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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: highly linear current mirrors Reply #15 - Dec 30^th, 2011, 2:47pm hello robg, yaa, with opamp it kills BW, but it wont add much noise because noise of the opamp will be again cascode noise what i mean is it gives better results compared to all from matching point of view. Bottom line is resistive degeneration better. Thanks, raj.
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sandman

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Re: highly linear current mirrors
Reply #16 - Jan 2^nd, 2012, 2:56am

raja.cedt wrote on Dec 30^th, 2011, 10:13am:

hello sandman,
why don't you post some rough sketch of your complete ckt. I am not that good at mixers. But being a analog guy i can comment on CM and opamps.

Coming to current mirrors what do you mean by linearity, i mean increase the input current and check at the o/p current plot these two and check the ratio and till what point of input current your gain drops till 1 db or some margin. Coming to current mirror if you keep on increasing current, Vov start increase and this intern effects the mirroring ration, if you are facing this problem i would have a cascode or some vds matching. Did you tried like this. One more thing whats the max input current of your current mirror?

Till now i didn't find any papers on CM noise, could you please tell me those names...

Thanks,
Raj.

Hi Raj,

I'm still working on sketching out an intelligent representation for my mixer, but it is just at I described in my messages above. If I can manage it, I will try to post a schematic sometime soon...

With reference to the linearity, I simply measure the OIP3 for a two-tone signal at the inputs of a current mirror. This is simplistic but sufficient because I have a sufficient back-off from the compression point and am not concerned about linearity near the compression zone. I'm also not very worried about the mixer's linearity because all I have are a set of well-driven passive mixers in the resistive region.

Increasing the Vov lessens the Vth mismatch and also the level of noise, as RobG pointed out above. Having a cascode reduces the impact of having a low λ, which should help with a more accurate mirror ratio, but I observed that this did not improve the linearity and only worsened it... which is why I'm trying to locate papers which treat the linearity of Current-Mirror Amplifiers. Any suggestions anyone?

I wonder if there is a technical term for such RF CM amplification circuits ... ?

Vds matching using an op-amp as a -ve feed-back error amplifier would only add more noise and I wonder if that will improve linearity ?

As for papers on noise, I found an old one from 1975 on IEEE - Noise Characteristics of Current Mirror Sinks/Sources A. BILOTTI & E. MARIANI, but I'm sure there are better ones out there.

cheers,

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sandman

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Re: highly linear current mirrors
Reply #17 - Jan 2^nd, 2012, 3:07am

RobG wrote on Dec 30^th, 2011, 11:42am:

sandman wrote on Dec 30^th, 2011, 9:33am:

I set out to explore the noise equations for a CM amplifier by hand but this has be expected-ly slow. A couple of papers treat noise in CM's but only through simulation results.

Where is the hangup? The noise source should just be current sources in parallel with the devices in the mirror. The analysis is pretty straight forward from there. As I mentioned earlier, about all you can do to reduce thermal noise is increase the overdrive (i.e. Vgs-Vt).

Apparently he can get the linearity with resistive degeneration so in the worst case he should be able to do far better with a cascode device.

RobG, thanks for your comments. While the treatment of a simple current mirror is straight-forward, I'm using a couple of feedback methods at the input which add their own noise to the input (or output) - hence the added complexity in the details...

I simulated a simple CM alongside a CM with output-stage cascode. The cascode had worse output linearity and the noise was somewhat increased than compared to resistive degeneration. From what I'm familiar with, non-linearity in the CM comes about due to mismatches in the mirror and the transistor non-linearity (non-linear capacitances, deviation from square law, other non-idealities which I'm yet to figure out!). 'Cascoding' the output only compensates for the poor λ. There's a lot more to linearity than just mismatches that you point out.
What are your thoughts on this ?

Source degeneration is one method which works, but that's because it reduces the gain but has a limit to the dynamic range that can be achieved with this method.

My input DC voltage is fixed, so there probably isn't much I could do, to play with the Vgs-Vth than increasing drain current, so (resistive) degeneration seems to be the only solution I can think of.

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raja.cedt

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Re: highly linear current mirrors
Reply #18 - Jan 2^nd, 2012, 5:14am

hello sandman,
I didn't understand why cascode doesn't improve the linearity, becaz you are saying that cascode improves o/p impedance means linearity got improved. Check your cascode is biased properly or not. Again using opamp for Vds also some kind of cascode only.

you can check some noise analysis here http://www.av.it.pt/ruilaa/papers/2002/icecs2002_noise.pdf

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RobG

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Re: highly linear current mirrors
Reply #19 - Jan 2^nd, 2012, 7:18am

sandman wrote on Jan 2^nd, 2012, 3:07am:

RobG wrote on Dec 30^th, 2011, 11:42am:

sandman wrote on Dec 30^th, 2011, 9:33am:

I set out to explore the noise equations for a CM amplifier by hand but this has be expected-ly slow. A couple of papers treat noise in CM's but only through simulation results.

