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Design >> Analog Design >> question in simulating amp
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Message started by glacieryy on May 11^th, 2006, 10:56am

Title: question in simulating amp
Post by glacieryy on May 11^th, 2006, 10:56am

Hi all,

I have a problem on how to simulate the frequency respoonse of amp in a SC integrator?
When doing this simulation,should I include the CMFB block?The CMFB block is implemented by SC curcuit.

Thanks in advance.

Regards
glacier

Title: Re: question in simulating amp
Post by sheldon on May 12^th, 2006, 7:02pm

Glacier,

It would be best to include the CMFB network in the simulation.
To include the CMFB network, you might try two approaches:

1) Use Spectre's analysis dependent switches to force the dc
operating point with common-mode feedback active and
the ac analysis with the common-mode feedback disabled.

2) Use SpectreRF's sampled pac analysis. Sample the frequency
response when the amplifier is in the normal mode of
operation.

Best Regards,

Sheldon

Title: Re: question in simulating amp
Post by glacieryy on May 12^th, 2006, 9:12pm

Hi Sheldon,
Thanks for your reply.
But may I ask a simple question?I am afraid that I do not understand the first way you have told me.Could you give me something in detail?
Thanks a lot.

Best wishes.
Glacier

Title: Re: question in simulating amp
Post by ywguo on May 15^th, 2006, 7:12pm

Glaicer,

I am not familiar SpectreRF. But I have a simple method. Make a CMFB macro model using R, C, and controlled voltage source, although it is not very accurate.

.SUBCKT CMFB_G7 P1 P1D P1DB P2 P2D P2DB CBIAS CMC CMO VDD VON VOP VSS
R1 CMC CMO1 200E3
R0 CMO2 CMC 200E3
V1 CMO 0 1.65
E1 CMO1 0 VOL='V(cbias)-(V(cmo)-V(VOP))' max=3.3 min=0
E2 CMO2 0 VOL='V(cbias)-(V(cmo)-V(VON))' max=3.3 min=0
C2 VON CMC 125E-15
C0 VOP CMC 125E-15

* END OF SUBCIRCUIT DEFINITION.
.ENDS CMFB_G7

where VOP and VON are the differential output of an opamp, CMO is the bias for common mode voltage, CBIAS is the system bias for tail current, CMC is tied to the tail transistor in the opamp.

Best regards,
Yawei