Reference current generator Circuit for current mirror

Discussion in 'Analog & Mixed-Signal Design' started by Babun Pal, Jul 22, 2016.

  1. Babun Pal

    Thread Starter New Member

    Jul 13, 2016

    In the above schematic attached, I have calculated rin and Iref and both the parameters are the same for fig (a). and fig (b).
    rin = R + I/gm and Iref = (Vdd - Vth)/ R.

    Could you please clarify me, which way of connecting the circuit is correct and why?

    Thank you.

    Kind regards.
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    Where is the current mirror? If you plan to connect it to the gate, than B is obviously a bad idea.
  3. Babun Pal

    Thread Starter New Member

    Jul 13, 2016
    Could you please explain me why fig.b is not a good idea for current mirror?

    Thank you.
  4. crutschow


    Mar 14, 2008
    If you draw the rest of the current-mirror you will see why.
  5. ci139


    Jul 11, 2016
    with fets - i guess - the both work while fig.a has a U.g setting voltage drop on R , it's mirrors´ current (I.d.Q.mir) would be similar to I.d.Q.ref
    then on the other hand the fig.b acts as a voltage ref (setting it´s U.g - though it requires some additinal R.d to work in practice as voltage ref) - so in the latter modified case you have U.g const that deafines for its mirror a similar I.d.Q.mir
    !!! but both (fig. a & b ) work only where U.g to GND (U.g.Q.ref to U.s.Q.mir) falls the non saturated linear region of Q.mir (has small or no resistance between it's source and GND !!!)

    the type of voltage mirrors work fine only for near fixed supply voltage and I. or U. ref being const

    if you need a true mirror you have to buold a complex amplifier
    - where each replicated node has it's own control/feedback/adjustment
    -- basically an input(eighter I or U)-controlled set of current sources

    if you dont need so true mirror - you can use your figures a,b @ the mirrors linear region.
  6. Bordodynov

    Well-Known Member

    May 20, 2015
    fig.a Iref = (Vdd - V1)/ R.
    fig.b Iref = (Vdd - V2)/ R. Vth<V1<V2 !!! Vth1<Vth2 (influence of the substrate).