2Review of Gm-C FiltersGm-C filters: voltage mode/current mode/log-domainUse amplifier dynamics as filtering elements:If making 10kHz filter, why make amplifiers run at 10MHz?Good propertiesHighest bandwidth / power consumedSmallest number of elements / area consumedLowest noise levels / power consumed (thermal)Utilizes capacitor matching (ie. C4)Electronically tunable
3Issues for Gm-C Filters Improvement byFloating-Gate TechniquesTuningNeed control schemes (direct or indirect) – adds significant amount of control overheadMostly compensates: slight adjustments due to transistor aging / T changes, etc.MatchingHuge issue for current-mode techniquesDesign to eliminate these issuesDistortionTechniques to improve linear range, but at a cost of lower gm/I (lower speed, higher noise, higher power)More techniques to improve linear rangeMost Gm-C techniques are fairly recent (80’s-90’s),and Floating-Gate techniques are even more recent (90’s - ).
4Other 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 lowernoise per unit bandwith (and more power) and distortion.Nonlinear systems can utilize noise shaping (Sigma-Delta Modulators)Common in sampled data systems.Switched Capacitor BlocksBlocks based upon traditional, discrete RC active fitlers.
5How to Build Resistances? Resistors in a CMOS process- Sometimes High resistance poly layer in a given process- Poly, diffusions, or Well, but larger area consumedFairly linear, can be large for frequencies under 1MHz.Not tunable: therefore RC > 20% mismatch, so we have a problem forprecission filters…so either laser trimming,EEPROM trimming, (could tune cap, but…)or imprecise filters, like anti-alaiasing filter.MOSFET as a Resistor
6MOSFET as a ResistorOhmic 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 subthresholdand 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.
7MOSFET 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 ortransisresistance approach as well)The amplifier must keep terminals nearly fixed toeliminate distrotion; therefore, in general the amplifiermust run a lot faster than expected by a simple GmC stage.2. Can use a combination of MOSFETs to linearize the behavior.