single stage amplifiers

289 Pages·2017·2.97 MB·English

Unit - I
SINGLE STAGE AMPLIFIERS
Outline – Part II
• Noise in analog ICs
• Matching in analog ICs
• Operational Amplifier design examples
• Analog design methodology
2
Thermal noise in passive components
Thermal noise is caused by the random thermally excited
vibration of the charge carriers in a conductor.
v2 Power spectral density [ V 2 / Hz ]
n
R
v2  4kTR  f [ V2 ]
n
2
i
n
4kT
i2   f [ A2 ]
n
R
R
There are no sources of noise in ideal capacitors or
inductors. In practice, real components have parasitic
resistance that does display thermal noise!
3
Noise sources in MOS transistors
Channel thermal noise: due to the random thermal motion of the
carriers in the channel
1/f noise: due to the random trapping and detrapping of mobile
carriers in the traps located at the Si-SiO interface and within the
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gate oxide.
Bulk resistance thermal noise: due to the distributed substrate
resistance.
Gate resistance thermal noise: due to the resistance of the
polysilicon gate and of the interconnections.
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Noise in circuits
To be independent from the gain of a given system, we use the concept
of input-referred noise. This allows comparing easily the noise
performance of different circuits (with different gains), and calculating
easily the Signal-to-Noise Ratio (SNR).
At the input of our linear two-port circuit, we use two noise generator
(one noise voltage source and one noise current source) to represent
the noise of the system regardless the impedance at the input of the
circuit and of the source driving the circuit.
v2
n,in
Noisy Noiseless
v2 i2 v2
circuit n,out n,in circuit n,out
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Input-referred voltage noise
The MOS transistor is represented by its small-signal equivalent circuit.
We can refer the noise sources inside the MOS transistor to the input,
obtaining an input-referred voltage noise.
2 2
v 1 K 1 g
in  4kTng  a  4kTR  4kT mb R
2  G 2 B
f g C WL f g
m ox m
Channel thermal Gate resistance Bulk resistance
1/f noise
noise thermal noise thermal noise
g ideally varies from 1/2 (w.i.) to 2/3 (s.i.)
K = 1/f noise parameter, technology dependent
a
Usually, the first two terms are the most important
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N-channel noise spectra
W = 2 mm, I = 0.5 mA, V = 0.8 V, V = 0 V
DS DS BS
1.E-07
L = 0.36um
L = 0.5um
L = 0.64um
L = 0.78um
]

)
z L = 1.2um
H
(
t
r
q
s
1.E-08
/
V

[

e
s
i
o
N
1.E-09
1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08
Frequency [ Hz ]
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Noise in a DP + Active CM
V V
DD DD
2I 2I
v2 v2 v2
in in tot
i2 i2
out out
v2 v2
load load
 g2 
v2  2  v2  2   m_load   v2
tot in  g2  load
 
m_in
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Noise in a DP + Active CM
K 1  K  L2 
v2  2 a_in  1 a_load load in  f
V
DD tot_1/f C2 W L f  K  L2 
 
ox in in a_in in load
2I
Make W L big and L  L
in in load in
v2
tot
 
 W 
 
  
load
2   L  
v2  4kTng   1 load  f
 
tot_th W  W 
2 C in I     
in ox L  in L  
in  in 
 W   W 
Make     
L L
   
in load
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Outline – Part II
• Noise in analog ICs
• Matching in analog ICs
• Operational Amplifier design examples
• Analog design methodology
10