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Published byMelissa Earwood Modified about 1 year ago

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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. C 4 ) Electronically tunable

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Issues for Gm-C Filters Most Gm-C techniques are fairly recent (80’s-90’s), and Floating-Gate techniques are even more recent (90’s - ). 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 techniquesDesign 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 Improvement by Floating-Gate Techniques

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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 > 20% mismatch, so we have a problem for precission filters…so either laser trimming, EEPROM trimming, (could tune cap, but…) or imprecise filters, like anti-alaiasing filter. MOSFET as a Resistor

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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 ViVi VaVa + VaVa - VcVc + VcVc - I out + - ViVi ViVi + ViVi - VaVa + VaVa - VcVc + VcVc - VcVc + VcVc - + - Simple Structure Balanced Differential Element

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Linearized MOSFET resistors ViVi + ViVi - VaVa + VaVa - VcVc + VcVc - VcVc + VcVc - I out + - + - In practice, one might use even lower input impedance elements GND

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Basic Resistive Feedback VcVc + VcVc - VcVc + VcVc - VbVb + VbVb - VbVb + VbVb - ViVi + ViVi - V out + - GND V out V in R1R1 R2R2

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Basic Integrator Structure VcVc + VcVc - VcVc + VcVc - VbVb + VbVb - VbVb - ViVi + ViVi - V out + - VbVb + C C Ideal Integrator if = VbVb + VbVb - GND V out V in R1R1 C

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Tow-Thomas SOS (Lowpass) V out GND C1C1 C2C2 V in R1R1 R2R2 R R R4R4 R3R3 Tuning can be interesting (tuning pots) R 4 needed for stability All amps must be sufficiently fast V2V2 V1V1

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Tow-Thomas SOS (Lowpass) V out GND C C V in R R R R R4R4 R = RC Q = R 4 / R

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