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Linearized MOSFET Resistors. Dr. Paul Hasler. Review of Gm-C Filters ... Resistors in a CMOS process - Sometimes High resistance poly layer in a given process ... – PowerPoint PPT presentation

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Title: Linearized%20MOSFET%20Resistors


1
Linearized MOSFET Resistors
  • Dr. Paul Hasler

2
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

3
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 - ).
4
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.
5
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
6
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.
7
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.
8
Linearized MOSFET resistors
Simple Structure
Balanced Differential Element
Vi
Vi
9
Linearized MOSFET resistors
In practice, one might use even lower input
impedance elements
GND
GND
GND
GND
10
Basic Resistive Feedback
GND
Vout
Vin
R1
R2
11
Basic Integrator Structure
C
GND
Vout
Vin
R1
C
C
Ideal Integrator if
12
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
13
Tow-Thomas SOS (Lowpass)
C
R
C
R
R
R4
R
Vin
R
GND
Vout
GND
GND
t RC
Q R4 / R
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