why these transistor are in the cutoff region?

transistor in cut off region

Hi all: I design a ClassAB Output OPA( can be found in the attachment). After the AC simulation, I found that the transistor MNA, MNB, MPA, MPB are in the cutoff region!

I do not know why these transistor are in the cutoff region? may be the cmfb circuit is not right?

Help me, please. thanx!!

The full schematic and the netlist of the OPA can be found at the attachment:
the balun.pdf can be used to AC simulation;
the classab_output.pdf is the MAIN OPA;
ClassAB_OUTPUT_ac.pdf is the circuit which can perform the AC simulation.
classab_output_ac.sp is the netlist which used to perform AC simulation;
the cmfb_bias.pdf and cmfb_switch.pdf can server as the cmfb circuit;
the csmc.lib is the library file of the OPA.
the inv.pdf is a inverter, it is used in the transient simulation.
the opa_bias_improve_2.pdf is the bias circuit;

First, I perform transient simulation, and save all of the operating point of the circuit, then I perform AC analysis and I found the transistor MNA, MNB, MPA, MPB are in the cutoff region. It is so strange!

cmos cutoff region

Hi all: may be nobody know the reason? this circuit can be found in the paper "A Compact Power-Efficient 3 V CMOS Rail-to-Rail Input/Output Operational Amplifier for VLSI Cell Libraries" and I modify the circuit to full differential OPA, as shown in the attachment!

Help me please, thanx!!

cutoff in relation to a transistor.

Dear Wjxcom,

If you know which transistor is in cut-off region, do you can't analysis its reason?
Just analyze its current and voltage.
It is complex to see your circuit, would you please attach it in PDF format?
Good luck

Regards,
Philip

wjxcom said:

Hi all: may be nobody know the reason? this circuit can be found in the paper "A Compact Power-Efficient 3 V CMOS Rail-to-Rail Input/Output Operational Amplifier for VLSI Cell Libraries" and I modify the circuit to full differential OPA, as shown in the attachment!

Help me please, thanx!!

Click to expand...

cutoff region transistor

Hi philipwang: I do not know why these transistors are in cutoff region.
At the same, I have already attach the file in pdf fromat, the name of this file is classab_output.pdf.

thanx you!!

determining if transistor is in cut-off region

For full differential OPA design, CMFB is important. Generally speak we should pay attention to:
1. CMFB bandwidth should not limit main amplifier bandwidth;
2. Capacitor should be selected carefully if capacitor is used.
Advise:
Use ideal CMFB replacing your CMFB to check whether problem coming from CMFB

philipwang said:

Dear Wjxcom,

If you know which transistor is in cut-off region, do you can't analysis its reason?
Just analyze its current and voltage.
It is complex to see your circuit, would you please attach it in PDF format?
Good luck

Regards,
Philip

wjxcom said:

Hi all: may be nobody know the reason? this circuit can be found in the paper "A Compact Power-Efficient 3 V CMOS Rail-to-Rail Input/Output Operational Amplifier for VLSI Cell Libraries" and I modify the circuit to full differential OPA, as shown in the attachment!

Help me please, thanx!!

Click to expand...

Click to expand...

Added after 11 minutes:

Sorry, no rar extract software in my computer since software copyright issue. Faint!

wjxcom said:

Hi philipwang: I do not know why these transistors are in cutoff region.
At the same, I have already attach the file in pdf fromat, the name of this file is classab_output.pdf.

thanx you!!

Click to expand...

Hi philipwang: I use ideal cmfb to replace the switch-capacitance cmfb circuit. After the simulation, I find the result with the ideal cmfb has no differential with the result with the -capacitance cmfb circuit. So I think the cmfb circuit has no problem!

thanx you!

Hi,Wjixcom,
Generally speak class_AB OPA is used as output buffer, it mainly handle "large" signal, so analysis in time domain is more important than ac. "small" signal analysis, is it right? These cut-off transistors limit the gain of differential input and differential output amplifier seriously?

Regards,

wjxcom said:

Hi philipwang: I use ideal cmfb to replace the switch-capacitance cmfb circuit. After the simulation, I find the result with the ideal cmfb has no differential with the result with the -capacitance cmfb circuit. So I think the cmfb circuit has no problem!

thanx you!

Click to expand...

Hi philipwang: you are right! and if these transistors are in cut-off region, the gain of the OPA is very seriously, less than 1dB!!

thanx you!

Added after 5 hours 39 minutes:

This circuit drives me crazy!
I think in steady, the current I=40u through Q6 and Q8 is divided between MPB and MNB, so MPB and MNB carry a current equal to 20u. and the w/l of MNB is equal to MB2; at the same, the w/l of MPB is equal to MA3.

But why MPB and MNB are in the cutoff region while MB2 and MA3 are in saturation region??

Help me please, thanx!!