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hi, i've seen this circuit in a book talking about current-mode design longtime ago
if i'm right, the R here is to increase the bandwidth, the pole will change and it also add a zero.
you can calculate the small signal transfer func from the input current to the voltage of the M2 gate
i think it should work, or maybe you can set it to the voltage of the net57. if it still not work, then maybe you should check the bias for these transistors
did you try setting Vdc to half the supply instand of zero?
the feedback will force the output to follow the input, so if you set the input to zero, the output will also be closed to zero volt, and since this cascode only have limited output swing, the nmos in the cascode won't work anymore..
did you check the region of the cascode? (since you have mentioned the output of the aux amp is fine)
i still think you have to match the vbias for the I15...
both the vbias and I15 are going to set the vgs of the M46. how do you match them?
and maybe the value of the L,C should be more larger to set the complex pole/zero couse by the LC to smaller frequency
for this inverting OPAMP, the feedback factor is Rin/(Rin+Rfb), actually the Vo/Vin=-A*B1/(1+A*B2)
A is the gain of your OPAMP,
B1=Rfb/(Rin+Rfb),
and B2=Rin/(Rin+Rfb)
and if you chang the A to A(s) as a one pole system A/(1+s/po), you will see the close loop gain is...
i guess what the stb simulation does is for the loop-gain, which will include the feedback factor. so the amplitude response in the bode plot will actually move up or down according the feedback factor, and the unity gain bandwidth changes..
hi, i've tried some simulation (sweep the Vgs for single transistor), and it seems that the Early voltage (just calculated from id/gds)will decrease when in weak inversion, however the gds will increase.
Re: Common-mode Gain
i dont know how you build the cm testbench,
but for the LC lpf method to test the open loop gain, actually there will be a inverse-U shape, i recall this from the book of 'The Designer's Guide to Spice and Spectre' .
if you change your opamp to a simple one-pole system...
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