF
ID= TL14
W= 2 mm
L=2 mm
MLEF
ID=TL16
W=2 mm
L=2 mm
MSUB
Er=2.54
H= 0.508 mm
T=0.035 mm
Rho= 1
Tand=0
ErNom=2.2
Name=SUB1
MLIN
ID=TL1 5
W=0.2 mm
L=0.5 mm
1
MRSTUB2
ID= TL3
Ri=0.17 3 mm
Ro=4.36 4 mm
Theta=60 De g
MLIN
ID= TL10
W= 1.4 4 mm
L=0.9 9 mm
MLIN
ID=TL17
W=0.2 mm
L= 0.5 mm
2
MTEE$
ID=TL2
3
MTEE$
ID=TL18
1
2
MCTRACE
ID= TL1
W= 0.2 mm
L=7.6 88 mm
R=5 mm
MCTRACE
ID= TL13
W= 0.2 mm
L=8.1 87 mm
R=5 mm
CAP
ID= C1
C=15 pF
1
MTEE$
ID= TL8
SUBCKT
ID=S1
NET="MGF1923"
MLIN
ID=TL5
W=1.4 mm
L= 0.5 mm
3
2
MLIN
ID=TL7
W=1.4 mm
L= 0.29 mm
MRSTUB2
ID=TL4
Ri=0.173 mm
Ro=4.364 mm
Theta=60 Deg
3
1
MLIN
ID=TL6
W=1.4 mm
L=0.5 mm
2
3
2
1
MTEE$
ID=TL 11
MLIN
ID=TL9
W=1.4 mm
L=0.5 mm
CAP
ID=C2
C= 1 5 pF
MLIN
ID= TL12
W= 1.4 mm
L=1 mm
S parameters for the design
Frequency: 6.8 GHz
S selected for maximum gain
L(Opt) : (0.771 , 134.097)
Load
Gen.
GT: 15.016 dB
NF: 3.362
S(Opt) : (0.68 , 173.5)
Selection of S as a compromise between GT and NF
Load
NF=1.5 dB
X
Source
L
X
Ga=10.5 dB
s
GT = 10.72 dB
L = 0.526142°
S = 0.5 133°
Input matching network
x
TLIN
ID=TL1
Z0=50 Ohm
EL=51.12
53.5 Deg
F0=6.8
6.5 GHz
s
107°
PORT
P=1
Z=50 Ohm
LOAD
ID=Z1
Z=50 Ohm
x
TLOC
ID=TL2
Z0=50 Ohm
EL=50.59
49.1 Deg
F0=6.8
6.5 GHz
S
Output matching network
x
TLIN
ID=TL1
Z0=50 Ohm
EL=51.12
48.1 Deg
F0=6.8
6.5 GHz
L
96.3°
PORT
P=1
Z=50 Ohm
LOAD
ID=Z1
Z=50 Ohm
x
TLOC
ID=TL2
Z0=50 Ohm
EL=50.59
50.9 Deg
F0=6.8
6.5 GHz
L
Scheme of the overall amplifier
MLEF
ID=TL4
W=3.225 mm
L=2.243 mm
MLIN
ID=TL6
W=1.386 mm
L=1 mm
3
2
PORT
P=1
Z=50 Ohm
MLIN
ID=TL1
W=0.3389 mm
L=1.47 mm
1
MTEE$
ID=TL5
SUBCKT
ID=S1
NET="transistor"
1
MTEE$
ID=TL8
2
1
MLIN
ID=TL3
W=0.3642 mm
L=1.617 mm
MSUB
Er=2.54
H=0.508 mm
T=0.035 mm
Rho=1
Tand=0
ErNom=2.2
Name=SUB1
MLIN
ID=TL7
W=1.387 mm
L=1 mm
2
PORT
P=2
Z=50 Ohm
3
MLEF
ID=TL9
W=1.443 mm
L=4.296 mm
Amplifier Layout
Tensione Vgs
Tensione Vds
Amplifier Response
Initial Response
Optimized
Response
1515
22
DB(GT())
(L)
DB(GT()) (L)
Ampli
Ampli
1414
6.5 GHz
1.494
dB
6.5
GHz
1313
1.8
1.8
DB(NF())
(R)
DB(NF()) (R)
Ampli
Ampli
1.6
1.6
1.335 dB
1212
1.4
1.4
1111
1.2
1.2
1010
11
6.5
z
6.5GH
GHz
10.86
10.89dB
dB
99
0.8
0.8
88
0.6
0.6
77
0.4
0.4
66
0.2
0.2
55
00
66
6.2
6.2
6.4
6.6
6.4
Frequency (GHz)
Frequency
(GHz)
6.8
6.8
77
Scheme of a power microwave amplifier
Reti di Polarizzazione
R0
Vin
MATCH
Out
MATCH
In
S
R0
L
The concept is identical to the ones seen before. In this case however the
values of S e L to be assigned have to maximize the power delivered to
the load (for specified biasing conditions). Instability must be obviously
avoided (commercial devices are generally pre-matched internally for
unconditional stability).
The active device must be characterized for large signal operation