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Comment by lisper | Hacker News Reader

> transistor logic which is relatively high current

Huh? The gate resistance of a FET is hundreds of megaohms, so the input current is measured in nanoamps. That doesn't seem like high current to me.

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That's the static resistance, but when it's switching there is current to charge/discharge the gate. Don't forget leakage currents too, which are quite high in modern CPUs --- they consume ~100A at ~1V when in active use.

Sure, but that is amortized over billions of transistors. If you had billions of vacuum tubes the current draw would be vastly higher.

Transistors in TTL logic use mA of input current because they're bipolar, not FET transistors.

In classic TTL the input currents are also very asymmetric, e.g. for a high level you mustn't draw more than a few dozen µA out of an input, for a low level you have to draw something like 1.5 mA or so.

This is different from current-steering based logic (all kinds of ECL, including CML/SCL) were the current in the circuit stays the same, but only takes a different path depending on state. Supply current is largely independent of circuit state with these.

That's true, but we're talking about computers here. Computer chips don't use TTL, they're all CMOS, which uses FETs (MOSFETs to be precise).

CMOS didn't exist when the first transistorized computers were made.

Ah, right. I thought the OP was referring to modern CPUs.

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