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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. For Same IOUT, L↓→W↓→GDS↑(Ro↓) →CDS ↓. Drop in Ro is not desired.

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Mos current mirror

MOS Current Mirror

Section 9.2

J. Ou

Trade offs in current mirror design
Trade-Offs in Current Mirror Design

  • Output resistance (1/gds)

  • CDS

  • W/L

  • Current

I out 100 ua
IOUT=100 uA

For Same IOUT,

L↓→W↓→GDS↑(Ro↓) →CDS ↓

Drop in Ro is not desired.

Use cascode to increase output resistance
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.

Design example
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

Add more vgs to minimize difference in vds
Add More VGS to Minimize Difference in VDS

VDS5=161.2 mV gives 50 nA

VDS4 is 155.7 mV, provides 49.82nA.

Need for a cascode
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.