So I've been perusing the "CMOS Circuit Design, Layout and Simulation" book and I had some question about the practical implementations of some of the things explained there : • Looking at some differential amplifier, they rely on bias voltages. In a preceding chapter, they do give some example of generating those, but I'm wondering a couple of things: ◦ Can the same bias generator be used for several "blocks" that use the same bias currents (like several diff amps) ? Because it's a rather "large" block in itself, so if it needs repeating each time, that definitely takes space. ◦ Then how do you deal with matching ? My understanding was that the transistor being biased needed to be matched to the corresponding one in the bias generator, but if you share a reference, they might be more appart ... even in the bias generator itself ( look at for instance ), there are many of them sharing the same gate potential and you can't possibly match all of them (makign they close / shared centroid / ... that kind of stuff)

Generally, you have a current source with multiple outputs, then you have your dedicated bias circuit for each block. Sometimes you can share biasing voltages, but be aware that there will be interference. Avoid routing biasing voltages.

Ok, so if I understand correctly, the transistor that the Vbias biases (which is effectively the current source), would be kept close to where the bias is generated and it's it's Drain or Source connection which carries the intended bias current that would be routed away.

It depends... as I see in this circuit, the transistors connected to vbiasp may be the output of a current mirror. They should be very close to the input transistor of the current mirror. The transistors which output the cascode biasing voltages doesn't really need to be close to the other transistors for matching purposes. Anyway, it all depends on the circuit you're trying to bias, after all.

Thanks, I'll re-read those chapters keeping this in mind.

@tnt: I think Luis has covered it pretty well. But just to add that from past experience, I used to build chips with chip-wide bias networks. Always move currents (not voltages) around the chip---the metal is very well insulated and wires have very little loss, so a microamp on one side of the chip is still a microamp on the other. Make sure the current-carrying wires are well shielded to avoid picking up noise (route them first so that all digital routing has to avoid them). Use current mirrors at the bias generator to make as many copies of the current as you need. Then convert the current back into a bias voltage at the destination.

@Tim Edwards 👍 Thanks.