Presentation on theme: "Linearized MOSFET Resistors Dr. Paul Hasler. Review of Gm-C Filters Gm-C filters: voltage mode/current mode/log-domain Use amplifier dynamics as filtering."— Presentation transcript:
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
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
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.
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
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.
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.