What is the significance of switching threshold in cmos?

[rp276]

what is the significance of switching threshold in cmos?? how it affects the circuit?? please help..

 

[rahdirs]

Re: switching threshold in cmos

@rp276:

• The switching threshold, Vm, is defined as the point where Vin = Vout. Switching threshold can be set by the ratio of relative driving strengths of the PMOS and NMOS transistors.
• To move Vm upwards, a larger value of ratio is required, which means making the PMOS wider. Increasing the strength of the NMOS, on the other hand, moves the switching threshold closer to GND.
  
  Why is switching threshold important???
• The effect of changing the Wp/Wn ratio is to shift the transient region of the VTC. Increasing the width of the PMOS or the NMOS moves VM towards VDD or GND
• respectively. This property can be very useful, as asymmetrical transfer characteristics are actually desirable in some designs.
• The attached image shows one advantage of understanding Vm.

By the way that below image is from Rabaey.