Thanks to Tranlee and Vamsi Mocherla.You are right! In order to drive low resistance,so the scale is very big!But the bigger is ,the better the performance!I think the scale is adjustive! So,I wonder that I may optimize the ratio between the scale of the two output transistors!(I think this is the key pont,for the sttucture is decided) By the way,in this case ,the gate vovltage of the two is campatible with their threshold and just to be sure of the low current!
There I hesistate that if I modify M21 and M24 to reduce the gate voltage of the output transistor, the current in the output satge will be handreds uA,the full stability will not be guaranteed especial for the technical corner!
Thanks to dumbfrog, in this figure,M21 and M24 are just for the current control.In such case ,the Mos's must be in saturation regime!I think in this case the gate voltage of the output transistors is limited if low current is needed!
Thanks to all!
If I don't alter the structure,So,how to optimize it?