I am designing a Wien-Bridge Oscillator on an IC and wanted to realize the resistors responsible for frequency-tuning using voltage controlled MOSFET devices (as a replacement for potentiostats). Since my node voltages (and thus the respective VDS of the transistors) are an AC-Sine voltage, the related small signal resistance of the devices (R_Transistor= rds = 1/gds = (delta Vds)/(delta I)) is not of fixed type but rather fluctuating with the drain-source voltage.
Does someone know of any designs or modfications that could be used to realize tunable pseudoresistors of fixed resistance value for AC-Sine voltages?
I personally have been thinking of operating the devices in subthreshold and with VDS_min > 4*VT = 100 mV to have at least the current independent of the voltage VDS, according to the BSIM model. This can be realized using a voltage-offset.
Speaking about minimizing the sine amplitude to reduce fluctuations, it can only be reduced to an amplitude of about +- 100 mV_pp due to design requirements...
Hope somebody can help me or at least provide some food for tought. All hints are well-appreciated :)
Thanks in advance!
Similar problems arise for feedback (reset) resistor in CMOS Charge Sensitive Amplifiers e.g. https://inspirehep.net/files/2739048b4ff76c464671e7ec9c10ab77
Indeed, subtreshold long, narrow MOSFETs and low voltage swing are used. One possible alternative might be use of rather high freq low duty factor modulated mosfet thus on average providing higher resistance. Some tricks to compensate non-linearity can be sued as well as you may see in that publication (you likely can more along that line).
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