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  1. #1
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    What is the drawback of operating in subthreshold

    Specifically, I'm building a constant-gm bias circuit (ie. delta-Vgs) with relatively low current consumption. What are the drawbacks of operating in subthreshold vs saturation?

    I know it will cost me more area to bring the operation to subthreshold (i.e. high W/L) but how about performance drawbacks, are there any?
    Mismatch effect is greater in subthreshold?

    Thanks in advance for your replies.

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  2. #2
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    Re: What is the drawback of operating in subthreshold

    Somewhere down the subthreshold slope, you bump into the
    leakage floor. If you're close to the "knee" (or maybe it's the
    elbow) you can expect worsening match, loss of current
    mirror fidelity (esp if any DIBL behavior), or loss of control
    (try to throttle below leakage floor, no bueno). I'd say you
    want to stay a decade above the highest observed leakage
    floor of a large sample size, multiple lots spaced over months
    / years, max junction temp plus margin. If you don't know
    these realities then you're on some sort of adventure, to
    be discovered later. Models tend to be poor at predicting DC
    leakage and its variation.

    A device operated subthreshold will have inferior Id/Cdg
    which is a proxy for bandwidth. Subthreshold is usually for
    low power concerns. You do gain a wider saturated region
    of operation and better Rout. Whether that compensates
    for lesser gm, you'd have to play around and see.

    If it were my circuit I'd start with ideal biasing of the signal
    chain and then worry about making it an on chip reference.
    There's no reason to push for subthreshold in the reference
    if the signal chain is going to operate at higher current
    densities. Or conversely. You also may, or may not, actually
    want constant-gm biasing. There may be other trends in
    the circuit that want rising gm w/ temp (for example junction
    capacitances go up, Rout goes down, constant bandwidth or
    constant gain would want gm made to compensate).


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  3. #3
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    Re: What is the drawback of operating in subthreshold

    Thank you for the explanation!
    I don't actually intentionally go to subthreshold but the current requirement kinda force me into this region.
    For a standard supply voltage variation (+/- 10%) and temperature variation (-40 to 125degC), what kind of spread on the current would you say you would expect on a constant-gm bias circuit? I think I'm getting higher than normal spread for some reason, and this is without mismatch (no Monte Carlo), just normal PVT spread.



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