Voltage Transfer Characteristics (VTC)
Vth: Inverter Threshold Voltage
0 < Vin < Vth Vout=‘1’
Vth < Vin < VDD Vout=‘0’
as the name implies, the invertor inverts the value of its input signal.
if input is 0 (low), then output is V+ (high)
if input is V+ (high), then output is 0 (low)
Voltage Transfer Characteristics (VTC)
Unlike the ideal inverter, the defination of voltage 'high' and 'low' are very clear cut. When we talking about non-ideal inverter, we have another way to define whether it is high or low (logic 1 or logic 0)
VOH:
the MAX value of output voltage Vo when the output is classified as '1' (high)
VOL:
the MIN value of output voltage Vo when the output is classified as '0' (low)
VIH:
the MAX value of input voltage Vin when the input is classified as '1' (high)
VIL:
the MIN value of input voltage Vin when the input is classified as '0' (low)
Noise Margins
Consider the situation that occurs ofthen in a digital system where an inverter (or a logic gate based on the inverter circuit) is driving another similar inverter. If the output of the driving inverter is high at VOH, we see that we have a 'margin of safety' equal to the difference between VOH and VIH. In other words, if for some reason a disturbing signal (noise) is superimposed on the output of the friving inverter, the driven inverter would not be 'bothered' as long as this noise does not decrease the voltage at its input below VIH. Thus we can say that the inverter has a noise margin for high input, NMH:
NMH = VOH - VIH
Similarly, if the output of the driving inverter is low at VOL, the driven inverter will provide a high output even if noise corrupts the VOL level at its input, raising it up to nearly VIL. Thus we can say that the inverter exhibits a noise margin for low input, NML:
NML = VIL - VOL
