gate is responsible for the formation of channel.if the gate voltage is greater than vt then channel will form.then why the reason for getting VDD-Vt rather than VDD at the source,as VDD is not responsible for formation of channel.
The potential difference between gate and source is responsible for the formation of channel.
When the source voltage reaches VDD-VTH, this potential difference is equal to VTH. When the voltage at source tries to go beyond VDD-VTH, VGS and VDS both become less than VTH... thus the MOSFET enters in to linear region (or subthreshold region). Hence the maximum source voltage = VDD-VTH.
Theoretically, the source voltage can reach at VDD, with the help of subthreshold current charging the output (source) node... but it'll take infinite time for this.
The potential difference between gate and source is responsible for the formation of channel.
ReplyDeleteWhen the source voltage reaches VDD-VTH, this potential difference is equal to VTH. When the voltage at source tries to go beyond VDD-VTH, VGS and VDS both become less than VTH... thus the MOSFET enters in to linear region (or subthreshold region).
Hence the maximum source voltage = VDD-VTH.
Theoretically, the source voltage can reach at VDD, with the help of subthreshold current charging the output (source) node... but it'll take infinite time for this.