PMOS resistor-biased cascoded current mirrors
Operating Point Analysis
Operating point DC measurement results (re-formatted for display):
n1 = 0.773 n2 = 0.650 n3 = 0.273 n4 = 0.771 vp = 1.3 v1#branch = 50 uA v2#branch = -50.0 uA (v2#branch/v1#branch) = 1.00
And our relevant transistors' device parameters at the DC OP (re-formatted for display):
device m3 m2 m1 model pmos pmos pmos gm 767.124 uS 445.368 uS 445.348 uS rds 12.168 kR 896.15 kR 892.41 kR id 50.0 uA 50.002 uA 50 uA vgs 0.498 0.527 0.527 vds 0.121 0.529 0.527 vth 0.440 0.337 0.337 vdsat 0.111 0.198 0.198
As seen given our close Vds for both source and mirror transistors, both branches are very well matched in performance.
DC Analysis (Sweep)
For measuring the variation of the mirrored output current under different applied loads. We apply a DC sweep to V2 (our load voltage) from 1.3V to 0V in 0.05V increments.
We are plotting the output current magnitude vs drain voltage.
Monte Carlo Analysis
In our Montecarlo analysis, we are measuring the effect of transistors’ mismatch on the performance of our current mirror i.e. how small variations in individual transistor parameters when added together can affect the performance of the overall circuit (please refer to the NMOS counterpart article for additional setup details for this simulation).
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 = 0.111 + 0.198 >> 0.309
- However as shown with the current sizing, the mirror output can go up to maximum 0.8V (at least ~0.5 vload voltage relative to vp) to yield its maximum Rout. (seen from the plot above where the slope is linear).
- Error Measurement: We see a variation of 49.926uA to 50.009uA over a linear operating vload range of 0.5 to 1.3V. This is equivalent to an error of 83nA or 0.166% relative to our reference.
Figures of Merit
Output Resistance Rout: 9.639MΩ (measured from 0.5 to 1.3V linear range).
Compliance Voltage Vload min: ~0.5V (from VP)
- Chapter 3 (pages 3-7)
- ICclopedia Toolchain.
- PTM 130nm CMOS SPICE models.