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Design >> Analog Design >> Distribute bias current or bias voltages ?
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Message started by harpoon on Feb 11^th, 2014, 2:16am

Title: Distribute bias current or bias voltages ?
Post by harpoon on Feb 11^th, 2014, 2:16am

hi all,

i am designing an array of amplifiers (say 100) and need to bias all of them in the same way.

i can generate a 100 bias currents and mirror them in the amplifiers (also need to generate cascode bias voltages) or i can generate a generic bias voltage and connect it to the gates of all the current mirrors in the amplifier.

i am keen on just distributing bias voltages (including cascodes) with a high series resistor (~10k) between the various bias lines.

comments anyone ?

Title: Re: Distribute bias current or bias voltages ?
Post by aaron_do on Feb 11^th, 2014, 2:55am

Its usually recommended to distribute current as the lower impedance picks up less noise.

Aaron

Title: Re: Distribute bias current or bias voltages ?
Post by harpoon on Feb 11^th, 2014, 3:27am

that is how i usually do it ... anyone in the "distributing voltages" camp ?

Title: Re: Distribute bias current or bias voltages ?
Post by carlgrace on Feb 11^th, 2014, 2:36pm

Aaron's right about voltage distribution being noisier but also think of crosstalk. If you have one voltage node connecting all your amplifiers together it is much easier for them to talk to each other.

One problem with distributing voltages is that you typically end up with errors because the "grounds" are far apart and there is some finite impedance between them.

One idea is to send a single current to amplifier 1, then generate an output current to amplifier 2, and so on. That way you are distributing currents but you don't need to route 100 wires from a master bias circuit.

Also, if you're designing an array, I highly recommend you do the layout so that it is "routed by construction". Try your best to limit top-level routing. It will save you time and the array will match better.

Title: Re: Distribute bias current or bias voltages ?
Post by RobG on Feb 11^th, 2014, 10:10pm

carlgrace wrote on Feb 11^th, 2014, 2:36pm:

A couple of observations - in addition to IR drops causing problems when matching voltages, process and temperature mismatches between components are issues if the distance between the devices is large. Beware, this can also bite you if you route current to create a voltage. I know of a group that tried to create local voltage references routing a current and ran into trouble because the local resistance didn't match the master one. Temperature, process, and packaging stress all conspire to make devices different across the die.

Avoid errors caused by the conversion device: if you are trying to create a voltage then you should generally route a voltage. (You might have to Kelvin connect things to avoid IR drop problems.) On the other hand if you want to create a current (for example, by biasing the gate of a local current source) you should route a current and mirror it locally.

When you have a huge array of devices it may make sense to do a little of both - make the layout tall and skinny so the biased devices are adjacent and have them all share the same local bias. You might be able to make this work for (guessing) 10 amplifiers without the end amplifiers debiasing. This means you will need ten of these sub-blocks to get 100 amplifiers, but you will only have to route ten bias lines. As Carl pointed out, you will have to be careful that the amplifiers don't crosstalk via the common bias line.

And for giggles I'm going to make Carl regret suggesting daisy chaining 100 biases since even a 5% error in the mirror gain will cause a 13,000% error in the last one. ;) I will leave it as an exercise for the reader to calculate the random mismatch and thermal noise errors of the last device (hint, ~sqrt(200)x). ;D

Title: Re: Distribute bias current or bias voltages ?
Post by harpoon on Feb 12^th, 2014, 12:29am

thanks everyone, all good ideas and suggestions ...

i think i may do the following ...

distribute current to 20 local bias blocks, and have each of these bias blocks will distribute the bias voltages to 5 amplifiers.

the distributed voltages are going to gates only, so i won't expect voltage drops.

also, at the receiving amplifiers, I intend to put a high value resistor in series to form a simple RC filter so that any high freq noise coupled into the bias line gets attenuated.

comments anyone ?

