We can analyse the behaviour of this circuit case by case, but the upshot is that if one or both of the inputs are grounded, then one or both of the p-types will conduct, driving the output high and in these cases the two n-types are never both conducting, so there is no path from the output to ground. Each input of the circuit, a or b, is connected to one of the n-types and one of the p types. In this circuit (as you see) there are two n-type transistors in series connecting the output z to ground, and two p-types in parallel connecting it to Vdd. We can't do much with just an inverter! So let's next look at this design for a NAND gate. It is this amplifying behaviour of logic gates that lets us build large, complex assemblies of gates with many stages and have the whole thing behave in a digital way. Despite the fact that a negligible current is drawn at the input, substantial current is available at the output. Also, the gates of the transistors are electrically insulated from the supply, so no current is drawn from the input a in a steady state.Ĭurrent does flow during transistions, because there is some capacitance between the gate and channel that must be charged or discharged when the gate voltage changes, and for a brief period both transistors will be partially turned on, allowing current to flow through them.įacts like this lead to the observation that the power dissipation of a CMOS logic circuit is proportional to the rate of state transitions.Īnother important observation is that if the input a is not actually at the rails, but close to them, then that will be enough to fully turn off one transistor and fully turn on the other, and the output will be very close to the opposite rail. It's worth noticing that in a steady state, only one of the transistors is conducting, so no quiescent current is drawn from the supply. We can make a truth table that summarises the behaviour of the circuit. On the other hand, if a is connected to Vdd, then it is the n-type transistor that conducts, connecting z to ground. If a is connected to ground, then the n-type transistor does not conduct, but the p-type does: this connects the output z to Vdd. This circuit has one input at a and one output at z.
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