Current mirror question

Discussion in 'General Electronics Chat' started by dMoser, Jan 3, 2012.

  1. dMoser

    Thread Starter Member

    Aug 25, 2011
    30
    1
    Hi,

    In a MOSFET-based current mirror, what ensures that M2 is also saturated?
    As IOUT equals IREF only if M2 is saturated.

    Thanks.

    [​IMG]
     
  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,686
    2,748
    I think your analysis is incorrect. Both devices must be operating in their linear regions, which means *not* saturated.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,018
    3,235
    Since the drain-source resistance of a MOSFET operated in the saturation region (the near horizontal portion of their characteristic curves) is very high, the two transistors (if matched) will have close to the same drain current, even if the drain voltage on one is different from the other. Of course the drain voltage on M2 must be large enough so that it is in the saturation region as is the drain current of M1.

    Note to joeyd999: The devices are indeed being operated in the saturation (flat) region, not the linear (sloped) region of the transistor characteristic curves. The saturation region of a MOSFET is defined somewhat differently from the saturation region of a bipolar transistor (for some obscure reason).
     
    Last edited: Jan 3, 2012
    joeyd999 likes this.
  4. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,686
    2,748
    You are correct. MOSFET saturation is when the Drain current is constant regardless of Vds, and dependent upon Vgs. Shootin' from the hip there, sorry.
     
Loading...