MOS Current Mirror Section 9.2 J. Ou

A Simple Current Mirror

Example 1

Example #2

Trade-Offs in Current Mirror Design Output resistance (1/gds) CDS W/L Current

I OUT =100 uA L(um)W(um)GDS (uS)CDS (fF) n n n For Same IOUT, L↓→W↓→GDS↑(Ro↓) →CDS ↓ Drop in Ro is not desired.

Use Cascode to Increase output Resistance Rout is approximately g m3 r o3 r o2 L1=L2, but L3 need not equal to L2. Design Criteria: Choose V b so that V Y and V X. We will learn how to generate Vb in the next class.

Design Example VDS of T4 is not matched to VDS of T5. T4 has VDS of mV. How come? Answer: VSD=0.2 was used as design criteria for T0 and T2. We need a transistor to absorb the difference between VDS of T4 (which is 0.6 V) and VSD of T2 (which is 0.2V) Bias I: 100 nA

Add More VGS to Minimize Difference in VDS VDS5=161.2 mV gives 50 nA VDS4 is mV, provides 49.82nA.

Need for a Cascode T1 is designed to have a VSD of 0.4, leading to Vout=0.6 V. T3 is designed to have VDS of 200 mV, but it must sustain Vout=0.6 V. So we need to add a transistor to absorb the difference in V.

Property Bias Output Voltage

Large Output Resistance

Small Signal Gain