ethan
|
Hi everybody,
When I built a test bench for my differentail fold-cascode OTA according to figure 6 in Ken's paper, I added two load capacitors right before the output Balun and disconnected two voltage sources (Vod and Voc) to leave output Balun "d" and "c" two terminals floating (since I thought there is no buffer in my OTA, it can only drive capacitance).
Question 1: for open loop gain simulation:
According to figure 6 of Ken's paper, I set one input sources (vsource) Vid with AC magnitude 1v, plus I added a small signal amplitude for transient parameter. Then, for another input source (vource), I set (in the first case) Vic with "DC voltage" =2.5v (which is VDD/2), I found the open loop gain is about 5500, plus perfect common-mode level for all inputs and outputs and perfect peak-to-peak swing for transient; in the second case, I set Vic with all zeros ("DC voltage"=0 or none), then the open loop gain is only 4800 (and the common-mode level for the output is not Vdd/2, it sits at zero level). In the third case, which is the traditional way, I directly attached vsources to differential inputs of the OTA and get the differential outputs AC results. In this way, I found the open loop gain is 3700.
Can anyone give me help to solve these conflictions? I am 100% sure the results and the measurments I did, since I have done several times for each case and couldn't figure out why.
Question 2: for closed-loop gain and loop gain, CMFB stability
Since my OTA is single stage without buffer, I only can use the configuration which is shown Ken's book --The Designer's Guide to Spice and Spectr, on page 119, figure 3.41 (with capacitive feedback for the fully-differential CMOS opamp). Am I right? But I am not quite understanding that, in that figure, there are two "isource". What's the their function in that configuration?
Plus, how can I measure Loop Gain by using either the structure (on book) or that figure 6 (on paper)?
Third, how can I check the CMFB stability based on these test benches?
I couldn't figure out the ways how to do these. Please excuse me for my retard.
Question 3:
If I use SC CMFB for my OTA, can I still use the test bench shown in figure 6 on paper to do PSS and PAC, PXF, PNOISE (or either use capacitive feedback test bench and do those)?
Please guide me. I really appreciate your help.
Ethan
|