Title: Seminar: Moderne Methoden der analogen MOSSchaltungstechnik 5' Noise in lowvoltage circuits
1Seminar Moderne Methoden der analogen
MOS-Schaltungstechnik5. Noise in low-voltage
circuits
2Noise in MOS Transistors
- Flicker (1/f) Noise
- Modeled as voltage source in series with the gate
- Inversely proportional to the frequency
(important al low frequencies) - Inversely proportional to the MOS area (large
transistors have less Flicker noise) - Inversely proportional to the specific
capacitance COX - It is caused from charge carriers trapped at the
interface between silicon and oxide or
fluctuation in the mobility of the charge
carriers - k is process-depending (about 10-25 V2F)
Design of Analog CMOS ICs B. Razavi
3Noise in MOS Transistors
- Thermal Noise
- Modeled as current source in parallel between
drain and source - It is due to the resistive channel
- Its spectrum is white (the same at all
frequencies) - ? is about 2/3
- The exact formula is
- but for long-channel transistors it is
4Noisy MOS
Thermal
Flicker
Output-referred
Input-referred
The input-equivalent noise current source was
neglected
5Noisy Common Source (passive load)
Total output voltage noise
Total input voltage noise
6Noisy Common Source (active load)
Total output voltage noise
Total input voltage noise
7Noisy Common Source (active load)
- Considering only the thermal noise
- Small gm2 reduces the total noise
- In saturation and
- W2/L2 should be small. By keeping the current
constant this increases Vov2 (reduction of the
output swing) - If gm1gm2
8Cascode amplifier
M2 is source degenerated through ro1 Gain form G2
to output AV2-ROUT/r01
Gain from G1 to output AV-gm1ROUT
Output-referred voltage noise
Input-referred voltage noise
9Cascode amplifier
The noise contribution of M2 can be neglected ?
Only M1 is noisy
10Cascode amplifier with passive load
M1
RL
M2
If RL is large its noise contribution can be
neglected
11Active Cascode
Gain G2/output
Input-referred
Can be neglected if A3ltltgm1r01
12Folded Cascode
Output-referred voltage noise
Gain G1/output
Gain G3/output
Gain G2/output
Input-referred voltage noise
More noise than cascode because of the M3-term!
M1
M2
M3
13Differential pair with passive loads
Noise analysis
- The noise contribution of M3 can be neglected
(symmetry) - For noise analysis node X is not virtual ground!
(uncorrelated sources)
14Differential pair with passive loads
Noise equivalent circuit
Superposition principle all uncorrelated noise
sources are considered separately. The output
noise power will be added.
15Differential pair with passive loads
Noise of Transistor M1
16Differential pair with passive loads
Noise of Resistor R1
Annahme
17Differential pair with passive loads
Noise of Transistor M2
Because of the symmetry
Noise of Resistor R2
Because of the symmetry
Total output noise
Total input referred noise
- Input-referred noise power is doubled compared
to single ended Common-Source - Input signal power is four times larger
- SNR is improved by 3 dB
18OTA circuits (1) Two-Stage
Two-stage OTA
Total transconductance
M1
M2
M3
19OTA circuits (2) OTA with current gain
- One-stage solution only the output node has a
high impedance - The current mirror has a gain B
- Mirror pole sp2gm2/(Cgs2Cgs3) (high frequency)
- The PM is high only if sp2gtgtsu
- Low-voltage suitable
- Transconductance (but not voltage gain) is
larger than standard single stage but smaller
than two-stage OTA
Same voltage gain as 1-stage!
B 2 to 4
20OTA circuits (2) OTA with current gain
21OTA circuits (3) current starving
- One-stage solution only the output node has a
high impedance - Mirror pole sp2gm2/(Cgs2Cgs3)
- M2 has less current than M1
- This reduces gm2 and thus sp2
- Compromise between single-stage and two-stage
- Low-voltage suitable
- Transconductance and voltage gain are larger
than single stage
B 2 to 4
A 0.5 to 0.9
A 0.8-V, 8-µW, CMOS OTA with 50-dB Gain and
1.2-MHz GBWin 18-pF Load L. Yao, M. Steyaert
and W. Sansen
22OTA circuits (3) current starving
23OTA circuits (3) current starving
Gain G4/Input
Gain G2/input
24OTA circuits (3) current starving
25Example noise analysis
Equivalent noise model
Active-RC Low pass filter
26Output noise voltage (1)
- Assumption all equivalent noise sources are
uncorrelated - Principle of superposition
- Effect of In1, Inf and In- (opamp -Terminal)
Integration impedance
27Output noise voltage (1)
- Low pass behavior
- 3-dB frequency
28Output noise voltage (2)
- Effect of In, Vn2 and Vn (opamp Terminal)
Gain of the opamp in non-inverting configuration
29Output noise voltage (2)
30Total output noise
Bode diagram of the noise transfer functions
31Calculation of the noise sources
Passive elements Rf100 kO Cf160 pF R110
kO R29.1 kO
opamp equivalent noise sources
32Output noise voltage (2)
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33The End