Analog Integrated Circuits Laboratory Current Mirror Hong-Yi Huang Nano Integrated Circuits and Systems Lab. Graduate Institute of Electrical Engineering Current Source

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 Current Source

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 Current Source

NMOS Current Source Let VBIAS = 0.8V VMIN=VBIAS – Vt ≒ 0.4V

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. Current Source

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. Current Source

Basic NMOS Current Mirror _ Second Order Effect If we increase the channel length then we get a more constant current source. Current Source

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 Current Source

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 × 0.15 + 0.4 = 0.7V Current Source

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 Current Source

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. Current Source

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. Current Source

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. Current Source

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) . Current Source

Low-Voltage Current Mirror From above-mentioned, we can increase the output resistance ro by increasing the width of MN3、MN4. Current Source

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. Current Source

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. Current Source

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? Current Source