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  1. #1
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    what's the purposes of those transistors?

    diff.jpg

    Transistors M11, M12 look like diode connected transistor, but inside there are M9, M10. I never seen this before.

    how does VCM ref affect the circuit?

    •   Alt9th January 2017, 04:00

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  2. #2
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    Re: what's the purposes of those transistors?

    It's called a headroom saver, s. here: headroom-saver.png

    The 2 transistors in series provide a higher output resistance than just one of such a current mirror.
    The gate connections of M11 & M12 are responsible for a relative low voltage "consumption" (≈ Vth+Vdssat) of the current mirror, that's why it is called a headroom saver.

    In the case of your Figure 3.10 this connection additionally provides voltage-to-current feedback (similar to a CMFB circuit).

    Vcmref via M13 and the diode-connected M14 supplies the necessary gate voltage for M9 & M10 .



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