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# How to design W/L of an OP(given circuit)

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#### tongkin

##### New Member
Greeting,

I would like to design W/L of every transistors. How could I start? If the Iref = 15uA

Thank you

#### crutschow

##### Well-Known Member
The W/L ratio is used in the design of integrated circuit transistors. It affects the gain, capacitance, and current capability of the transistor. If you are doing that type of design, then you need the Spice model parameters for the particular integrated circuit fabrication process you will use to build the IC and then perform Spice simulations of the circuit to optimize the W/L ratios for each transistor.

#### BrownOut

##### Banned
Decide what your terminal voltages and current requirements are, then use iD=un,p*Cox*W/L(vGS-Vt)^2. Or use the more general equation, which I can't remember at this time.

#### tongkin

##### New Member
The W/L ratio is used in the design of integrated circuit transistors. It affects the gain, capacitance, and current capability of the transistor. If you are doing that type of design, then you need the Spice model parameters for the particular integrated circuit fabrication process you will use to build the IC and then perform Spice simulations of the circuit to optimize the W/L ratios for each transistor.

I have a Spice simulation with a fabrication process.

#### tongkin

##### New Member
Decide what your terminal voltages and current requirements are, then use iD=un,p*Cox*W/L(vGS-Vt)^2. Or use the more general equation, which I can't remember at this time.

thanks,

How about Q1 and Q2, how could I start to design?

because the Vgs of Q1 and Q2 is unknow

#### BrownOut

##### Banned
Quiescent current through Q1 and Q2 will be I/2. Solve the equation above to get an expressoin for Vgs.

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