how to drive differential pair in op amp

[srikanth89]

I need to drive the nmos differential pair of an op amp. Method i know is to give input common mode voltage. is there any way without that because, previous module in our project is charge scaling capacitive DAC. where only step changes occur like 0.45v, .9v, 1.35v. inputs to differential pair are analog signal and converted analog signal from DAC. so the differential input pair transistors are not in saturation.

I can provide schematic if needed.

 

[diemilio]

Usually the input common mode voltage is set when you close the feedback loop in your amplifier so you don't need to specify it externally. Now, if you are using your amp in open loop configuration you probably don't have the appropriate topology because your amplifier is saturating (by saturating I mean that your transistors are either going into triode or cutoff) for some of the voltages that it's supposed to be able to handle.

Please provide the schematic, that way it will be easier to understand what you are trying to do.

Regards,

diemilio

 

[srikanth89]

this is the schematic we have used

 

[diemilio]

OK, that's fine... but how are you connecting the amp to the output of the DAC??

I'm guessing you are using it either as a buffer stage or a non-inverting stage, so there should be negative feedback which will increase your ICMR. If this is what your doing then what you need to check is your ICMR+ and ICMR- values, which for your topology are:

ICMR+ = Vdd-VsgP3+VtN0
ICMR- = VdsatN2+VgsN0

What you need to do is set this values to the MAX and MIN output values of your DAC, that way you'll guarantee that the amp is not saturated.

Hope this helps,

diemilio