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    In what applications, low-Vth MOSFET and medium-Vth MOSFET are useful?

    Hi, all.
    Low-Vth MOSFET and medium-Vth MOSFET are provided by TSMC 0.18um CMOS process. but I never use them, all circuits I have designed apply standard-vth MOSFET.
    So I am very interested where low-Vth MOSFET and medium-Vth MOSFET are useful.

    Ziqi

    •   Alt1st September 2012, 15:17

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    Re: In what applications, low-Vth MOSFET and medium-Vth MOSFET are useful?

    E.g. for MOS diodes in tail current sources, in order not to loose too much Vds .


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    •   Alt1st September 2012, 15:24

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    Re: In what applications, low-Vth MOSFET and medium-Vth MOSFET are useful?

    in places where the resistive drop across the MOS must be insignificant you go for low threshold
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    •   Alt1st September 2012, 16:02

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    Re: In what applications, low-Vth MOSFET and medium-Vth MOSFET are useful?

    Quote Originally Posted by erikl View Post
    E.g. for MOS diodes in tail current sources, in order not to loose too much Vds .
    Quote Originally Posted by jeffrey samuel View Post
    in places where the resistive drop across the MOS must be insignificant you go for low threshold
    I agree with you, both Erikl and Jeffrey. Although low-Vth MOSFET benefits low voltage drop, this technique is rarely introduced in textbooks or papers. So I suspect there are some drawbacks comparing with standard-Vth MOSFET. could you give me a sense?



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    Re: In what applications, low-Vth MOSFET and medium-Vth MOSFET are useful?

    each and every device has its own ups and downs based on the need served they are utilised by the public be it high low or standard ones

    books mostly use more prevalent form of any device as they are easier to understand and practically testing them in amateur state is easier and simpler

    low Vth devices the variations are very sensitive to voltage ip at mV levels that makes it not more popular for usage in labs and books for teaching newbies
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    Re: In what applications, low-Vth MOSFET and medium-Vth MOSFET are useful?

    If Vth is low then Vth mismatches will cause more drain current error.


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    Re: In what applications, low-Vth MOSFET and medium-Vth MOSFET are useful?

    Quote Originally Posted by KonstanU View Post
    If Vth is low then Vth mismatches will cause more drain current error.
    yeah true

    in the case of low power MOS we use them as the low Vth is enough to keep the device in on state
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    Re: In what applications, low-Vth MOSFET and medium-Vth MOSFET are useful?

    Quote Originally Posted by Ziqi View Post
    ... So I suspect there are some drawbacks comparing with standard-Vth MOSFET. could you give me a sense?
    Low-Vth MOSFETs in "off" mode actually are still in weak inversion aka subthreshold mode, which means they have substantial leakage currents, considerably higher than standard-Vth MOSFETs.

    So in order to disable circuits for very low stand-by current consumption, they should only be used in series with standard-Vth MOSFETs, which can be disabled to small leakage currents. Hence low-Vth MOSFETs usually aren't left alone between GND & VDD, but as auxiliary devices for current sourcing/sinking with low Vds losses.

    If used as NMOS tail-current source for a degenerated differential NMOS stage, this has the advantage that only a low back-gate Vgb for the differential NMOS stage exists, so allowing for larger gm of this stage.


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