deciding channel for current mirror

[Sambhav_1]

Hi,

How do we decide the channel length of the diode connected device in designing current mirror? Do we decide it in such a way that we will get minimum variation in current across PVT or while mirroring the current in other branch?

 

[crutschow]

How do we decide the channel length of the diode connected device in designing current mirror?

Depends upon the ratio of the controlling current to the mirror output current.

 

[Sambhav_1]

Depends upon the ratio of the controlling current to the mirror output current.

so you mean that we should increase the length of the device till we get the accurate copy(or ratio) of current?

 

[martinnl]

Assuming you have a simple current mirror you should start with two things:

1. Keep the Veff in a reasonable range. This sets the transistor region and you want to be in saturation to achieve good matching and high immunity to variations.

2. Keep the Vds the same for the diode and the output transistor. What do you expect at the drain of the output transistor? If you expect a fixed 1 V make sure the diode has 1 V on the drain as well.

If you have a varying voltage on the output, the output voltage is very low etc. You have to look in to more advanced current mirror topologies.

Also check how sensitive you really are to changes in the output current, does it really have to be super accurate?