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Design >> Analog Design >> How to compensate for gate leakage
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Message started by ic_engr on Nov 10^th, 2009, 12:45pm

Title: How to compensate for gate leakage
Post by ic_engr on Nov 10^th, 2009, 12:45pm

Hello All,

I have a circuit which provides a BandPass Filter function. The Low-Cutoff is designed to be very low ~0.5Hz. The DC is set by using feedback network resistors realized using diode connected transistors. In the feedback, due to possible large signal swing I have used 4 transistors with one 10pF capacitor (C2) in the middle of the feedback, to filter so that the resistance of the feedback (two diode transistors) is relatively constant. The total resistance is in the order of 25GΩ. A replica of the diode transistors is also connected to the reference voltage VREF to provide matching is offset.
Note VREF is set to 450mV. Having matched impedances between VREF and feedback nework my output of the opamp is stting DC=450mV. Which is what I want.

However, due to the gate leakage of the diff pair (PMOS) I have a gate current sourcing and thus VN & VP sit around 784mV!!. The gate current sourcing is 12.8pA.

I need to cancel the affect of the gate leakage so that I am around 450mV on both VN & VP. Any ideas how I can sink a replica gate leakage current to make my VN & VP be around 450mV. My circuit now suffers from low DC open loop gain and also high input swing cause distortion.

Any suggestions ???.

ic_engr.

Title: Re: How to compensate for gate leakage
Post by loose-electron on Nov 10^th, 2009, 5:52pm

take a look in the design kit for higher voltage transistors (often called IO transistors) and see if their leakage is less.

other options are available in dropping the impedance of you system down and then the leakage is less of an effect.

Title: Re: How to compensate for gate leakage
Post by Mayank on Nov 10^th, 2009, 9:17pm

Leakage : Understandable.....With such high resistances of around 1G in the line, even a current of pA magnitude will produce a drop of the order of a few mV.....
Which technology (& minimum length) are you working in ?? I would suggest you to use Thick Gate Oxide MOSFETs....

As for the Distortion, When you use diode connected MOSFETs as resistors, dont you think they will be highly non-linear with large swings at the output....Why dont you use passive Resistance structures....As in Switched-Cap implementation, or other linearized active MOSFET resistances....Everything comes with a trade-off...SC requires clock....linearized MOSFET resistances require triode-biasing. You have to weigh the pros & the cons and choose one which best suites your application....

--Mayank

Title: Re: How to compensate for gate leakage
Post by ic_engr on Nov 11^th, 2009, 11:11am

Mayank,

I do have thick gate transistors, but I cannot use these on my Opamp differential input due to low voltage operation, as the VT also goes higher for thick gate transistors.

Yes, for large swings the resistance will be highly non-linear. With SC we would have other problems.

Do you have any literature for linear MOSFETS as resistors? May be that would be a better compromise.

Thanks

ic_engr.

Title: Re: How to compensate for gate leakage
Post by thechopper on Nov 11^th, 2009, 5:56pm

Hi

Try this one..."Continuous-time MOSFET-C Filters in VLSI", Y. Tsividis
M. Banu, J. Khoury. IEEE JSSC Feb 1986

This includes several techniques for linearizing MOSFETS for filter apps...

Hope this helps

Tosei

Title: Re: How to compensate for gate leakage
Post by Mayank on Nov 11^th, 2009, 8:55pm

Hi,
Point taken --> If you want a low-Voltage operation,Thick Gate O₂ cant be used....

Along with what Tosei suggested, check the Czarnul ckt also out -->

Z. Czarnul, "Modification of Banu-Tsividis
continuous-time integrator structure," Circuits
and Systems, IEEE Trans. Ckt. Syst., pp. 714-
716, July 1986.

All of these structures are aimed at reducing 2nd OR higher Harmonic Distortion.

1 more thing --> Do a quick 'rapid IP3' run on your config (the one with Diode-connected Resistive Loads) and quote the 1dB compression point....What's your acceptable spec. on it ?

Anyways,Just out of curiosity, what's your application which demands such low cut-offs ~ 0.5Hz ???

--Mayank.