Title: Re: Distribute bias current or bias voltages ?
Post by aaron_do on Feb 12^th, 2014, 2:13am

Last point, which has already been touched on, but a voltage always comes with it's reference, so when you distribute a voltage, ideally you need to distribute its reference too. Although a capacitive load won't result in any DC drop along the line, there could be an AC drop. So any noise/transient could be a problem...

BTW, I'm not saying anything about the method you've chosen since it entirely depends on your design/layout.

Aaron

Title: Re: Distribute bias current or bias voltages ?
Post by carlgrace on Feb 12^th, 2014, 8:30am

First off, I do in fact regret suggesting you use daisy chaining. How boneheaded!

That said your plan sounds solid. My advisor once said "route currents globally and voltages locally" and that sounds like what you're doing.

The bias line filtering is a good idea. Besides the resistor you can put moscaps referenced to whatever supply the transistors are referenced to in order to do more filtering. (by just putting the resistor you're relying on the gate cap of the bias xtr which might be small).

Title: Re: Distribute bias current or bias voltages ?
Post by RobG on Feb 12^th, 2014, 9:33am

harpoon wrote on Feb 12^th, 2014, 12:29am:

the distributed voltages are going to gates only, so i won't expect voltage drops.

This is done all the time with good success but as aaron said, voltages come in pairs so you are also routing to the source - which is actually where the problem comes from since it is resistance in the source line that causes debiasing or worse, if signal or noise currents flow through that line the IR drop in it will modulate the gate-source voltage. This is not a reason to avoid routing a common gate voltage - just make sure you check the effects of metal resistance in the source return line because it can be unexpectedly high. Making the overdrive of the current source as large as possible will help and is always a good idea.

Quote:

also, at the receiving amplifiers, I intend to put a high value resistor in series to form a simple RC filter so that any high freq noise coupled into the bias line gets attenuated.

I've also done this - you will have a slow startup but that might not be important. Gate leakage could be important, yet it may not be modeled so check (I don't know if gate leakage would create noise problems with a large series resistance - base currents in BJTs do. Does anyone know?).

One thing I found totally counter-intuitive is that routing a small current is less noisy than routing a large current if you are filtering the resulting voltage generated (which could be either an IR or Vgs). The reason is that the AC impedance the current sees is 1/(sC) and the magnitude of the total thermal noise is less with a smaller current than with a larger current. Think about it - a current that is twice as large has sqrt(2) more noise. But I suppose if you made it too small you would run into 1/f noise and mismatch problems.

Of course, all this advice could be total overkill - it depends on your application.

And anyone familiar with Carl Grace's work knows he isn't a bonehead :).

Title: Re: Distribute bias current or bias voltages ?
Post by harpoon on Feb 13^th, 2014, 4:42am

thanks everyone for participating ... really interesting ideas and approaches ... :)

Title: Re: Distribute bias current or bias voltages ?
Post by tzg6sa on Feb 13^th, 2014, 4:46am

RobG wrote on Feb 12^th, 2014, 9:33am:

One thing I found totally counter-intuitive is that routing a small current is less noisy than routing a large current if you are filtering the resulting voltage generated (which could be either an IR or Vgs).

It was interesting to read and think about it. It is counter intuitive indeed! But it is only true if you use the same resistor to form the RC filter and convert the current to voltage. If you add an RC filter with a separate, high enough R to make the filtering pole approximately independent from the current-to-voltage converter resistor, then you get back the familiar higher signal power, higher signal-to-noise ratio.
The new, higher value resistor should not increase your noise, since it will be still kT/C. Of course it needs more area.

Title: Re: Distribute bias current or bias voltages ?
Post by RobG on Feb 14^th, 2014, 7:44am

Horror Vacui wrote on Feb 13^th, 2014, 4:46am:

I don't quite understand your assumptions. I'll revisit this and show the math. I think mine may neglect noise below the cutoff frequency.

