Why is the output current not equal to the reference current in this current mirroring circuit? (I have been explicitly told that they are not equal). Both of the transistors are equal and in the saturation region. I thought that the voltage applied to the drain of the second MOSFET (Q4) did not alter the output current as long as it remained in saturation?

Current Mirror


As a hint, what effects in a MOSFET cause the channel current to depend on the drain-source voltage, even in saturation mode?

  • \$\begingroup\$ Yeah this makes perfect sense, I guess I wasn't looking at it as literally as I was supposed to. When considering channel length modulation, the drain voltage is always going to have a small impact on the drain current, regardless of gate voltage correct? \$\endgroup\$ – amateurhour Oct 26 '20 at 1:00
  • \$\begingroup\$ @ThePhoton can you improve this answer? \$\endgroup\$ – Voltage Spike Oct 26 '20 at 3:18
  • \$\begingroup\$ @VoltageSpike, as this appears to be a homework (or homework-like) question, I prefer to give a hint in the direction of an answer rather than a complete answer. \$\endgroup\$ – The Photon Oct 26 '20 at 3:25
  • 2
    \$\begingroup\$ @ThePhoton - is there a reason you put this as an answer and not a comment? \$\endgroup\$ – Puffafish Oct 26 '20 at 8:44
  • \$\begingroup\$ @Puffafish, because the comments area is for helping the OP improve the question, not for answering or leading them to an answer. \$\endgroup\$ – The Photon Oct 26 '20 at 16:10

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