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subthrehold operation

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Octago

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When the transistors operate in subthreshold region, in this case, the transistors are not fully conducted, the current flowing through the transistors are pretty small.

Therefore, my question is:
1, for the subthreshold operation, what is the range of operating current?
2, is it possible for the circuit which is operating in subthreshold region to drive a large capactitive load? or how can I optimize this circuit to enable it to drive a large load?
 

Re: logarithmic current to voltage converter

the sub-threshold current is depend on the device size. If it can drive a large cap load is depend on how long you need the voltage settling and the cap leakage current level.
 

    Octago

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Re: logarithmic current to voltage converter

you can add in series of this configuration a low noise amplifier which boost the small amplitude of your signal so you still get the logarithmic conversion with output signal boosted.
 

    Octago

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Re: logarithmic current to voltage converter

You should consider a diode as an alternate.

Diodes working from pA to some tens of uA so about 7 decades. Only the series resistance make a derivation from the log(i)->v principle. In MOS you have the subthreshold transition region, substrate effects, gate leakage and drain voltage dependence. All complicate a circuit for an accurate log(i)->v conversion.

With BiCMOS and an NPN you can get 9-10 decades depending on the process. A further option is to use the substrate vertical P+/NWELL/PSUB bipolar. Because of the vertical current gain only a fraction of the emitter current into the P+ is flowing out of the NWELL base. That lowers the effect of the NWELL base resistance of the log(i)->v accuracy. If the current gain in a non-epi CMOS process is between 5-20 you can extend the range by a further decade.
 

    Octago

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logarithmic current to voltage converter

And to drive a load, you will need some sort of buffer between this stage and the next. For example a simple source follower (either PMOS or NMOS) will only present a tiny capacitive load to this stage (to keep it fast) but can drive a much larger capacitive load on the output.
 

    Octago

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