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What's wrong with this bias circuit?

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wjxcom

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Hi all: I am a fresh man to design OPA, After I reading this two paper
1."Design Procedure for Two-Stage CMOS Operational Amplifiers Employing Current Buffer" Jirayuth Mahattanakul, IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS 2005, November, 2005
2."A Compensation Strategy for Two-Stage CMOS Opamps Based on Current Buffer" IEEE Trans. Circuits and Systs. I,vol.44, pp.257-262, Mar.1997,

I design a OPA in the file opa.pdf which can be found in the attachment, at the same, I design a bias circuit in the file opa_bias_improve.pdf which can be found in the attachment too, but afer I simulating the circuit use Hspice, I found the CurrentBuffer in the opa.pdf i.e. the transistor M6 is in the cutoff region, not in the saturation region. The netlist is in the file use_opa_bias_improve.sp.

At the same time, I found that the Vth of M6 is almost equal to the Vgs of M6, even larger than Vgs of M6, no matter what I change the w/l of the MB4 and MB6 which lie in the file opa_bias_improve.pdf.

After I use the other bias circuit in the file opa_bias.pdf which can be found in the attachment too, afer I simulating the circuit use Hspice, I found the CurrentBuffer in the opa.pdf i.e. the transistor M6 can be in the saturation region. The netlist is in the file use_opa_bias.sp

I do not know why, so I beg help. Help me please, thanx!!!!
 

wjxcom

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Hi all: may be I don't show my doubt, my meaning is if I use the bias circuit in the file opa_bias.pdf, the transistor M6 can work normally, but if I ues the bias circuit in the file opa_bias_improve.pdf, M6 is in the cutoff region, so I want to know why!

Help me please, thanx!!!
 

mince

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One thing that could be causing the trouble is that the gate length of M6 is only 0.6u while the gate length of M11 is 4u. It would be a good idea to have these two transistors having the same Vgs-Vth voltage. Since they will conduct the same current they should be identical in size.

Right now since W/L of M6 is so large compared to M11, the circuit is pulling it into cutoff or triode instead of saturation.
 

hehongyu2001

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vnu=vgsmb10-vdsmb8+vgsmb8
other trace vnu=vgsmb10+vdsmb6
you must satify: vdsmb6=-vdsmb8+vgsmb8
so the w/L of mb6 should be smaller . mb4 will be cut off. so vnu=0
if the w/L of mb6 is large, vdsmb8 is large, mb10 is at triode region.
so vnu= vgsmb8 or so. it is too low.
 

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