CMOS Vgs Self-Referenced Current Source

This is a CMOS constant current source using the gate-to-source voltage (Vgs) of a MOSFET as a reference. A reference current is generated through Vgs (also Vg) of a reference MOSFET and through a resistance from the gate to source (also ground). The resulting current is mirrored to two branches, one replica is fed back as the drain current of the reference MOSFET and the other copy is fed to the output. The output current is then Iout = Vgs / Riset = Vgs / (Rilim + Riadj) Ignoring body effect and neglecting channel length modulation the MOSFET characteristic equation is Id = ½ µn·Cox (W/L)(Vgs - Vt)² In Multisim KP = µnCox Id = ½ KP (W/L)(Vgs - VTO)² For the saved circuit the reference MOSFET Q1 have threshold voltage of 0.75 V, all other MOSFETs retain their default parameter values. It is desired to have an output current of 50 µA. Vt1 = 0.75 V = VTO1 Iout = Iset = 50 µA = Id1 = Id2 = Id3 = Id4 = Id5 Proceeding, a convenient gate-to-source (Vgs) reference voltage should be determined first, in the saved circuit this is equal to 1.75 V. Vref = 1.75 V = Vgs1 With this data the (W/L) ratio for Q1 can now be determined (W/L)1 = 2Id1 / KP1 (Vgs1 - VTO1)² (W/L)1 = 2(50 µA) / (20 µA/V²) (1.75 V - 0.75 V)² (W/L)1 = (100 µA) / (20 µA/V²) (1 V)² (W/L)1 = (100 µA) / (20 µA/V²) (1 V²) (W/L)1 = 5 The 100 µm width of Q1 is retained and the length becomes 20 µm. The value of the current setting resistor (Riset = Rilim + Riadj) is simply Riset = Rilim + Riadj = Vgs1 / Iout Riset = 1.75 V / 50 µA Riset = 35 kΩ 30 kΩ is alloted to Rilim and a 10 kΩ variable resistance is assigned to Riadj. The variable resistance has a saved position of 50% (50 kΩ) which results to an exact output current of 50 µA.