NMOS diode-biased cascoded current mirror

From ICclopedia

Schematic Diagram[edit]

NMOS diode-biased cascoded current mirror.svg

SPICE Simulations[edit]

Operating Point Analysis[edit]

Here the generation of our cascode bias is accomplished via the use of thin diode-connected device (M5). The W/L of M5 is chosen to yield the necessary cascode bias in order to equalize the drain voltages of M1 and M2 for best current match (as was done in the case of the NMOS resistor-biased cascoded current mirror).

Operating point DC measurement results (re-formatted for display):

n1 = 0.520
n2 = 0.650
n3 = 1.055
n4 = 0.520
vp = 1.3 
v1#branch =  I1 + I2  =  (-50u) + (-50u)  
v1#branch = -100uA 
v2#branch = -49.981uA
(v2#branch/50uA) = 0.9996

And our relevant transistor' device parameters at the DC OP (re-formatted for display)

device        m5           m4           m3           m2           m1                 
model       nmos         nmos         nmos         nmos         nmos                  
gm    117.497 uS   833.258 uS   836.715 uS   556.736 uS   556.733 uS                 
gds     1.641 MR    23.994 kR    29.214 kR    874.72 kR    874.38 kR                  
id     49.964 uA    50.000 uA    49.981 uA    49.986 uA    49.986 uA                  
vgs     1.055  V      0.535 V      0.535 V      0.520 V      0.520 V                  
vds     1.055  V      0.123 V      0.129 V      0.520 V      0.520 V                  
vth     0.388  V      0.509 V      0.509 V      0.388 V      0.388 V                  
vdsat   0.542  V      0.082 V      0.081 V      0.146 V      0.146 V

As seen given our cascode bias and sizing, we attain equal Vds for both source (m1) and mirror transistors (m2) and hence both branches are matched in performance.

DC Analysis (Sweep)[edit]

For measuring the variation of the mirrored output current under different applied loads. We apply a DC sweep to V2 (our load voltage) from 0 to 1.3V in 0.05V increments.

We are plotting the output current magnitude vs drain voltage. (our load voltage at n2)

NMOS diode-biased cascoded current mirror dc analysis.svg

Monte Carlo Analysis[edit]

In our Montecarlo analysis, we are measuring the effect of transistors’ random mismatch on the output current of our mirror (i.e. how small random variations in individual transistor parameters when added together can result in an overall output current error).

Further details concerning Montecarlo simulation settings have been described under the NMOS resistor-biased cascoded current mirror article and also apply here.

NMOS diode-biased cascoded current mirror mc analysis.svg


As compared to the canonical resistor-biased cascoded mirror, the use of a thin-diode to bias our cascode allows us to make do without any resistors and hence minimize area. However, the use of a thin-diode in place of a resistor-bias does come at the expense of an additional current branch and also a larger variation of the cascode bias over PVT and montecarlo. Hence in summary, with a diode-biased cascoded current mirror typically you can attain lower area at the cost of performance and larger a current consumption (although it really depends on the specific application).

For many low-supply/headroom applications, the diode-biased topology can be a good choice to attain reduced area -- where a bias resistor may otherwise prove too large.

For the cascoded current mirror, we can make the following observations

  • The absolute minimum voltage needed for the mirror output branch to be in the sat region is the vdsat of both mirror and cascode devices: vmin_abs = 0.146 + 0.081 >> 0.227
  • However as shown with the current sizing, the mirror requires at least ~0.5 load voltage to yield its maximum Rout. (seen from the plot above where the slope is linear).
  • Error Measurement: We see a variation of 49.92uA to 49.98uA over a linear operating range of 0.5 to 1.3V. This is equivalent to an error of 66.1nA or 0.132% relative to our reference. (keep in mind this is only for the typical corner, the real performance variation should be assessed over PVT where the cascode bias variation can be measured).

Figures of Merit[edit]

Output Resistance Rout: 18.911MΩ (measured from 0.5 to 1.3V linear range).

Compliance Voltage Vmin: ~0.5V (from ground)