PMOS differential amplifier with current mirror load

Discussion in 'Homework Help' started by elnovato, Jul 29, 2013.

  1. elnovato

    Thread Starter New Member

    Jul 29, 2013

    As shown in the attached circuit, I have to calculate the current I_out of a differential amplifier with current mirror load where the output is fed back with M5 to one input.

    Given are the transconductances of all transistors (where g_m1 to g_m4 are all equal), g_DS2 and g_DS4 (I suppose it's the drain to source conductivity) and the voltages V_ref and V_dd.

    I have derived the small signal amplification but I doubt this is useful in that case. I suspect that the feedback forces the gate of M2 to V_ref, but I'm not sure.

    Any hints are highly appreciated.
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    To find Iout current you can use a small signal analysis and find the amplifier voltage gain.
    Treat Vref as a Vin voltage and Vout as a voltage across R resistor. Also treat M5 as a source follower (common-drain) amplifier. So you will end up with Vout voltage across R resistor finally and Iout = Vout/R.
  3. elnovato

    Thread Starter New Member

    Jul 29, 2013
    Thanks for your answer, I'll will try that.
  4. WBahn


    Mar 31, 2012
    Are you tasked with finding the nominal output current, or the actual output current given non-ideal devices? The former is very simple: what does the gate voltage on M2 need to be to make the four devices on the left (M1-M4) happy? The latter is hard to do unless you know the impedance of the load connected to the output.

    Now, if you are tasked with finding the output impedance of this current source, that is a difference matter.