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OTA-C filter input range

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edabrduser

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I am trying to design a gm-c LP filter with a cutoff of 100kHz. I used a basic single-ended folded-cascode stage and a poly capacitor.

When I apply an input signal of few uV the output is in the range of 10's of mV. My actual signal amplitude is in the range of 100mV, what can be done to improve the input range of the filter. Thanks.
 

LvW

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In case of a 1st order lowpass you have an R||C output arrangement, don't you?
Reduce R and increase C correspondingly.
In addition you can reduce the transconductance gm.
 

edabrduser

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In case of a 1st order lowpass you have an R||C output arrangement, don't you?
Reduce R and increase C correspondingly.
In addition you can reduce the transconductance gm.
I didn't use any R at the output. Just a capacitor with one-end at the output of the OTA and the other end grounded.

How are practical gm-c filters built? When I see papers they just use Gm cells and capacitors. Because the Gm cells are operated in open-loop the gain is quite high and the input signal range is severely limited.
 

pfd001

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I didn't use any R at the output. Just a capacitor with one-end at the output of the OTA and the other end grounded.

How are practical gm-c filters built? When I see papers they just use Gm cells and capacitors. Because the Gm cells are operated in open-loop the gain is quite high and the input signal range is severely limited.
You can improve the linearity of the gm cell.
 

edabrduser

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You can improve the linearity of the gm cell.
I am not concerned about the linearity of the filter. I have a signal that is few 10's of mV in amplitude. The gm-C filter can take a maximum of 10's of uV before it saturates ( as it being operated in open loop)
 

LvW

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As I have mentioned before: You can reduce gm.
Or you can use OTA's with feedback.
Example: OTA with 100% negative feedback (R=0 between output and neg. input) and with a Cload at the output is equivalent to a 1st order lowpass with time constant T=sCload/gm.
Remember, the most efficient and most popular circuit is design is based on active realization of passive topologies (active L technique).
 

abcyin

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the gm cell is not so linear as you expected, so I suggested you to use the OTA-C filter with feedback configuration, which will meet the linearity requirement. and also some voltage gain could be realized together with the feedback circuit.
 
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