Where is the hangup? The noise source should just be current sources in parallel with the devices in the mirror. The analysis is pretty straight forward from there. As I mentioned earlier, about all you can do to reduce thermal noise is increase the overdrive (i.e. Vgs-Vt).

Apparently he can get the linearity with resistive degeneration so in the worst case he should be able to do far better with a cascode device.

RobG, thanks for your comments. While the treatment of a simple current mirror is straight-forward, I'm using a couple of feedback methods at the input which add their own noise to the input (or output) - hence the added complexity in the details...

I simulated a simple CM alongside a CM with output-stage cascode. The cascode had worse output linearity and the noise was somewhat increased than compared to resistive degeneration. From what I'm familiar with, non-linearity in the CM comes about due to mismatches in the mirror and the transistor non-linearity (non-linear capacitances, deviation from square law, other non-idealities which I'm yet to figure out!). 'Cascoding' the output only compensates for the poor λ. There's a lot more to linearity than just mismatches that you point out.
What are your thoughts on this ?

Source degeneration is one method which works, but that's because it reduces the gain but has a limit to the dynamic range that can be achieved with this method.

My input DC voltage is fixed, so there probably isn't much I could do, to play with the Vgs-Vth than increasing drain current, so (resistive) degeneration seems to be the only solution I can think of.

OK, based on what you said you are losing linearity from things like non-linear capacitance, non-square law etc. That is a hard problem for us to help with since we don't know your topology. Personally, I'm surprised those things affect the mirror accuracy but we all learn new things. I assume these bad effects happen at high frequency and large signal (transient) which always changes what we know.

Try it with the tolerances increased for sure!

As for the noise analysis, you should be able to just do the mirror effects as the noise sources should not influence one another.

Increasing current should help with noise and it sounds like it may be your only option. It will reduce your output impedance which might affect other things.

These are things that generally work. It is entirely possible that there is something special about your circuit that is negating some of these effects. It is hard to help more without seeing your actual circuit.

When I see things like this I usually dive into the equations and try to figure out what is going on taking into account different effects, but I understand that might be too difficult in your case. Starting with ideal devices is also helpful, although these days the model is so complex that it is difficult to do.

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aaron_do

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Re: highly linear current mirrors
Reply #20 - Jan 2^nd, 2012, 5:13pm

Hi sandman,

Did I understand correctly that your current mirror is operating at IF? In which case, nonlinear capacitance would not affect the IIPn.

Quote:

I simulated a simple CM alongside a CM with output-stage cascode.

Also, IIPn is a very sticky issue IMO. How simple was the current mirror you simulated? You will often find that the nonlinearity of one stage compensates the nonlinearity of another, especially if there isn't much gain seperating the stages (In some circuits, we deliberately add nonlinearity for such purposes). For a fair comparison, I guess you should keep the design as simple as possible.

Lastly, have you tried simulating a CM with both input and ouput-stage cascode? If only the output stage has a cascode, then the drain voltage of the input stage would change with the input signal level resulting in some mismatch.

cheers,
Aaron

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sandman

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Re: highly linear current mirrors
Reply #21 - Jan 4^th, 2012, 1:08am

raja.cedt wrote on Jan 2^nd, 2012, 5:14am:

hello sandman,
I didn't understand why cascode doesn't improve the linearity, becaz you are saying that cascode improves o/p impedance means linearity got improved. Check your cascode is biased properly or not. Again using opamp for Vds also some kind of cascode only.

Hi Raj,

I simulated a an improved Wilson. The bias is dependent on the external DC. If this is correct, then the cascode improves the output impedance, but that only goes to reduce the mismatch. Mismatch could only be a subset of the non-linearities at IF or RF if it degraded the IP3 when compared to the simple CM. Do you have an understanding of the other non-linear mechanisms in current mirrors operated at IF or RF ?

cheers,

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sandman

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Re: highly linear current mirrors
Reply #22 - Jan 4^th, 2012, 1:20am

aaron_do wrote on Jan 2^nd, 2012, 5:13pm:

Did I understand correctly that your current mirror is operating at IF? In which case, nonlinear capacitance would not affect the IIPn.

Quote:

I simulated a simple CM alongside a CM with output-stage cascode.

How simple was the current mirror you simulated? You will often find that the nonlinearity of one stage compensates the nonlinearity of another, especially if there isn't much gain seperating the stages (In some circuits, we deliberately add nonlinearity for such purposes). For a fair comparison, I guess you should keep the design as simple as possible.

Lastly, have you tried simulating a CM with both input and ouput-stage cascode? If only the output stage has a cascode, then the drain voltage of the input stage would change with the input signal level resulting in some mismatch.

Hi Aaaron,

I simulated both a simple CM and an improved Wilson CM (input and output cascode). The biasing there appears straightforward.

