why fc does decrease with increasing R anymore

[chichi]

i am designing a amplifier with an opamp,i am using the sub-threshold mos transistor and capacitor feedback to generate a low cut off frequency for the amplifier,as fc are proportional to 1/RC,i modify gate voltage to mos transistors,therewith the high mos resistance is controlled ,and the fc is further on controlled,but the problem is that the fc stop decreasing with increasing resistance at about 1Hz,and a smaller fc can not be achieved anymore,can anybody answer that question?

 

[dick_freebird]

Perhaps there are minimum conductances in the
matrix, that make the MOS channel "bypassed" at
higher than that "effective impedance"? Do not know
if the DC "gmin" effect on operating point, is carried
forward into the AC analysis. You might try raising
that (or similar convergence-mode conductance) and
resimulate with (say)gmin=1E-15 instead of the usual
1E-12.

What is the channel on resistance at 1Hz? Thinking
with 1pF comp cap and 1E12, that's almost the magic
number.

 

[zopeon]

.i modify gate voltage to mos transistors,therewith the high mos resistance is controlled ,and the fc is further on controlled.

Hi,
In weak inversion the Mosfet transconductance is given by "Id/nKT" i.e. independant of Vgs. So assuming you are using MOS as R or simillar to OTA-C implementation, the Res = 1/gm. So if you are using Vgs to control your Res perhaps its not changing much. As we can achieve lower fc using a gm of 40pS from Series parallel OTA and higher C.
I hope I understood your question correct, If I am wrong correct me.

Hope this helps.

-Zopeon

 

[chichi]

i did not think too deep into it,but i think your answer is right,it helps me in figuring it out ..thanks