Linearized MOSFET Resistors

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Linearized MOSFET Resistors

• Dr. Paul Hasler

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Review of Gm-C Filters
Gm-C filters voltage mode/current
mode/log-domain Use amplifier dynamics as
filtering elements If making 10kHz filter,
why make amplifiers run at 10MHz?

• Good properties
• Highest bandwidth / power consumed
• Smallest number of elements / area consumed
• Lowest noise levels / power consumed (thermal)
• Utilizes capacitor matching (ie. C4)
• Electronically tunable

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Issues for Gm-C Filters
Improvement by Floating-Gate Techniques
Tuning Need control schemes (direct or indirect) adds significant amount of control overhead Mostly compensates slight adjustments due to transistor aging / T changes, etc.
Matching Huge issue for current-mode techniques Design to eliminate these issues
Distortion Techniques to improve linear range, but at a cost of lower gm/I (lower speed, higher noise, higher power) More techniques to improve linear range
Most Gm-C techniques are fairly recent
(80s-90s), and Floating-Gate techniques
are even more recent (90s - ).
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Other Filter Techniques
Utilizing higher frequency elements / additional
elements, to improve distortion (as well as
1/f noise, etc.). Two techniques Amplifiers
(Op-amps), that run at much faster frequencies
than filter cutoff. Can use feedback to
widen the linear range. Significant power
increase. Oversampling Using a wider bandwidth
than necessary to lower
noise per unit bandwith (and more power) and
distortion. Nonlinear
systems can utilize noise shaping (Sigma-Delta
Modulators) Common in sampled
data systems. Two techniques Switched
Capacitor Blocks Blocks based upon
traditional, discrete RC active fitlers.
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How to Build Resistances?
Resistors in a CMOS process - Sometimes
High resistance poly layer in a given process
- Poly, diffusions, or Well, but larger area
consumed Fairly linear, can be large for
frequencies under 1MHz. Not tunable therefore
RC gt 20 mismatch, so we have a problem for
precission filtersso either laser trimming,
EEPROM trimming, (could tune cap, but)
or imprecise filters, like anti-alaiasing filter.
MOSFET as a Resistor
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MOSFET as a Resistor
Ohmic Region how linear will that be, well only
over a small region. We have a gate voltage,
so it is tunable, but of course, we
still need a method of tuning. MOSFET has an
ohmic region both in subthreshold
and above threshold operation. Resistance is
not exactly a constant, except for a fixed
source voltage. resistance changes with
source / drain voltage. Could imagine an nFET
and a pFET in parallel, but still not a
precission element.
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MOSFET as a Resistor
Two things to improve the situation. 1.
Typically built around an amplifier to fix one of
the terminals (mostly op-amps, but could
also be a Norton or
transisresistance approach as well)
The amplifier must keep terminals nearly fixed to
eliminate distrotion therefore, in
general the amplifier must run a lot
faster than expected by a simple GmC stage.
2. Can use a combination of MOSFETs to linearize
the behavior.
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Linearized MOSFET resistors
Simple Structure
Balanced Differential Element
Vi
Vi
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Linearized MOSFET resistors
In practice, one might use even lower input
impedance elements
GND
GND
GND
GND
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Basic Resistive Feedback
GND
Vout
Vin
R1
R2
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Basic Integrator Structure
C
GND
Vout
Vin
R1
C
C
Ideal Integrator if
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Tow-Thomas SOS (Lowpass)
C1
R3
C2
R
R1
R4
R2
Vin
R
GND
V1
Vout
GND
V2
GND
R4 needed for stability
Tuning can be interesting (tuning pots)
All amps must be sufficiently fast
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Tow-Thomas SOS (Lowpass)
C
R
C
R
R
R4
R
Vin
R
GND
Vout
GND
GND
t RC
Q R4 / R