Title: Re: Distribute bias current or bias voltages ?
Post by tzg6sa on Feb 17^th, 2014, 12:48am

Here is my simple calculation:
If we use higher current to generate the same voltage, then the resistance would be proportionally lower.
v=i*R & vn^2=c*i*R^2 p=1/RC
v'=2i*R/2 & vn'^2=c*2i*(R/2)^2=c*i*R^2 /2 = vn^2 /2 --> vn'=vn/sqrt(2) p'=1/(R/2)/C=2/RC
So in the double current case we have 1/sqrt(2) voltage noise if there is no capacitance. But if there is a cap, then the bandwidth of the RC filter increases. This results in increased noise. But if the filtering pole is independent of the current-to-voltage generating resistor, then higher current means lower noise.

Title: Re: Distribute bias current or bias voltages ?
Post by loose-electron on Feb 17^th, 2014, 2:08pm

if you are going a distance distribute by currents

Voltage distribution will pick up the ground to ground voltage difference as noise.
(modulated bias circuits are the result)

Voltage distribution will suffer from distance across the die matching issues.

Yes you can locally filter a voltage bias but you can not get around the matching over distance problem. Also you start to fight the RC filter problem of setting the pole low enough in frequency while keeping the area small. Small C can lead to big thermal noise contribution to your bias circuit.

In some big current array devices, I have done a two stage bias system of multiple current bias distributions to local reference diodes which then provided a bias point for the local devices. How big that "bias circle" was a function of the proximity matching data.

Title: Re: Distribute bias current or bias voltages ?
Post by harpoon on Feb 18^th, 2014, 5:21am

loose-electron wrote on Feb 17^th, 2014, 2:08pm:

In some big current array devices, I have done a two stage bias system of multiple current bias distributions to local reference diodes which then provided a bias point for the local devices. How big that "bias circle" was a function of the proximity matching data.

i like this idea ... distribute current globallly ... and then again locally.

Title: Re: Distribute bias current or bias voltages ?
Post by RobG on Feb 18^th, 2014, 8:11am

Horror Vacui wrote on Feb 17^th, 2014, 12:48am:

I think I know what you are talking about - it amounts to whether or not you are interested in the whole spectrum or just larger than 1/RC. Here's my math, all mistakes due to early morning mental fog.

Title: Re: Distribute bias current or bias voltages ?
Post by RobG on Feb 18^th, 2014, 8:14am

harpoon wrote on Feb 18^th, 2014, 5:21am:

loose-electron wrote on Feb 17^th, 2014, 2:08pm:

i like this idea ... distribute current globallly ... and then again locally.

This is what I and to some extent Carl where getting at, but you don't want too many daisy chained since the errors add.

Title: Re: Distribute bias current or bias voltages ?
Post by carlgrace on Feb 23^rd, 2014, 9:06pm

RobG wrote on Feb 12^th, 2014, 9:33am:

[quote
And anyone familiar with Carl Grace's work knows he isn't a bonehead :).

Shucks, Rob, that is really nice of you to say. But when I make a bonehead comment it is still a bonehead comment no matter what I've done in the past. ;)

Title: Re: Distribute bias current or bias voltages ?
Post by Lex on Mar 3^rd, 2014, 2:10am

In image sensors where there can be millions of amplifiers and ADC's on several micron pitch, I've seen mostly voltage routing. The two-stage-bias-system that is mentioned, I've seen on silicon as well, thought it has the disadvantage that it can create 'block patterns' as the part of the noise is correlated. Block patterns are more visible than the smooth gradient that would normally arise. (Hence more disturbing to the eye.) Of course, it depends on the application whether this is a problem.

Another obvious thing is whether changing from voltage to current routing costs power. In the image sensor example, the extra current draw would also be quite costly.

Title: Re: Distribute bias current or bias voltages ?
Post by RobG on Mar 3^rd, 2014, 5:18am

Great insight with the "block noise" Lex.