I agree with the IP3 being a naive assumption about device linearity. Besides, I also have a 1Vpp differential signal at the input ! I'm also considering the compression point. The input signal bandwidth is 0MHz to 500MHz. If this is low frequency for the capacitances to come into effect and non-linear capacitances are a big part of non-linearity, then I wonder why the improved R_out (reduced mismatch) didn't improve the linearity... Any suggestions?

cheers,

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aaron_do

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Re: highly linear current mirrors
Reply #23 - Jan 4^th, 2012, 1:39am

Hi sandman,

I didn't realise this was such a wideband system. In that case, nonlinear capacitance may have some effect. How much effect will depend on the ratio of the reactive to resistive impedance at the nodes which are swinging. At the input side of the CM, nonlinear cap may not have much effect due to the low input impedance, but at the output side, nonlinear cap may be very important due to the high output resistance.

One thing though, how are you checking the nonlinearity for such a wideband system? If you use IIP3, what are the two tone frequencies? I guess ultimately you are concerned with the spectrum mask requirements as well as EVM. Whatever it is it sounds like you've got quite a task ahead of you.

The P1dB is mainly determined by when the devices hit the triode region. Maybe you just need to optimize your biasing?

cheers,
Aaron

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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: highly linear current mirrors Reply #24 - Jan 4^th, 2012, 6:55am Running 500MHz through a current mirror and expecting BW and linearity is probably not going to happen. As frequency goes up, more and more of that high impedance output current source is getting lost to parasitic capacitance. I would consider other options.
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RobG

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Re: highly linear current mirrors
Reply #25 - Jan 4^th, 2012, 7:58am

loose-electron wrote on Jan 4^th, 2012, 6:55am:

Running 500MHz through a current mirror and expecting
BW and linearity is probably not going to happen.

As frequency goes up, more and more of that
high impedance output current source is getting
lost to parasitic capacitance.

I would consider other options.

I believe the output is terminated with an impedance much lower than the mirror output impedance, so mirror output impedance should have negligible effects. In fact, current mode should be faster since the voltage swings are small. That is what Gilbert's multipliers etc exploit.

rg

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RobG

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Re: highly linear current mirrors
Reply #26 - Jan 4^th, 2012, 8:50am

This topic is getting beyond my comfort range, but if it is nonlinear capacitance then using the narrowest device possible and still maintaining headroom requirements for the mirror should work better than resistor degeneration. Remember, you have to keep your gate/vdd voltage the same for this to be a fair comparison.

I suspect that there is more than one thing going on. I suggest replacing everything but the mirror with ideal devices and get the mirror working. You might even have to replace part of the mirror (e.g. the cascode devices if you use them). Vccs with the same gm can replace a mosfet. You may have to set the minimum and maximum currents and add tiny capacitances (0.1 fF) to keep the circuit from converging to unrealistic operating points.

Also try it at a lower frequency. Keep idealizing things until you get something that works and then use that as a starting point. Over the years I've wasted many months trying to get a non-working circuit to work by just trying different things. Almost always I would have been better off starting from an ideal circuit and working my way towards the final implementation one mosfet at a time.

rg

rg

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loose-electron

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Re: highly linear current mirrors
Reply #27 - Jan 4^th, 2012, 10:15am

RobG wrote on Jan 4^th, 2012, 7:58am:

loose-electron wrote on Jan 4^th, 2012, 6:55am:

Running 500MHz through a current mirror and expecting
BW and linearity is probably not going to happen.

As frequency goes up, more and more of that
high impedance output current source is getting
lost to parasitic capacitance.

I would consider other options.

I believe the output is terminated with an impedance much lower than the mirror output impedance, so mirror output impedance should have negligible effects. In fact, current mode should be faster since the voltage swings are small. That is what Gilbert's multipliers etc exploit.

rg

Then its a common source amplifier structure, not a current mirror.

No schematics = No clear definition = No good answers...

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aaron_do

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Re: highly linear current mirrors
Reply #28 - Jan 4^th, 2012, 4:56pm

Quote:

I believe the output is terminated with an impedance much lower than the mirror output impedance, so mirror output impedance should have negligible effects. In fact, current mode should be faster since the voltage swings are small. That is what Gilbert's multipliers etc exploit.

rg

Quote:

Then its a common source amplifier structure, not a current mirror.

In order to see whether nonlinear cap at the output can have an effect, he only needs to run an AC analysis (maybe in this case a PAC analysis if the mixer is included) to check the bandwidth at the output node. If it is much more than 500 MHz, then he needn't worry.

However, I don't think it's clear that the terminating resistance is low. He mentioned the mirror is followed by a "current passive mixer", not current-commutating devices (i.e. Gilbert Cell). Therefore, the terminating resistance of the CM will depend on the termination of the passive mixer, as well as things like LO drive, switch size etc. Although "current passive mixer" implies the passive mixer is followed by a low impedance, that is kind of atypical at RF (common-gate amplifier maybe?), so a schematic would help.

cheers,
Aaron

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