2022-11-04 10:32 Tags: # CMOS gate switching  The switching speed CMOS gates operate at is very important for their power consumption. If we change the input voltage slowly over time, the output voltage follows the [Digital CMOS Voltage Transfer Characteristic](Digital%20CMOS%20Voltage%20Transfer%20Characteristic.md):  Current characteristic is the same.  This current flowing in the inverter is not doing anything useful (e.g. charging capacitors). This is called the [crowbar current](crowbar%20current.md).  If it takes longer for the input to change, the crowbar current will take longer and the amount of energy consumed will be larger. We need fast transitions on the input of an inverter to minimise the [crowbar current](crowbar%20current.md). ## Fast Transition at the Input Consider the input and output as similar speeds:  The current peak is actually much smaller than for a slow transition, since the PMOS remains in the [triode region](MOSFET%20Operating%20Regions.md#Triode). We can essentially ignore this very small [crowbar current](crowbar%20current.md).  ## Slower Output Transition Consider slowing the output change by adding a load capacitor to the output.  This capacitor will increase the power usage (as current is needed to charge and discharge it) however the [crowbar current](crowbar%20current.md) will be much smaller since the PMOS spends the majority of its time in the triode region.  ## Changing the Crowbar Current Usually can't be done since a slow transition on the input of one gate would mean a slow transition at the input of another gate. It is more important to make sure the in and output transition times are balanced. [^1] --- # References [^1]: [vr-4602-wk08-sc02-cmosswitching](../../Spaces/University/ELEC4602%20–%20Microelectronics%20Design%20and%20Technology/Lectures/W8/vr-4602-wk08-sc02-cmosswitching.mp4)