I'm desingning an operational amplifier, and I'm implementing a current source with a MOS transistor M1.
All the projects that I have seen have a small length to M1, probably due to the matching, I think.
But if I have a big L to M1, I have a greater impedance to the current source, and this is good. What
should I do??? Why dont use a big L???
Thanks

The M1 is the current source, as shown in the figure.
Why should I use a small L to it?? Why don’t use a big L to M1 and MB?

Your M1 current source should have a large L for better output impedance. However, if you make L of M1 large you limit your input common-mode range (since now M1 has a large vdsat, your PMOS inputs will need to have a lower input common-mode for M1 to be in saturation). The high output impedance of M1 will improve your input common-mode rejection (CMRR). So, it is a trade-off between input common-mode compliance (i.e. range) and CMRR.

-Sid

Monkeybad wrote on Aug 2^nd, 2007, 7:04pm:

Yes, you are absolutely right. The drain-source current is proportional to W/L and the output resistance to 1/L, so by increasing W and L with the same factor you can achieve a higher output resistance. But a larger transistor also has higher capacitances, so the effective output capacitance of your current source will increase. If you do not expect to have any voltage swing at the output node of the current source, then the extra capacitance will not matter. But if you expect large voltage swings at high frequenies, it might actually degrade the output impedance. A larger transistor also takes up more area which might be valuable. Summarizing you can say that, increasing W and L with the same factor for the output transistor in this current source results in higher output resistance, larger area and larger output capacitance.