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Advancements in Gm-C Filters for High-Speed Applications

Explore innovative techniques in Gm-C filters, utilizing linearized MOSFET resistors and other filter methods for improving performance. Learn about floating-gate techniques and advanced filter designs like oversampling. Discover the role of resistors in a CMOS process and the use of MOSFETs as resistors for precision elements.

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Advancements in Gm-C Filters for High-Speed Applications

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  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 Most Gm-C techniques are fairly recent (80’s-90’s), and Floating-Gate techniques are even more recent (90’s - ).

  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 > 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

  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. + + + Vc Vc Vc - + + + - Vi Iout Iout Iout Iout - - - Vc Vc Vc - Vi + + Va Va - - Va Va Linearized MOSFET resistors Simple Structure Balanced Differential Element Vi Vi

  9. + + Vc Vc + - + - + Iout Iout Iout Iout Vi - - Vc Vc - Vi + Va - Va Linearized MOSFET resistors In practice, one might use even lower input impedance elements GND GND GND GND

  10. + + + + Vc Vc Vb Vb + Vi - - - - Vc Vc Vb Vb - Vi + - Vout Vout Basic Resistive Feedback GND Vout Vin R1 R2

  11. + + + + + Vb Vc Vb Vb Vc + Vi - - - - - Vc Vc Vb Vb Vb - Vi + - Vout Vout 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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