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From what I understand, the voltage drop across a transistor is something like 0.5V and the minimum voltage for (conventional) current to pass from the emitter to the collector is 0.1-0.2V. If that is true, then how can CPUs operate at voltages as low as 1.5V if they have billions of transistors? Wouldn't current only get past the first 3 transistors?

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    \$\begingroup\$ Note that CPUs don’t run off BJTs but rather FETs \$\endgroup\$
    – rr1303
    Nov 14, 2020 at 22:52
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    \$\begingroup\$ And all of those billions of transistors are not in series. \$\endgroup\$ Nov 14, 2020 at 22:54
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    \$\begingroup\$ In MOSFET logic there is no sequential voltage drop, so you just need enough voltage to switch one transistor. \$\endgroup\$ Nov 14, 2020 at 22:56
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    \$\begingroup\$ Wikipedia might be a better place to start. \$\endgroup\$ Nov 14, 2020 at 23:02
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    \$\begingroup\$ The Blue smoke makes it work! Once the Blue smoke escapes it needs replacing. \$\endgroup\$
    – mhaselup
    Nov 15, 2020 at 2:18

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The less important part of the answer is that in order to go fast and consume less power, CMOS chips use transistors designed to operate at very low voltages (by 1990 standards).

The more important part is that you are thinking of transistor arranged in series, like this:

schematic

simulate this circuit – Schematic created using CircuitLab

But in a modern CMOS chip, the transistors are in parallel -- actually series-parallel, but only short series strings. Kind of like this. It's common to have three or four in series, and some in parallel, but not more than that:

schematic

simulate this circuit

Search on "CMOS logic circuit" for more information.

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