mos current mirror section 9.2 j. ou. a simple current mirror
TRANSCRIPT
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MOS Current Mirror
Section 9.2J. Ou
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A Simple Current Mirror
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Example 1
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Example #2
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Trade-Offs in Current Mirror Design
• Output resistance (1/gds)• CDS• W/L• Current
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IOUT=100 uA
L(um) W(um) GDS (uS) CDS (fF)
2 109.63 51.82 100.39
800n 47.4 56.5 17.13
180n 17.02 92.9 1.079
120n 13.33 147 0.411
For Same IOUT, L↓→W↓→GDS↑(Ro↓) →CDS ↓
Drop in Ro is not desired.
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Use Cascode to Increase output Resistance
Rout is approximately gm3ro3ro2
L1=L2, but L3 need not equal to L2.
Design Criteria: Choose Vb so that VY and VX.
We will learn how to generate Vb in the next class.
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Design Example
VDS of T4 is not matched to VDS of T5. T4 has VDS of 999.1 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
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Add More VGS to Minimize Difference in VDS
VDS5=161.2 mV gives 50 nAVDS4 is 155.7 mV, provides 49.82nA.
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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.
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Property Bias Output Voltage
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Large Output Resistance
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Small Signal Gain