1. Analog Integrated Circuits Laboratory
Current Mirror
Hong-Yi Huang
Nano Integrated Circuits and Systems Lab.
Graduate Institute of Electrical Engineering
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2. Outline NMOS Current Source Basic NMOS Current Mirror
Cascode Current Mirrors
Improve Current Mirror
High-Swing Cascode Current Mirror
Wilson Current Mirror
Low-Voltage Current Mirror
Experiment Steps
Questions
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3. NMOS Current Source For NMOS get into saturation region：
VGS ≥ Vt, VGD ≤ Vt
VGD = VBIAS – VDS ≤ Vt , we get VDS ≥ VBIAS-Vt
Define VBIAS-Vt is VMIN
So VDS ≥ VMIN is a Current Source
K=dimensional factor
NCH=doping concentration
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4. NMOS Current Source Let VBIAS = 0.8V VMIN=VBIAS – Vt ≒ 0.4V

5. Basic NMOS Current Mirror
In fact, in analog IC design, using Current Mirror (as seen in the figure above) to get current source from a current reference is a typical method.
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6. Basic NMOS Current Mirror _ Second Order Effect
If we take Short Channel Effect into analysis ：
From the formula, if we want to reduce the Short Channel Effect, increase the channel length will increase rO, so that the current source become more ideal.
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7. Basic NMOS Current Mirror _ Second Order Effect
If we increase the channel length then we get a more constant current source.
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8. Cascode Current Mirror
Use cascode structure to enhance ro.
Vo = Va + Vc = Va + ro3[ Io + gmb3Va +gm3Va ]， and Va = Io × ro1
Vo = Io[ ro1 + ro3 + gmb3 × ro1 × ro3+ gm3 × ro1 × ro3 ]，
We get ro = Vo / Io = ro1 + ro3 + gm3 × ro1 × ro3 × (1+η3)；
Where η3 = gmb3 / gm3
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9. Cascode Current Mirror
Now we analyze the VMIN of Cascode Current Mirror
VDS(sat) = VGS – Vt，
VGS = VDS(sat) + Vt ，
and VR = 2VGS = 2VDS(sat) + 2Vt；
Vo = VR - Vt；
We get Vo = 2VDS(sat) + Vt = VMIN
In this case, VMIN ≒ 2 × = 0.7V
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10. High-Swing Cascode Current Mirror
In order to reduce VMIN，adjust the Channel Length of MN4 as four times to the other NMOS，according to the current formula in saturation：
(VGS4 – Vt) = 2(VGS2 – Vt) = 2VDS(sat)，so VGS4 = 2VDS(sat) + Vt
we get Vo = VGS4 – Vt
Vo = (2VDS(sat) + Vt ) – Vt = 2VDS(sat) = VMIN
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11. High-Swing Cascode Current Mirror
From the simulation, we can see that the VMIN of High-Swing Cascode Current Mirror is smaller than Cascode Current Mirror.
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12. Wilson Current Mirror
From the small signal analysis we get the output resistance to be
ro = = Vo / Io = ro3 + ro2 +[ ]
if we increase the ro3 means we can increase output resistance obviously.
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13. Wilson Current Mirror
We increase the width of MN3 in order to increase the output resistance, from the simulation, the larger width produce smaller slope current curve, means we increase the output resistance.
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14. Low-Voltage Current Mirror
Now, we introduce a Low-Voltage Current Mirror.
From the small signal, the output resistance：
ro = Vo / Io = 1 + ro1(1 + gm3ro3 + gmb3ro3) .
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15. Low-Voltage Current Mirror
From above-mentioned, we can increase the output resistance ro by increasing the width of MN3、MN4.
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16. Experiment Steps
Use a single MOS as a current source as page 3, simulate output current ID. Try to increase the channel length to see what change in ID.
Use a simple type current mirror as page 5, get the output current Io from the reference current IREF. Try to increase the channel length to see what change in ID.
Use a cascode current mirror as page 8, get the output current Io from the reference current IREF. Try to increase the channel length to see what change in ID.
Use high swing cascode current mirror as page 10, get the output current Io from the reference current IREF. Try to increase the channel length to see what change in ID.
Use wilson current mirror as page 12, get the output current Io from the reference current IREF. Try to increase the channel width of MN3 to vary gm3 and to see what change in ro.
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17. Steps
Use low voltage current mirror as page 14, get the output current Io from the reference current IREF. Try to increase the channel width of MN3 to vary gm3 and to see what change in ro.
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18. Questions
Assuming IREF=100uA, how can we use IREF to generate 500uA、300uA current source (sink) respectively ?
An advantage of the High-Swing Cascode Current Mirror is that its VMIN is 2VDS(sat), but what is the drawback for this structure?
Can you describe the voltage operation range of a Low-Voltage Current Mirror?
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