Dear all,
below is the very know to you the class AB OP-amp,
It is by literature or simulation, the output voltage swing towrds rails. However, if we assume that output transistors should be in the saturation region as a condition of operation, then it looks impossible to have rail to rail output swing.
My question, suppose I am using single supply operation with VDD = 3.3 V, and I want the output signal to swing between 0.3 and 3 V,
take now the cade where Vo = Vomin = 0.3 V and prove please that MN9 is in the saturation region,
here is my solution, for MN9 to be in saturation then VDS(sat)(MN9) >=VGS(MN9)-Vthn,,,,
say we are at the border of sat then VDS(sat)(MN9) =VGS(MN9)-Vthn
Subsitute the value of VDS sat(MN9) and Vthn results in VGS <= 0.9 V
to find VGS(MN9) from kirschoff voltage loop at the gate of MN9 to VDD , thus VGS= VDD-VDS sat(MP4)-VDS sat(MP6)- VDS sat(MP8)
if you assume typical values of VDS sat(MP4)=VDS sat(MP6)-=VDS sat(MP8)=150 mV and Vthn = 0.6 V.....then VGS-Vthn =2.25 V
Since 2.25 V is greater than 0.9 V, it means driver transistor MN9 is at triode region even before appraoching the maximum value
Thank you in advance
Regards