![]() The NAND gate seems a curious choice for our first useful gate, but it is actually a natural one, because it is simple, and because it shares with all simple CMOS gates the property of being anti-monotonic: as inputs change from 0 to 1, that can only make more of the n-type transistors in the bottom half of the gate conduct, and fewer of the p-types in the top half and that can only cause the output, if it changes at all, to change from 1 to 0. We call the function NAND, because the output is low only if both a and b are high. ![]() On the other hand, of both inputs are high, the two n-types both conduct, driving the output low, but the p-types do not conduct and the output is disconnected from Vdd. 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. Chances are that no matter if someone's building a whole CPU or something more special-purpose and limited, that someone will be doing so in terms of already-assembled-and-composable gates rather than transistors directly, if only for the sake of one's own sanity.This circuit has one input at a and one output at z. ![]() Building a whole general-purpose CPU from individual transistors is much easier when those transistors are already arranged on a little board you can plug into your bigger board. "Chips" are just discrete components, whether crafted from a single chunk of silicon or itself built out of discrete components and treated as a single discrete unit. One of the key advantages of a general-purpose CPU is that it's general purpose and can be made to do all sorts of different things, but there are certainly plenty of cases where that ain't necessary and you'd need a fraction of that capability at most. There are multiple ways to skin a cat, though (perhaps literally this would be the post-apocalypse, after all!), and you're right that there are numerous ways to put transistors to use besides building full-blown CPUs. ![]() There are examples elsewhere in these comments (linked by myself and others) of CPUs with similar (if not greater) transistor counts soldered together by hand out of TTL chips or even out of individual discrete transistors. ![]()
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