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How to bias the gm and id of telescopic opamp?

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neo

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Telescopic opamp again!

In designing the telescopic opamp:
1. can i make the input and output common-mode voltage the same, e.g. 0.9V for 1.8V power supply, because i need to design a pipeline adc, so the common-mode voltage for each stage should be same (is it a must?).
2. after i calculte the gm and id for each transistor, how can i bias them (the most involved point is to bias the three NMOS in a stack, e.g for the NMOS input telescopic), is it the right procedure that firstly using the ideal "vdc" to drive the gates, then designing bias circuit to bias the gates? or you can tell me some generic methods to do so.
can you give some heuristic advices?
 

Telescopic opamp again!

for 1.8v telescope op, it is hard to bias input common -mode voltage to 0.9v, although you want common-mode voltage for each stage to be same, but i don't think it need that the input and output common-mode voltage should be same.
 

Re: Telescopic opamp again!

1. it's hard, in fact it's no need, so focus more on pipeline ADC
2. it's ok, as you want
 

Telescopic opamp again!

yeah, it is not a must the input CM equals the output CM in the pipeline stages. it is only a common mode signal, right?
 

Re: Telescopic opamp again!

1.I don't think it's possible to design VOCM=VICM for the telescopic amp,especially for the 1.8v power supply.
2.for the pipeline ADC, There is not direct relationship between the VOCM and VICM.
3.the bias voltage is depended on the overdrive voltage of every transistor and if your circuit is fully differential, the cmfb circuit is needed!

neo said:
In designing the telescopic opamp:
1. can i make the input and output common-mode voltage the same, e.g. 0.9V for 1.8V power supply, because i need to design a pipeline adc, so the common-mode voltage for each stage should be same (is it a must?).
2. after i calculte the gm and id for each transistor, how can i bias them (the most involved point is to bias the three NMOS in a stack, e.g for the NMOS input telescopic), is it the right procedure that firstly using the ideal "vdc" to drive the gates, then designing bias circuit to bias the gates? or you can tell me some generic methods to do so.
can you give some heuristic advices?
Click to expand...
 

Re: Telescopic opamp again!

1. Sure you can and it is recommended. Otherwise you would end up with many
different common mode voltages which complicates design.
2. To achieve optimum biasing, which means if you have three NMOS or PMOS stacked, you are targeting to have a output headroom about 3 Vdsat above ground or Vdd-3Vdsat. Take any textbook of Ken Martin or Razavi's analog circuit design and you should be able to find one.
 

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