Common Mode Range Calculations Opamp CMOS

[Puppet1]

common mode range

How do you calculate input and output common mode range of say a opamp ?

I am confused as to where the Vt drop and Vds drops are and where the Vds - sat is used and how to calculate these.

Please help.

 

[Hero]

icmr of op amp

Hi,

For CMRR in dB look at data sheet for selected OP. You don't need to measure this data take it from data sheet.

 

[safwatonline]

common mode range op amp

well input CM has a minimum value (for NMOS) which is related to the tail current (i.e. the minimum CM is the minimum voltage at which the tail current source is still operating in saturation Vcm-min=Vgs-pair+Veff-tail=Veff-pair+Veff-tail+Vth
taking an input signal of amplitude X for example
then Vcm-min=Veff-pair+Veff-tail+Vth+X

while the maximum CM at the input is the maximum voltage at which the diff. pair is in saturation
Vgs-Vth<Vds then Vg-Vth<Vd then Vcm-max=Vg-max=Vdmin-Vth


for output CM range u set a Veff for each MOS then the maximum-output-CMR=Vdd-(sum of Veff of all MOS in stack)
regards

 

[qiushidaren]

icmr of the opam

Excuse Safwatonline, your explanation is quite nice, but I can't catch this eqution Vcm-max=Vg-max=Vdmin-Vth, so could you explain it for me, thank you.

 

[safwatonline]

op amp common mode range

u want to consider the maxmum input common mode , in any MOS for saturation Vgs-Vth<Vds then Vg-Vs-Vth<Vd-Vs
then Vg<Vd+Vth for worst case u put the Vg as maximum and u put at the same time Vd as minimum and given those condition the inequality must be satisfied
so Vgmax<Vdmin+Vth
Vgmax is the max gate voltage which is Vcm-max+Vinput-signal-max

Vdmin can be VoutCM-Vout-signal-max

 

[shaikhsarfraz]

common-mode range

For ICMR, have a unity feedback Op Amp.
On the other input of the opamp give a ramp signal from -5 to 5.

Plot the output. The region with unity slope is the ICMR

 

[Crusader370]

op amp data sheet cmos

Now I am really confused for the ICMR. Is it where all the transistors are in saturation, or where it can track the input DC voltage?

 

[shaikhsarfraz]

calculating common mode input range

sweep one of the input.

PLot the drain current of Current Sink

The region where current sink goes into saturation, gives the ICMR min

 

[beckchm]

op amp icmr

just for single-end op , you can connect the op like a unity-gain system

 

[safwatonline]

range of opamp

calculation aint simulation or measurments!!