PMOS 3T resistor-biased cascoded current mirrors

= Schematic Diagram =



= Circuit Netlist = * dev * ---                                                                I1    n_pos n3      50uA V1   n_pos 0       1.3V V2   n2    0       0.65V R1   n3    n1      10.7kΩ M1   n1    n1 0  0 nmos W=20u L=3u M2   n4    n1 0  0 nmos W=20u L=3u M3   n2 n3 n4 0    nmos W=10u L=0.35u

= SPICE Simulations =

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).



= Results =

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)

= References =


 * Designing Analog Chips (Hans Camenzind)


 * Chapter 3 (pages 3-7)


 * Project files.

= Toolchain =


 * ICclopedia Toolchain.


 * PTM 130nm CMOS SPICE models.