拉扎维模拟cmos集成电路课件Lecture_17

EE 510 Lecture 17 Handout
Spring 2013
Noise (Ch. 7)
1. Introduction An understanding of noise and its effects on circuit performance is helpful in the development of a good design. Our study pertains to inherent noise sources that are associated with the basic physical charge conduction mechanisms of electrical components. Such forms are thermal, shot, and flicker noise. In contrast to these kinds of noise, interference noise results from various external, unwanted sources, described by such terms as “interference,” “cross talk,” “hum,” “EMI,” and “RFI.” Such noise sources may or may not be random, and they are addressed in design by careful wiring layout, shielding, and other strategies. 2. Terminology A. Power Spectral Density (PSD) The PSD function, S x (f), is a description of the noise power spectrum. S x (f) for a noise waveform x(t) is the average power carried by x(t) in a one-hertz bandwidth about frequency f.
Vn2 = 4kTR ;
f ≥0
(ii) MOSFET thermal noise results primarily in the channel as a result of thermally induced, random motions in charge carriers.
2 In = 4kT γ g m
PSD for white noise
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EE 510 Lecture 17 Handout B. Spectral Shaping
Spring 2013
Theorem. If a signal with spectrum S X (f) is applied to a linear time-invariant system with transfer function H(s), then the output spectrum is given by SY ( f ) = S X ( f ) H ( f ) Examples of Spectral Shaping:
2 In = 2qI D
C. Flicker Noise Flicker noise is found in active devices and carbon resistors. It occurs only when a DC current exists. At the interface between the gate oxide and the silicon substrate, there appear dangling bonds that give rise to extra energy states. Flicker noise results from charge carriers at the interface being randomly trapped and subsequently released by these energy states. This noise source is more significant in MOSFETs than BJTs.
To illustrate this concept, consider the following CS stage where thermal and flicker noises result in the given output noise voltage.
The total output noise voltage per unit bandwidth is 2 1 2 4kT 2 K Vn2,out = 4kT 3 g m + C WL f g m + R RD ox D
Calculations of input-referred noise voltage (figure a) and noise current (figure b) are illustrated as follows:
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EE 510 Lecture 17 Handout
Definition for the Gaussian PDF: 1 − (x − m ) 2σ 2
2
p X ( x) =
σ 2π
exp
where m and σ and are the mean and standard deviation of the distribution. D. Correlated and Uncorrelated Sources
K 1 2 2 4kT g m = gm 3 C oxWL f C fC = K 3 gm C oxWL 8kT
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EE 510 Lecture 17 Handout 4. Circuit Realization of Noise Sources
Spring 2013
Usually, noise sources are referred to the input of a circuit to provide a comparison basis with the level of the input signal.
Vn2 =
K 1 C oxWL f
; K ≈ 10 − 25 V 2 F
The following plot shows the effects of both flicker and spectral noise. The 1/f noise corner frequency (f c ) is a measure of where flicker noise at low frequencies becomes relatively more important than thermal noise.
Spring 2013
Shot noise occurs because of the discrete nature of charge carriers in semiconductors. It is proportional to the semiconductor’s DC bias current. As an example, the white noise spectral current for a diode is described as follows:
Referring this noise voltage to the input is done using the following calculation: Vn2,out Av2 Vn2,out
2 2 gm RD
V
2 n ,in=来自=Vn2,in = 4kT
K 2 1 4kT + + 2 3 g m C oxWL f g m RD
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EE 510 Lecture 17 Handout 3. Types of Noise
Spring 2013
We consider three different noise sources that are observed in different electrical components. A. Thermal (Johnson, Nyquist) Noise (i) Resistor thermal noise is attributed to the thermally induced, random motions in a conductor, which in turn, give rise to observed voltage fluctuations. This is a form of white noise. Thermal noise circuit representations:
Spring 2013
The amplitude distribution for noise is described by its probability density function, PDF, as follows: P X (x) dx = probability of x < X < x + dx ; X = measured value of x(t) at time t Illustration of the amplitude distribution of noise:
The resistive regions associated with the MOSFET’s gate, source, and drain also display thermal noise, as accounted for in the following model:
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EE 510 Lecture 17 Handout B. Shot Noise