NMOS diode-biased cascoded current mirror
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).
The W/L ratio of the thin diode-connected device (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 3T resistor-biased cascoded current mirror).
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.
As before our cascoded rout is approximately given by:
r_out = ro2 x a3 = ro2 x (gm3 x ro3)
Note: the bias current I2 (used for the thin diode to generate the cascode bias) is typically just a mirrored version of the main bias current I1.
Schematic Diagram[edit]
Circuit Netlist[edit]
* dev <nets> <values> * ----------------------------------- I1 vp n1 50uA I2 vp n5 50uA V1 vp 0 1.3V V2 n2 0 0.5210616V M1 n3 n1 0 0 nmos W=20u L=3u M2 n4 n1 0 0 nmos W=20u L=3u M3 n2 n5 n4 0 nmos W=10u L=0.35u M4 n1 n5 n3 0 nmos W=10u L=0.35u M5 n5 n5 0 0 nmos W=4u L=3u
SPICE Simulations[edit]
Operating Point Analysis[edit]
Operating point DC measurement results (re-formatted for display):
n1 = 0.521 n2 = 0.521 n3 = 0.267 n4 = 0.267 n5 = 0.740 vp = 1.3 v1#branch = I1 + I2 = (-50u) + (-50u) v1#branch = -100uA v2#branch = -49.98uA (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 240.279 uS 859.945 uS 859.945 uS 552.793 uS 552.793 uS gds 1.261 MR 150.73 kR 150.73 kR 215.65 kR 215.65 kR id 49.989 uA 49.980 uA 49.980 uA 49.985 uA 49.985 uA vgs 0.740 V 0.473 V 0.473 V 0.521 V 0.521 V vds 0.740 V 0.255 V 0.255 V 0.266 V 0.266 V vth 0.388 V 0.453 V 0.453 V 0.388 V 0.388 V vdsat 0.310 V 0.079 V 0.079 V 0.147 V 0.147 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)
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 3T resistor-biased cascoded current mirror article and also apply here.
Results[edit]
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 = 0.147 + 0.079 >> 0.226
- However as shown with the current sizing, the mirror requires at least ~0.4 load voltage to yield its maximum Rout. (seen from the plot above where the slope is linear).
- Error Measurement: We see a variation of 49.97326uA to 49.99395uA over a performance operating range of 0.4 to 1.3V. (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: 43.50MΩ (measured from 0.4 to 1.3V linear range).
Compliance Voltage Vmin: ~0.4V (from ground)
In summary,
| Topology | Vmin (V) | Compliance Voltage (V) | r_out (kΩ) | Bias Current (uA) | Area (um^2) |
|---|---|---|---|---|---|
| Simple NMOS | 0.226 | 0.35 | 869.17 | 50 | 20 |
| NMOS Source Degenerated | 0.3815 | 0.45 | 1521 | 50 | 20 + 2 x A_r3k |
| NMOS 3T Res-biased Cascoded | 0.221 | 0.5 | 18911 | 50 | 123.5 + 1 x A_r10p7k |
| NMOS 4T Res-biased Cascoded | 0.226 | 0.4 | 43605 | 50 | 127 + 1 x A_r4p4k |
| NMOS Diode-biased Cascoded | 0.226 | 0.4 | 43500 | 100 | 139 |
Please keep in mind, the area figures for the diode-biased cascoded example above are not considering the area of devices needed to mirror the I1 to I2 current
References[edit]
- Designing Analog Chips (Hans Camenzind)
- Chapter 3 (pages 3-8)
Toolchain[edit]
- ICclopedia Toolchain.
- PTM 130nm CMOS SPICE models.
