It is the only way to do it.Regarding your suggested two-stage op-amp, I see you are using the minimum channel length, is it possible? I only use the minimum channel length in digital design
Look on power consumption of microprocessors, they are counted in hundreds of watts. Look on TX power in wifi or 5G. Again it is in watts. Miliamps are not unusual numbers.Also for me I never had a current value like 20 mA or 2.4 mA for biasing the amplifier, you can say I have no experience but would such value accepted in IC community? especially I will convert it to fully differential amplifier where it will about have twice the power.
It might be possible to get 250MHz with 2mA fully differential design but with fancy architecture only (which I will not share).
Regarding load cap. Yes, feedback cap should be included, but I believe this cap is below 1pF, isn't it? So, in total it would be ca 3-5 pF. At least for 10bit pipeline we used ca 500fF caps, so 12 bit might have 4 times bigger only.
In filter feedback? So another cap in filter is 50pF? Nevertheless, I don't believe you need more than 300fF in filter feedback. You don't add gain in AAF and your noise constraints for sure are not so strict. Or they are?I use 5 PF
In filter feedback? So another cap in filter is 50pF? Nevertheless, I don't believe you need more than 300fF in filter feedback. You don't add gain in AAF and your noise constraints for sure are not so strict. Or they are?
Don't use Salen-Key topology.Consider the circuit as an example of the Sallen-Key filter below, suppose that C1 = C2 = 5 pF.
In discrete design is hard to find good Capacitor in range of single pF, and discrete opamp has terminal capacitance in order of single pF due to package, pads, esd, etc.It is recommended not to use a capacitr less than 10 PF otherwise the op-amp parasitic will dominate it and shift the filter frequency, that what told by TexasInstrument filter design procedure,
Since my task is to have tunable filter by using MOS resistor,
I know this implementation. It might be tricky due to diode leakage and swing is limited to +/-0.3V. So, you will use only 20% of available swing. Also body effect in source follower might affect performance.
It is one of the method in material science.what the hell is Impedance spectroscopy.
If you create one tunable unit and use it to create all resistors, you will be able to trim cut-off frequency and keeping Q factor and gain constant. However, if you want to vary all filter parameters you will suffer for old proverb "if sthing is for everything then is useless" and need to trim every component independently.how it will be possible to have fc independent on Q and gain (K) when they share all components (except for R1) ?
I would say in non-tunable and since fc is not depending on R1 then one can use to adjust the gain and compensate for Q change.
For tunable filter we need to shift R1 as well to keep constant gain ratio that is not changing by selected fc.
you create one tunable unit and use it to create all resistors, you will be able to trim cut-off frequency and keeping Q factor and gain constant.
So wide range of cut-off frequencies might force you to use bank of caps and limit maximum resistance due to noises.
Mentioned in linked article (there is also a book of those two guys) pseudo resistor implementation might be not suitable for low resistance values.
I might be wrong of course. What I remember when I designed such filter last time, R1=R3=Ru and R2=4Ru and Ru was trimable. Gain was set by ratio of R2/R1=4 independent to Ru value. Q-factor was set also by ratio of proper RCs fixed by trimable units.Whih combination setup leads to these properties? I tried several assumptions on the transfer function formula to make them independent but concluded that is not possible. Which values ratios satisfy this condition ?
If you study topic deeply you find this statement false. Active RC filters are the only choice in many applications. Also, in case of set lower fc you will not need so high GBW. So opamp power can be scaled as well. In very high frequencies it is a problem. But RF is a problem itself.Integrated Active filters are very power-hungry that makes them not suitable for high fc
With gm-C is not possible to get more than 50dB attenuation (usually it is 40dB). Swing is limited to input pairs linearity (100mV-200mV maybe), sacrificing the noises. They are better in very high frequency applications (fc>100MHz) but still with moderate parameters.So if I would compare MOS-C filter with gm-C filter, I think gm-C win with honour
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