~ELECTRONICS~

Part 2

The CMOS Inverter

The most important CMOS gate is the CMOS inverter. It consists of only two transistors, a pair of one N-type and one P-type transistor. The applet demonstrates how the inverter works.

Voltage levels are shown in colors as above: a logical '1' corresponding to electrical level VCC is shown in red, a logical '0' (corresponding to 0V or GND) in blue.

Again, a floating wire (not connected to either VCC or GND) is shown in orange. Because of parasitic effects, the voltage level on such wire may reach some undefined voltage between VCC and GND after some time. A floating wire will cause problems, when its voltage is around VCC/2, because a gate voltage around VCC/2 on either N-type and P-type transistors implies that the transistor is conducting. The applet illustrates why this is a serious problem: When both transistors are conducting, there is a direct path from VCC to GND, and this implies a short-circuit condition (shown in light green), which dissipates much energy and may destroy the device.

Click anywhere in the applet to toggle the input voltage for the inverter from GND to VCC to Z (unknown) to GND.

If the input voltage is '1' (VCC) the P-type transistor on top is nonconducting, but the N-type transistor is conducting and provides a path from GND to the output Y. The output level therefore is '0'. On the other hand, if the input level is '0', the P-type transistor is conducting and provides a path from VCC to the output Y, so that the output level is '1', while the N-type transistor is blocked.

If the input is floating, both transistors may be conducting and a short-circuit condition is possible:


 

Part 3