dc analysis of OTA(cmfb level choosing)

[vhdl00]

common mode dc analysis

Hi all,
I am trying to do dc analysis of open loop OTA. at the beginning I was doing dc differential sweep to make sure all the device working in saturation region, after I has been told to use CMFB to set up the operation point, I am kind of confused: how to choose Vcm , do we have to choose it randomly(such as in the middle of the rail to rail)? or we have to do dc common mode sweep to find it ?

Thanks.

 

[ccw27]

Well your CMFB will take the output voltage and compare to a Vcm, this is your desired output voltage. Your CMFB will feedback and adjust accordingly so that your output is around Vcm. Gray and Meyer book has a good review on this topic.

Hope this helps

 

[vhdl00]

Hi ccw27,
thanks.
i was reading the grey and meyer's book. I remember the difference between voc and vcm have to be small, otherwise the control device will be out of sat region?

 

[suria3]

If you do the dc common mode sweep it will be better to find the dc center voltage.
Here you will know where actually your output swing will start to clip with having from the lowest to highest dc voltage from output. Then you take the average value of them. This will be you dc common mode voltage. Normally, if you deal with amplifier design with 1.8V power supply, your common mode dc will be 1.2V.
Hope this helps you.

 

[montage2000]

at the middle of useful range

 

[jordan76]

If you do the dc common mode sweep it will be better to find the dc center voltage.
Here you will know where actually your output swing will start to clip with having from the lowest to highest dc voltage from output. Then you take the average value of them. This will be you dc common mode voltage. Normally, if you deal with amplifier design with 1.8V power supply, your common mode dc will be 1.2V.
Hope this helps you.

Hi suria3,

Could you tell why normally the common mode dc voltage will be 1.2V for 1.8V power supply amplifier design? Thanks!

regards,
jordan76

 

[suria3]

If you do the dc common mode sweep it will be better to find the dc center voltage.
Here you will know where actually your output swing will start to clip with having from the lowest to highest dc voltage from output. Then you take the average value of them. This will be you dc common mode voltage. Normally, if you deal with amplifier design with 1.8V power supply, your common mode dc will be 1.2V.
Hope this helps you.

Hi suria3,

Could you tell why normally the common mode dc voltage will be 1.2V for 1.8V power supply amplifier design? Thanks!

regards,
jordan76

Jordan,

Choosing CM=1.2V in general depend on type of circuits that we used to design. Like for my case, when normally dealing with Receiver system design, which has the VDD=1.8V, the signal transmission throughout the system will be from 1.8V to 0.6V. In a differential amplifier design in RX system, signal normally will saturate at current source (tail) at 600mV. So, this swing now is 1.8V to 0.6V and the average is 1.2V which is CM. So, totally it all depend on the output common mode of an amplifier when we sweep the input.

Thx