Voltage Transfer Characteristic (VTC)
The voltage-transfer characteristic (VTC) is a crucial representation of a gate’s electrical behavior in digital circuits, illustrating the relationship between input and output voltages. When a logical variable serves as input to an inverting gate, producing the variable “out,” the VTC depicts how changes in the input voltage (Vin) translate to changes in the output voltage (Vout). An example of an inverter VTC is shown in Figure 1.
The VTC provides key information
Nominal Voltage Levels: The nominal high and low voltages, VOH and VOL, are readily identifiable from the VTC. These values represent the levels of Vout corresponding to Vin = VOL and Vin = VOH, respectively.
Gate Threshold Voltage (VM): VM, the gate threshold voltage, is the point where the VTC intersects the line defined by Vout = Vin. VM represents the midpoint of the switching characteristics and is significant for analyzing circuits with feedback or sequential circuits.
Even when an ideal input value is applied to a gate, the output signal often deviates from the expected value. These deviations can be influenced by factors like noise or the load on the gate’s output, determined by the number of gates connected to it. In reality, a logic level is represented not as a single nominal voltage, but as a range of acceptable voltages, separated by a region of uncertainty.
What are VIH and VIL, and why are they significant in digital circuits?
VIH and VIL represent the points on the voltage-transfer characteristic where the gain (= dVout / dVin) equals -1. They delimit the regions of acceptable high and low voltages, respectively, on the VTC. These voltage levels are important for ensuring proper circuit operation.
What is the “undefined region” in the context of digital circuits, and why should steady-state signals avoid this region?
The undefined region, also known as the transition width (TW), is the range between VIH and VIL on the VTC. Steady-state signals should avoid this region to ensure proper circuit operation, as signals in this region might lead to unpredictable behavior due to the rapid changes in output voltage.
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