Need Help:Voltage reference

[leonken]

I am designing a simple power supply independent voltage reference.
a single diode-connected mos transistor biased by a current source.
The current source is a widlar current source consist of 4 mos transistors.
The problem is the output reference voltage is power supply voltage dependent. The simulated reference voltage is about 400mV.
When the power supply voltage vary from 3V to 5V, the reference voltage also changed about 40mV.
Has anybody have some good ideas to stabilized the reference voltage ?
for instance, variation range only several mV versus power supply.
Thank you

 

[terryssw]

your circuit sensitivity is already quite low for such very simple voltage reference (Vdd change 2000mV to vout change 40 mV, 2% only). This is due to the finite output resistance of the current source. Try cascode one or increase the channel length.

 

[sunking]

An other method is add the longth of L about MOS Tr.

 

[leonken]

I selected the 0.35um process.What is a suitable length for me ?
E.X. L=50um or 30um .

 

[maxwellequ]

The output resistance of the MOS transistors increases considerably until 2 or 3 um. From that point forward as you increase the L, your rds will not increase MUCH more (it will keep increasing but not much).

My advise to you is that you try it (keep the same W/L in the MOS transistors and you increase W). The best solution would really be to use cascodes.